Genetic regulation of biological nitrogen fixation

The micro-ecological feature of colonies is a potential strategy for Phaeocystis globosa bloom formation

Phaeocystis globosa is among the dominant microalgae associated with harmful algal blooms. P. globosa has a polymorphic life cycle and its ecological success has been attributed to algal colony formation, however, few studies have assessed differences in microbial communities and their functional profiles between intra- and extra-colonies during P. globosa blooms. To address this, environmental and metagenomics tools were used to conduct a time-series analysis of the bacterial composition and metabolic characteristics of intra- and extra-colonies during a natural P. globosa bloom. The results show that bacterial composition, biodiversity, and network interactions differed significantly between intra- and extra-colonies. Dominant extra-colonial bacteria were Bacteroidia and Saccharimonadis, while dominant intra-colonial bacteria included Alphaproteobacteria and Gammaproteobacteria. Despite the lower richness and diversity observed in the intra-colonial bacterial community, relative to extra-colonies, the complexity and interconnectedness of the intra-colonial networks were higher. Regarding bacterial function, more functional genes were enriched in substance metabolism (polysaccharides, iron element and dimethylsulfoniopropionate) and signal communication (quorum sensing, indoleacetic acid-IAA) pathways in intra- than in extra-colonies. Conceptual model construction showed that microbial cooperative synthesis of ammonium, vitamin B12, IAA, and siderophores were strongly related to the P. globosa bloom, particularly in the intra-colonial environment. Overall, our data highlight the differences in bacterial structure and functions within and outside the colony during P. globosa blooms. These findings represent fundamental information indicating that phenotypic heterogeneity is a selective strategy that improves microbial population competitiveness and environmental adaptation, benefiting P. globosa bloom formation and persistence.

Nitrogen fixation in the widely distributed marine γ-proteobacterial diazotroph Candidatus Thalassolituus haligoni

The high diversity and global distribution of heterotrophic bacterial diazotrophs (HBDs) in the ocean has recently become apparent. However, understanding the role these largely uncultured microorganisms play in marine N2 fixation poses a challenge due to their undefined growth requirements and the complex regulation of the nitrogenase enzyme. We isolated and characterized Candidatus Thalassolituus haligoni, a member of a widely distributed clade of HBD belonging to the Oceanospirillales. Analysis of its nifH gene via amplicon sequencing revealed the extensive distribution of Cand. T. haligoni across the Pacific, Atlantic, and Arctic Oceans. Pangenome analysis indicates that the isolate shares >99% identity with an uncultured metagenome-assembled genome called Arc-Gamma-03, recently recovered from the Arctic Ocean. Through combined genomic, proteomic, and physiological approaches, we confirmed that the isolate fixes N2 gas. However, the mechanisms governing nitrogenase regulation in Cand. T. haligoni remain unclear. We propose Cand. T. haligoni as a globally distributed, cultured HBD model species within this understudied clade of Oceanospirillales.

Identifying functional multi-host shuttle plasmids to advance synthetic biology applications in Mesorhizobium and Bradyrhizobium

Ammonia availability has a crucial role in agriculture as it ensures healthy plant growth and increased crop yields. Since diazotrophs are the only organisms capable of reducing dinitrogen to ammonia, they have great ecological importance and potential to mitigate the environmental and economic costs of synthetic fertilizer use. Rhizobia are especially valuable being that they can engage in nitrogen-fixing symbiotic relationships with legumes, and they demonstrate great diversity and plasticity in genomic and phenotypic traits. However, few rhizobial species have sufficient genetic tractability for synthetic biology applications. This study established a basic genetic toolbox with antibiotic resistance markers, multi-host shuttle plasmids and a streamlined protocol for biparental conjugation with Mesorhizobium and Bradyrhizobium species. We identified two repABC origins of replication from Sinorhizobium meliloti (pSymB) and Rhizobium etli (p42d) that were stable across all three strains of interest. Furthermore, the NZP2235 genome was sequenced and phylogenetic analysis determined its reclassification to Mesorhizobium huakuii. These tools will enable the use of plasmid-based strategies for more advanced genetic engineering projects and ultimately contribute towards the development of more sustainable agriculture practices by means of novel nitrogen-fixing organelles, elite bioinoculants, or symbiotic association with nonlegumes.

Integrated Proteomics and Metabolomics Reveal Altered Metabolic Regulation of Xanthobacter autotrophicus under Electrochemical Water-Splitting Conditions

Biological-inorganic hybrid systems are a growing class of technologies that combine microorganisms with materials for a variety of purposes, including chemical synthesis, environmental remediation, and energy generation. These systems typically consider microorganisms as simple catalysts for the reaction of interest; however, other metabolic activity is likely to have a large influence on the system performance. The investigation of biological responses to the hybrid environment is thus critical to the future development and optimization. The present study investigates this phenomenon in a recently reported hybrid system that uses electrochemical water splitting to provide reducing equivalents to the nitrogen-fixing bacteria Xanthobacter autotrophicus for efficient reduction of N2 to biomass that may be used as fertilizer. Using integrated proteomic and metabolomic methods, we find a pattern of differentiated metabolic regulation under electrochemical water-splitting (hybrid) conditions with an increase in carbon fixation products glycerate-3-phosphate and acetyl-CoA that suggests a high energy availability. We further report an increased expression of proteins of interest, namely, those responsible for nitrogen fixation and assimilation, which indicate increased rates of nitrogen fixation and support previous observations of faster biomass accumulation in the hybrid system compared to typical planktonic growth conditions. This work complicates the inert catalyst view of biological-inorganic hybrids while demonstrating the power of multiomics analysis as a tool for deeper understanding of those systems.

Proteolytic control of FixT by the Lon protease impacts FixLJ signaling in Caulobacter crescentus

Responding to changes in oxygen levels is critical for aerobic microbes. In Caulobacter crescentus, low oxygen is sensed by the FixL-FixJ two-component system which induces multiple genes, including those involved in heme biosynthesis, to accommodate microaerobic conditions. The FixLJ inhibitor FixT is also induced under low oxygen conditions and is degraded by the Lon protease when the oxygen levels are sufficient, which together provides negative feedback proposed to adjust FixLJ signaling thresholds during changing conditions. Here, we address whether degradation of FixT by the Lon protease contributes to phenotypic defects associated with loss of Lon. We find that ∆lon strains are deficient in FixLJ-dependent heme biosynthesis, consistent with elevated FixT levels as deletion of fixT suppresses this defect. Transcriptomics validate this result as, along with heme biosynthesis, there is diminished expression of many FixL-activated genes in ∆lon. However, stabilization of FixT in ∆lon strains does not contribute to restoring any known Lon-related fitness defect, such as cell morphology defects or stress sensitivity. In fact, cells lacking both FixT and Lon are compromised in viability during growth in standard aerobic conditions. Our work highlights the complexity of protease-dependent regulation of transcription factors and explains the molecular basis of defective heme accumulation in Lon-deficient Caulobacter.

IMPORTANCE

The Lon protease shapes protein quality control, signaling pathways, and stress responses in many bacteria species. Loss of Lon often results in multiple phenotypic consequences. In this work, we found a connection between the Lon protease and deficiencies in heme accumulation that then led to our finding of a global change in gene expression due in part to degradation of a regulator of the hypoxic response. However, loss of degradation of this regulator did not explain other phenotypes associated with Lon deficiencies demonstrating the complex and multiple pathways that this highly conserved protease can impact.

Formation of NifA-PII complex represses ammonium-sensitive nitrogen fixation in diazotrophic proteobacteria lacking NifL

Biological nitrogen fixation catalyzed by nitrogenase contributes greatly to the global nitrogen cycle. Nitrogenase expression is subject to regulation in response to nitrogen availability. However, the mechanism through which the transcriptional activator NifA regulates nitrogenase expression by interacting with PII nitrogen regulatory proteins remains unclear in diazotrophic proteobacteria lacking NifL. Here, we demonstrate that in Rhodopseudomonas palustris grown with ammonium, NifA bound deuridylylated PII proteins to form an inactive NifA-PII complex, thereby inhibiting the expression of nitrogenase. Upon nitrogen limitation, the dissociation of uridylylated PII proteins from NifA resulted in the full restoration of NifA activity, and, simultaneously, uridylylation of the significantly up-regulated PII protein GlnK2 led to the increased expression of NifA in R. palustris. This insight into how NifA interacts with PII proteins and controls nitrogenase expression sets the stage for creating highly efficient diazotrophs, reducing the need for energy-intensive chemical fertilizers and helping to diminish carbon emissions.

Plant roots affect free-living diazotroph communities in temperate grassland soils despite decades of fertilization

Fixation of atmospheric N2 by free-living diazotrophs accounts for an important proportion of nitrogen naturally introduced to temperate grasslands. The effect of plants or fertilization on the general microbial community has been extensively studied, yet an understanding of the potential combinatorial effects on the community structure and activity of free-living diazotrophs is lacking. In this study we provide a multilevel assessment of the single and interactive effects of different long-term fertilization treatments, plant species and vicinity to roots on the free-living diazotroph community in relation to the general microbial community in grassland soils. We sequenced the dinitrogenase reductase (nifH) and the 16S rRNA genes of bulk soil and root-associated compartments (rhizosphere soil, rhizoplane and root) of two grass species (Arrhenatherum elatius and Anthoxanthum odoratum) and two herb species (Galium album and Plantago lanceolata) growing in Austrian grassland soils treated with different fertilizers (N, P, NPK) since 1960. Overall, fertilization has the strongest effect on the diazotroph and general microbial community structure, however with vicinity to the root, the plant effect increases. Despite the long-term fertilization, plants strongly influence the diazotroph communities emphasizing the complexity of soil microbial communities' responses to changing nutrient conditions in temperate grasslands.

Holed up, but thriving: Impact of multitrophic cryoconite communities on glacier elemental cycles

Cryoconite holes (water and sediment-filled depressions), found on glacier surfaces worldwide, serve as reservoirs of microbes, carbon, trace elements, and nutrients, transferring these components downstream via glacier hydrological networks. Through targeted amplicon sequencing of carbon and nitrogen cycling genes, coupled with functional inference-based methods, we explore the functional diversity of these mini-ecosystems within Antarctica and the Himalayas. These regions showcase distinct environmental gradients and experience varying rates of environmental change influenced by global climatic shifts. Analysis revealed a diverse array of photosynthetic microorganisms, including Stramenopiles, Cyanobacteria, Rhizobiales, Burkholderiales, and photosynthetic purple sulfur Proteobacteria. Functional inference highlighted the high potential for carbohydrate, amino acid, and lipid metabolism in the Himalayan region, where organic carbon concentrations surpassed those in Antarctica by up to 2 orders of magnitude. Nitrogen cycling processes, including fixation, nitrification, and denitrification, are evident, with Antarctic cryoconite exhibiting a pronounced capacity for nitrogen fixation, potentially compensating for the limited nitrate concentrations in this region. Processes associated with the respiration of elemental sulfur and inorganic sulfur compounds such as sulfate, sulfite, thiosulfate, and sulfide suggest the presence of a complete sulfur cycle. The Himalayan region exhibits a higher potential for sulfur cycling, likely due to the abundant sulfate ions and sulfur-bearing minerals in this region. The capability for complete iron cycling through iron oxidation and reduction reactions was also predicted. Methanogenic archaea that produce methane during organic matter decomposition and methanotrophic bacteria that utilize methane as carbon and energy sources co-exist in the cryoconite, suggesting that these niches support the complete cycling of methane. Additionally, the presence of various microfauna suggests the existence of a complex food web. Collectively, these results indicate that cryoconite holes are self-sustaining ecosystems that drive elemental cycles on glaciers and potentially control carbon, nitrogen, sulfur, and iron exports downstream.

Biohydrogen utilization in legume-rhizobium symbiosis reveals a novel mechanism of accelerated tetrachlorobiphenyl transformation

Symbiosis between Glycine max and Bradyrhizobium diazoefficiens were used as a model system to investigate whether biohydrogen utilization promotes the transformation of the tetrachlorobiphenyl PCB77. Both a H2 uptake-positive (Hup+) strain (wild type) and a Hup- strain (a hupL deletion mutant) were inoculated into soybean nodules. Compared with Hup- nodules, Hup+ nodules increased dechlorination significantly by 61.1 % and reduced the accumulation of PCB77 in nodules by 37.7 % (p < 0.05). After exposure to nickel, an enhancer of uptake hydrogenase, dechlorination increased significantly by 2.2-fold, and the accumulation of PCB77 in nodules decreased by 54.4 % (p < 0.05). Furthermore, the tetrachlorobiphenyl transformation in the soybean root nodules was mainly testified to be mediated by nitrate reductase (encoded by the gene NR) for tetrachlorobiphenyl dechlorination and biphenyl-2,3-diol 1,2-dioxygenase (bphC) for biphenyl degradation. This study demonstrates for the first time that biohydrogen utilization has a beneficial effect on tetrachlorobiphenyl biotransformation in a legume-rhizobium symbiosis.

Restoration of the Functional nif Gene Cluster by Complex Recombination Events during Heterocyst Development in the Nitrogen-Fixing Cyanobacterium Calothrix sp. NIES-4101

In the genome of the heterocystous cyanobacterium Calothrix sp. NIES-4101 (NIES-4101), the four genes essential for nitrogen fixation (nifB, nifH, nifD and nifK) are highly fragmented into 13 parts in a 350-kb chromosomal region, and four of these parts are encoded in the reverse strand. Such a complex fragmentation feature makes it difficult to restore the intact nifBHDK genes by the excision mechanism found in the nifD gene of the Anabaena sp. PCC 7120 heterocyst. To examine the nitrogen-fixing ability of NIES-4101, we confirmed that NIES-4101 grew well on a combined nitrogen-free medium and showed high nitrogenase activity, which strongly suggested that the complete nifBHDK genes are restored by a complex recombination process in heterocysts. Next, we resequenced the genome prepared from cells grown under nitrogen-fixing conditions. Two contigs covering the complete nifHDK and nifB genes were found by de novo assembly of the sequencing reads. In addition, the DNA fragments covering the nifBHDK operon were successfully amplified by PCR. We propose that the process of nifBHDK restoration occurs as follows. First, the nifD-nifK genes are restored by four excision events. Then, the complete nifH and nifB genes are restored by two excision events followed by two successive inversion events between the inverted repeat sequences and one excision event, forming the functional nif gene cluster, nifB-fdxN-nifS-nifU-nifH-nifD-nifK. All genes coding recombinases responsible for these nine recombination events are located close to the terminal repeat sequences. The restoration of the nifBHDK genes in NIES-4101 is the most complex genome reorganization reported in heterocystous cyanobacteria.

Deconstructing synthetic biology across scales: a conceptual approach for training synthetic biologists

Synthetic biology allows us to reuse, repurpose, and reconfigure biological systems to address society's most pressing challenges. Developing biotechnologies in this way requires integrating concepts across disciplines, posing challenges to educating students with diverse expertise. We created a framework for synthetic biology training that deconstructs biotechnologies across scales-molecular, circuit/network, cell/cell-free systems, biological communities, and societal-giving students a holistic toolkit to integrate cross-disciplinary concepts towards responsible innovation of successful biotechnologies. We present this framework, lessons learned, and inclusive teaching materials to allow its adaption to train the next generation of synthetic biologists.

Integrated Hfq-interacting RNAome and transcriptomic analysis reveals complex regulatory networks of nitrogen fixation in root-associated Pseudomonas stutzeri A1501

The RNA chaperone Hfq acts as a global regulator of numerous biological processes, such as carbon/nitrogen metabolism and environmental adaptation in plant-associated diazotrophs; however, its target RNAs and the mechanisms underlying nitrogen fixation remain largely unknown. Here, we used enhanced UV cross-linking immunoprecipitation coupled with high-throughput sequencing to identify hundreds of Hfq-binding RNAs probably involved in nitrogen fixation, carbon substrate utilization, biofilm formation, and other functions. Collectively, these processes endow strain A1501 with the requisite capabilities to thrive in the highly competitive rhizosphere. Our findings revealed a previously uncharted landscape of Hfq target genes. Notable among these is nifM, encoding an isomerase necessary for nitrogenase reductase solubility; amtB, encoding an ammonium transporter; oprB, encoding a carbohydrate porin; and cheZ, encoding a chemotaxis protein. Furthermore, we identified more than 100 genes of unknown function, which expands the potential direct regulatory targets of Hfq in diazotrophs. Our data showed that Hfq directly interacts with the mRNA of regulatory proteins (RsmA, AlgU, and NifA), regulatory ncRNA RsmY, and other potential targets, thus revealing the mechanistic links in nitrogen fixation and other metabolic pathways.

IMPORTANCE

Numerous experimental approaches often face challenges in distinguishing between direct and indirect effects of Hfq-mediated regulation. New technologies based on high-throughput sequencing are increasingly providing insight into the global regulation of Hfq in gene expression. Here, enhanced UV cross-linking immunoprecipitation coupled with high-throughput sequencing was employed to identify the Hfq-binding sites and potential targets in the root-associated Pseudomonas stutzeri A1501 and identify hundreds of novel Hfq-binding RNAs that are predicted to be involved in metabolism, environmental adaptation, and nitrogen fixation. In particular, we have shown Hfq interactions with various regulatory proteins' mRNA and their potential targets at the posttranscriptional level. This study not only enhances our understanding of Hfq regulation but, importantly, also provides a framework for addressing integrated regulatory network underlying root-associated nitrogen fixation.

Green Roof Substrate Microbes Compose a Core Community of Stress-Tolerant Taxa

Extensive green roofs provide for many ecosystem services in urban environments. The efficacy of these services is influenced by the vegetation structure. Despite their key role in plant performance and productivity, but also their contribution to nitrogen fixation or carbon sequestration, green roof microbial communities have received little attention so far. No study included a spatiotemporal aspect to investigate the core microbiota residing in the substrates of extensive green roofs, although these key taxa are hypothesized to be amongst the most ecologically important taxa. Here, we identified the core microbiota residing in extensive green roof substrates and investigated whether microbial community composition is affected by the vegetation that is planted on extensive green roofs. Eleven green roofs from three different cities in Flanders (Belgium), planted either with a mixture of grasses, wildflowers and succulents (Sedum spp.; Sedum-herbs-grasses roofs) or solely species of Sedum (Sedum-moss roofs), were seasonally sampled to investigate prokaryotic and fungal communities via metabarcoding. Identifying the key microbial taxa revealed that most taxa are dominant phylotypes in soils worldwide. Many bacterial core taxa are capable of nitrogen fixation, and most fungal key taxa are stress-tolerant saprotrophs, endophytes, or both. Considering that soil microbes adapted to the local edaphic conditions have been found to improve plant fitness, further investigation of the core microbiome is warranted to determine the extent to which these stress-tolerant microbes are beneficial for the vegetational layer. Although Sedum-herbs-grasses roofs contained more plant species than Sedum-moss roofs, we observed no discriminant microbial communities between both roof types, likely due to sharing the same substrate textures and the vegetational layers that became more similar throughout time. Future studies are recommended to comprehensively characterize the vegetational layer and composition to examine the primary drivers of microbial community assembly processes.

Degradation of Complex Carbon Sources in Organic Fertilizers Facilitates Nitrogen Fixation

Biological nitrogen fixation is crucial for agriculture and improving fertilizer efficiency, but organic fertilizers in enhancing this process remain debated. Here, we investigate the impact of organic fertilizers on biological nitrogen fixation through experiments and propose a new model where bacterial interactions with complex carbon sources enhance nitrogen fixation. Field experiments showed that adding organic fertilizers increased the nitrogenase activity by 57.85%. Subculture experiments revealed that organic fertilizer addition enriched genes corresponding to complex carbon and energy metabolism, as well as nifJ involved in electron transfer for nitrogenase. It also enhanced bacterial interactions and enhanced connectors associated with complex carbon degradation. Validation experiments demonstrated that combinations increased nitrogenase activity by 2.98 times compared to the single. Our findings suggest that organic fertilizers promoted nitrogen fixation by enhancing microbial cooperation, improved the degradation of complex carbon sources, and thereby provided utilizable carbon sources, energy, and electrons to N-fixers, thus increasing nitrogenase activity and nitrogen fixation.

Rhizobia-diatom symbiosis fixes missing nitrogen in the ocean

Nitrogen (N2) fixation in oligotrophic surface waters is the main source of new nitrogen to the ocean1 and has a key role in fuelling the biological carbon pump2. Oceanic N2 fixation has been attributed almost exclusively to cyanobacteria, even though genes encoding nitrogenase, the enzyme that fixes N2 into ammonia, are widespread among marine bacteria and archaea3-5. Little is known about these non-cyanobacterial N2 fixers, and direct proof that they can fix nitrogen in the ocean has so far been lacking. Here we report the discovery of a non-cyanobacterial N2-fixing symbiont, 'Candidatus Tectiglobus diatomicola', which provides its diatom host with fixed nitrogen in return for photosynthetic carbon. The N2-fixing symbiont belongs to the order Rhizobiales and its association with a unicellular diatom expands the known hosts for this order beyond the well-known N2-fixing rhizobia-legume symbioses on land6. Our results show that the rhizobia-diatom symbioses can contribute as much fixed nitrogen as can cyanobacterial N2 fixers in the tropical North Atlantic, and that they might be responsible for N2 fixation in the vast regions of the ocean in which cyanobacteria are too rare to account for the measured rates.

Nitrogen fertilization regulates crosstalk between marandu palisadegrass and Herbaspirillum seropedicae: An investigation based on 15N isotopic analysis and root morphology

Strategies seeking to increase the use efficiency of nitrogen (N) fertilizers and that benefit plant growth through multiple mechanisms can reduce production costs and contribute to more sustainable agriculture free of polluting residues. Under controlled conditions, we investigated the compatibility between foliar inoculation with an endophytic diazotrophic bacterium (Herbaspirillum seropedicae HRC54) at control and low, medium and high N fertilization levels (0, 25, 50 and 100 mg of N kg-1 as urea, respectively) in Marandu palisadegrass. Common procedures in our research field (biometric and nutritional assessments) were combined with isotopic techniques (natural abundance - δ15N‰ and 15N isotope dilution) and root scanning to determine the contribution of fixed N and recovery of N fertilizer by the grass. Overall, the combined use of 15N isotopic techniques revealed that inoculation not only improved the recovery of applied N-urea from the soil but also provided fixed nitrogen to Marandu palisade grass, resulting in an increase in the total accumulated N. When inoculated plants grew at control and low levels of N, a positive cascade effect encompassing root growth stimulation (nodes of smaller diameter roots), better soil and fertilizer resource exploitation and increased forage production was observed. In contrast, increasing N reduced the contributions of N fixed by H. seropedicae from 21.5% at the control level to 8.6% at the high N level. Given the minimal to no observed growth promotion, this condition was deemed inhibitory to the positive effects of H. seropedicae. We discuss how to make better use of H. seropedicae inoculation in Marandu palisadegrass, albeit on a small scale, thus contributing to a more rational and efficient use of N fertilizers. Finally, we pose questions for future investigations based on 15N isotopic techniques under field conditions, which have great applicability potential.

A distinct, high-affinity, alkaline phosphatase facilitates occupation of P-depleted environments by marine picocyanobacteria

Marine picocyanobacteria of the genera Prochlorococcus and Synechococcus, the two most abundant phototrophs on Earth, thrive in oligotrophic oceanic regions. While it is well known that specific lineages are exquisitely adapted to prevailing in situ light and temperature regimes, much less is known of the molecular machinery required to facilitate occupancy of these low-nutrient environments. Here, we describe a hitherto unknown alkaline phosphatase, Psip1, that has a substantially higher affinity for phosphomonoesters than other well-known phosphatases like PhoA, PhoX, or PhoD and is restricted to clade III Synechococcus and a subset of high light I-adapted Prochlorococcus strains, suggesting niche specificity. We demonstrate that Psip1 has undergone convergent evolution with PhoX, requiring both iron and calcium for activity and likely possessing identical key residues around the active site, despite generally very low sequence homology. Interrogation of metagenomes and transcriptomes from TARA oceans and an Atlantic Meridional transect shows that psip1 is abundant and highly expressed in picocyanobacterial populations from the Mediterranean Sea and north Atlantic gyre, regions well recognized to be phosphorus (P)-deplete. Together, this identifies psip1 as an important oligotrophy-specific gene for P recycling in these organisms. Furthermore, psip1 is not restricted to picocyanobacteria and is abundant and highly transcribed in some α-proteobacteria and eukaryotic algae, suggesting that such a high-affinity phosphatase is important across the microbial taxonomic world to occupy low-P environments.

Two diversities meet in the rhizosphere: root specialized metabolites and microbiome

Plants serve as rich repositories of diverse chemical compounds collectively referred to as specialized metabolites. These compounds are of importance for adaptive processes, including interactions with various microbes both beneficial and harmful. Considering microbes as bioreactors, the chemical diversity undergoes dynamic changes when root-derived specialized metabolites (RSMs) and microbes encounter each other in the rhizosphere. Recent advancements in sequencing techniques and molecular biology tools have not only accelerated the elucidation of biosynthetic pathways of RSMs but also unveiled the significance of RSMs in plant-microbe interactions. In this review, we provide a comprehensive description of the effects of RSMs on microbe assembly in the rhizosphere and the influence of corresponding microbial changes on plant health, incorporating the most up-to-date information available. Additionally, we highlight open questions that remain for a deeper understanding of and harnessing the potential of RSM-microbe interactions to enhance plant adaptation to the environment. Finally, we propose a pipeline for investigating the intricate associations between root exometabolites and the rhizomicrobiome.

Metal nutrition and transport in the process of symbiotic nitrogen fixation

Symbiotic nitrogen fixation (SNF) facilitated by the interaction between legumes and rhizobia is a well-documented and eco-friendly alternative to chemical nitrogen fertilizers. Host plants obtain fixed nitrogen from rhizobia by providing carbon and mineral nutrients. These mineral nutrients, which are mostly in the form of metal ions, are implicated in various stages of the SNF process. This review describes the functional roles played by metal ions in nodule formation and nitrogen fixation and specifically addresses their transport mechanisms and associated transporters within root nodules. Future research directions and potential strategies for enhancing SNF efficiency are also discussed.

Hidden diversity and potential ecological function of phosphorus acquisition genes in widespread terrestrial bacteriophages

Phosphorus (P) limitation of ecosystem processes is widespread in terrestrial habitats. While a few auxiliary metabolic genes (AMGs) in bacteriophages from aquatic habitats are reported to have the potential to enhance P-acquisition ability of their hosts, little is known about the diversity and potential ecological function of P-acquisition genes encoded by terrestrial bacteriophages. Here, we analyze 333 soil metagenomes from five terrestrial habitat types across China and identify 75 viral operational taxonomic units (vOTUs) that encode 105 P-acquisition AMGs. These AMGs span 17 distinct functional genes involved in four primary processes of microbial P-acquisition. Among them, over 60% (11/17) have not been reported previously. We experimentally verify in-vitro enzymatic activities of two pyrophosphatases and one alkaline phosphatase encoded by P-acquisition vOTUs. Thirty-six percent of the 75 P-acquisition vOTUs are detectable in a published global topsoil metagenome dataset. Further analyses reveal that, under certain circumstances, the identified P-acquisition AMGs have a greater influence on soil P availability and are more dominant in soil metatranscriptomes than their corresponding bacterial genes. Overall, our results reinforce the necessity of incorporating viral contributions into biogeochemical P cycling.

Precipitation affects soil nitrogen fixation by regulating active diazotrophs and nitrate nitrogen in an alpine grassland of Qinghai-Tibetan Plateau

Soil asymbiotic nitrogen (N) fixation provides a critical N source to support plant growth in alpine grasslands, and precipitation change is expected to lead to shifts in soil asymbiotic N fixation. However, large gaps remain in understanding the response of soil asymbiotic N fixation to precipitation gradients. Here we simulated five precipitation gradients (10 % (0.1P), 50 % (0.5P), 70 % (0.7P), 100 % (1.0P) and 150 % (1.5P) of the natural precipitation) in an alpine grassland of Qinghai-Tibetan Plateau and examined the soil nitrogenase activity and N fixation rate for each gradient. Quantitative PCR and high-throughput sequencing were used to measure the abundance and community composition of the soil nifH DNA (total diazotrophs) and nifH RNA reverse transcription (active diazotrophs) gene. Our results showed that the soil diazotrophic abundance, diversity and nifH gene expression rate peaked under the 0.5P. Soil nitrogenase activity and N fixation rate varied in the range 0.032-0.073 nmol·C2H4·g-1·h-1 and 0.008-0.022 nmol·N2·g-1·h-1 respectively, being highest under the 0.5P. The 50 % precipitation reduction enhanced the gene expression rates of Azospirillum and Halorhodospira which were likely responsible for the high N fixation potential. The 0.5P treatment also possessed a larger and more complex active diazotrophic network than the other treatments, which facilitated the resistance of diazotrophic community to environmental stress and thus maintained a high N fixation potential. The active diazotrophic abundance had the largest positive effect on soil N fixation, while nitrate nitrogen had the largest negative effect. Together, our study suggested that appropriate precipitation reduction can enhance soil N fixation through promoting the abundance of the soil active diazotrophs and decreasing soil nitrate nitrogen, and soil active diazotrophs and nitrate nitrogen should be considered in predicting soil N inputs in the alpine grassland of Qinghai-Tibetan Plateau under precipitation change.

Metabolic trade-offs constrain the cell size ratio in a nitrogen-fixing symbiosis

Biological dinitrogen (N2) fixation is a key metabolic process exclusively performed by prokaryotes, some of which are symbiotic with eukaryotes. Species of the marine haptophyte algae Braarudosphaera bigelowii harbor the N2-fixing endosymbiotic cyanobacteria UCYN-A, which might be evolving organelle-like characteristics. We found that the size ratio between UCYN-A and their hosts is strikingly conserved across sublineages/species, which is consistent with the size relationships of organelles in this symbiosis and other species. Metabolic modeling showed that this size relationship maximizes the coordinated growth rate based on trade-offs between resource acquisition and exchange. Our findings show that the size relationships of N2-fixing endosymbionts and organelles in unicellular eukaryotes are constrained by predictable metabolic underpinnings and that UCYN-A is, in many regards, functioning like a hypothetical N2-fixing organelle (or nitroplast).

Soil carbon and nitrogen cycles driven by iron redox: A review

Soil carbon and nitrogen cycles affect agricultural production, environmental quality, and global climate. Iron (Fe), regarded as the most abundant redox-active metal element in the Earth's crust, is involved in a biogeochemical cycle that includes Fe(III) reduction and Fe(II) oxidation. The redox reactions of Fe can be linked to the carbon and nitrogen cycles in soil in various ways. Investigating the transformation processes and mechanisms of soil carbon and nitrogen species driven by Fe redox can provide theoretical guidance for improving soil fertility, and addressing global environmental pollution as well as climate change. Although the widespread occurrence of these coupling processes in soils has been revealed, explorations of the effects of Fe redox on soil carbon and nitrogen cycles remain in the early stages, particularly when considering the broader context of global climate and environmental changes. The key functional microorganisms, mechanisms, and contributions of these coupling processes to soil carbon and nitrogen cycles have not been fully elucidated. Here, we present a systematic review of the research progress on soil carbon and nitrogen cycles mediated by Fe redox, including the underlying reaction processes, the key microorganisms involved, the influencing factors, and their environmental significance. Finally, some unresolved issues and future perspectives are addressed. This knowledge expands our understanding of the interconnected cycles of Fe, carbon and nitrogen in soils.

A CRISPR interference system for engineering biological nitrogen fixation

A grand challenge for the next century is in facing a changing climate through bioengineering solutions. Biological nitrogen fixation, the globally consequential, nitrogenase-catalyzed reduction of atmospheric nitrogen to bioavailable ammonia, is a vital area of focus. Nitrogen fixation engineering relies upon extensive understanding of underlying genetics in microbial models, including the broadly utilized gammaproteobacterium, Azotobacter vinelandii (A. vinelandii). Here, we report the first CRISPR interference (CRISPRi) system for targeted gene silencing in A. vinelandii that integrates genomically via site-specific transposon insertion. We demonstrate that CRISPRi can repress transcription of an essential nitrogen fixation gene by ~60%. Further, we show that nitrogenase genes are suitably expressed from the transposon insertion site, indicating that CRISPRi and engineered nitrogen fixation genes can be co-integrated for combinatorial studies of gene expression and engineering. Our established CRISPRi system fills an important gap for engineering microbial nitrogen fixation for desired purposes.IMPORTANCEAll life on Earth requires nitrogen to survive. About 78% of the atmosphere alone is nitrogen, yet humans cannot use it directly. Instead, we obtain the nitrogen we need for our survival through the food we eat. For more than 100 years, a substantial portion of agricultural productivity has relied on industrial methods for nitrogen fertilizer synthesis, which consumes significant amounts of nonrenewable energy resources and exacerbates environmental degradation and human-induced climate change. Promising alternatives to these industrial methods rely on engineering the only biological pathway for generating bioaccessible nitrogen: microbial nitrogen fixation. Bioengineering strategies require an extensive understanding of underlying genetics in nitrogen-fixing microbes, but genetic tools for this critical goal remain lacking. The CRISPRi gene silencing system that we report, developed in the broadly utilized nitrogen-fixing bacterial model, Azotobacter vinelandii, is an important step toward elucidating the complexity of nitrogen fixation genetics and enabling their manipulation.

Rice N-biofertilization by inoculation with an engineered photosynthetic diazotroph

Photosynthetic diazotrophs expressing iron-only (Fe-only) nitrogenase can be developed into a promising biofertilizer, as it is independent on the molybdenum availability in the soil. However, the expression of Fe-only nitrogenase in diazotrophs is repressed by the fixed nitrogen of the soil, limiting the efficiency of nitrogen fixation in farmland with low ammonium concentrations that are inadequate for sustainable crop growth. Here, we succeeded in constitutively expressing the Fe-only nitrogenase even in the presence of ammonium by controlling the transcription of Fe-only nitrogenase gene cluster (anfHDGK) with the transcriptional activator of Mo nitrogenase (NifA*) in several different ways, indicating that the engineered NifA* strains can be used as promising chassis cells for efficient expression of different types of nitrogenases. When applied as a biofertilizer, the engineered Rhodopseudomonas palustris effectively stimulated rice growth, contributing to the reduced use of chemical fertilizer and the development of sustainable agriculture.

Intensive N2 fixation accelerates microbial turnover in cropland soils

Biological nitrogen fixation (BNF) is strongly affected by the carbon (C) and nitrogen (N) stoichiometry in soil and depends on the input of organic C. Due to the high metabolic costs of nitrogenase activity, however, the response of BNF to organic C input and its impact on microbial turnover remain unclear. To address this knowledge gap, we combined 15N2 tracing with high-throughput sequencing by adding glucose or glucose plus mineral N fertilizer for a 12-day incubation in three cropland soils. Glucose addition alone strongly changed the BNF activity (0.76-2.51 mg N kg-1 d-1), while BNF was completely absent after mineral N fertilization. This switch-on of BNF by glucose addition supported equally high rates of microbial growth and organic C mineralization compared with the direct mineral N assimilation by microorganisms. Glucose-induced BNF was predominantly catalyzed by Azotobacter-affiliated free-living diazotrophs (>50 % of the total nifH genes), which increased with diverse nondiazotrophs such as Nitrososphaera, Bacillus and Pseudoxanthomonas. Structural equation models (SEMs) and random forest (RF) analyses consistently revealed that the soil C:N ratio and Azotobacter-affiliated diazotrophic abundances were the key factors affecting glucose-induced BNF. Our findings emphasize the importance of free-living diazotrophs for microbial turnover of organic C in soil.

Proteome profiling of Paenibacillus sonchi genomovar Riograndensis SBR5T under conventional and alternative nitrogen fixation

Paenibacillus sonchi SBR5T is a Gram-positive, endospore-forming facultative aerobic diazotrophic bacterium that can fix nitrogen via an alternative Fe-only nitrogenase (AnfHDGK). In several bacteria, this alternative system is expressed under molybdenum (Mo)-limiting conditions when the conventional Mo-dependent nitrogenase (NifHDK) production is impaired. The regulatory mechanisms, metabolic processes, and cellular functions of N2 fixation by alternative and/or conventional systems are poorly understood in the Paenibacillus genus. We conducted a comparative proteomic profiling study of P. sonchi SBR5T grown under N2-fixing conditions with and without Mo supply through an LC-MS/MS and label-free quantification analysis to address this gap. Protein abundances revealed overrepresented processes related to anaerobiosis growth adaption, Fe-S cluster biosynthesis, ammonia assimilation, electron transfer, and sporulation under N2-fixing conditions compared to non-fixing control. Under Mo limitation, the Fe-only nitrogenase components were overrepresented together with the Mo-transporter system, while the dinitrogenase component (NifDK) of Mo‑nitrogenase was underrepresented. The dinitrogenase reductase component (NifH) and accessory proteins encoded by the nif operon had no significant differential expression, suggesting post-transcriptional regulation of nif gene products in this strain. Overall, this was the first comprehensive proteomic analysis of a diazotrophic strain from the Paenibacillaceae family, and it provided insights related to alternative N2-fixation by Fe-only nitrogenase. SIGNIFICANCE: In this work, we try to understand how the alternative nitrogen fixation system, presented by some diazotrophic bacteria, works. For this, we used the SBR5 lineage of P. sonchi, which presents the alternative system in which the nitrogenase cofactor is composed only of iron. In addition, we tried to unravel the proteome of this strain in different situations of nitrogen fixation, since, for Gram-positive bacteria, these systems are little known. The results achieved, through LC-MS/MS and label-free quantitative analysis, showed an overrepresentation of proteins related to different processes involved with growth under stressful conditions in situations of nitrogen deficiency, in addition to suggesting that some encoded proteins by the nif operon may be regulated at post-transcriptional levels. Our findings represent important steps toward the elucidation of nitrogen fixation systems in Gram-positive diazotrophic bacteria.

Soybean-mediated suppression of BjaI/BjaR1 quorum sensing in Bradyrhizobium diazoefficiens impacts symbiotic nitrogen fixation

The acyl-homoserine lactones (AHLs)-mediated LuxI/LuxR quorum sensing (QS) system orchestrates diverse bacterial behaviors in response to changes in population density. The role of the BjaI/BjaR1 QS system in Bradyrhizobium diazoefficiens USDA 110, which shares homology with LuxI/LuxR, remains elusive during symbiotic interaction with soybean. Here this genetic system in wild-type (WT) bacteria residing inside nodules exhibited significantly reduced activity compared to free-living cells, potentially attributed to soybean-mediated suppression. The deletion mutant strain ΔbjaR1 showed significantly enhanced nodulation induction and nitrogen fixation ability. Nevertheless, its ultimate symbiotic outcome (plant dry weight) in soybeans was compromised. Furthermore, comparative analysis of the transcriptome, proteome, and promoter activity revealed that the inactivation of BjaR1 systematically activated and inhibited genomic modules associated with nodulation and nitrogen metabolism. The former appeared to be linked to a significant decrease in the expression of NodD2, a key cell-density-dependent repressor of nodulation genes, while the latter conferred bacterial growth and nitrogen fixation insensitivity to environmental nitrogen. In addition, BjaR1 exerted a positive influence on the transcription of multiple genes involved in a so-called central intermediate metabolism within the nodule. In conclusion, our findings highlight the crucial role of the BjaI/BjaR1 QS circuit in positively regulating bacterial nitrogen metabolism and emphasize the significance of the soybean-mediated suppression of this genetic system for promoting efficient symbiotic nitrogen fixation by B. diazoefficiens.IMPORTANCEThe present study demonstrates, for the first time, that the BjaI/BjaR1 QS system of Bradyrhizobium diazoefficiens has a significant impact on its nodulation and nitrogen fixation capability in soybean by positively regulating NodD2 expression and bacterial nitrogen metabolism. Moreover, it provides novel insights into the importance of suppressing the activity of this QS circuit by the soybean host plant in establishing an efficient mutual relationship between the two symbiotic partners. This research expands our understanding of legumes' role in modulating symbiotic nitrogen fixation through rhizobial QS-mediated metabolic functioning, thereby deepening our comprehension of symbiotic coevolution theory. In addition, these findings may hold great promise for developing quorum quenching technology in agriculture.

Proteolytic control of FixT by the Lon protease impacts FixLJ signaling in Caulobacter crescentus

Responding to changes in oxygen levels is critical for aerobic microbes. In Caulobacter crescentus, low oxygen is sensed by the FixL-FixJ two-component system which induces multiple genes, including heme biosynthesis, to accommodate microaerobic conditions. The FixLJ inhibitor FixT is also induced under low oxygen conditions and is degraded by the Lon protease, which together provides negative feedback proposed to adjust FixLJ signaling thresholds during changing conditions. Here, we address if the degradation of FixT by the Lon protease contributes to phenotypic defects associated with loss of Lon. We find that ∆lon strains are deficient in FixLJ-dependent heme biosynthesis, consistent with elevated FixT levels as deletion of fixT suppresses this defect. Transcriptomics validate this result as there is diminished expression of many FixLJ-activated genes in ∆lon. However, no physiological changes in response to microaerobic conditions occurred upon loss of Lon, suggesting that FixT dynamics are not a major contributor to fitness in oxygen limiting conditions. Similarly, stabilization of FixT in ∆lon strains does not contribute to any known Lon-related fitness defect, such as cell morphology defects or stress sensitivity. In fact, cells lacking both FixT and Lon are compromised in viability during adaptation to long term aerobic growth. Our work highlights the complexity of protease-dependent regulation of transcription factors and explains the molecular basis of defective heme accumulation in Lon-deficient Caulobacter.

Deciphering the spatial distribution and function profiles of soil bacterial community in Liao River estuarine wetland, Northeast China

Soil microbes play vital roles in estuarine wetlands. Understanding the soil bacterial community structure and function profiles is essential to reveal the ecological functions of microbes in estuarine wetlands. Herein, soil samples were collected from Liao River estuarine wetland, Northeast China, along the river to the estuarine mouth, and soil bacterial communities were explored. Results showed that soil physiochemical properties, bacterial community structure and functions exhibited distinct variations influenced by geographical location. Bacterial phyla in soils were dominated by Proteobacteria and Bacteroidetes, while Gillisia and Woeseia were the predominant genera. Soil pH, electrical conductivity and nitrogen-related nutrients were the important factors affecting bacterial community structure. Based on PICRUSt prediction, the genes related to metabolism of nitrogen, sulfur and methane showed spatial distribution patterns, and the abundances of most biomarker genes increased as the distance from estuarine mouth extended. These findings could enrich the understanding of soil microbiome in estuarine wetlands.

Effect of humic substances on nitrogen cycling in soil-plant ecosystems: Advances, issues, and future perspectives

Nitrogen (N) cycle is one of the most significant biogeochemical cycles driven by soil microorganisms on the earth. Exogenous humic substances (HS), which include composted-HS and artificial-HS, as a new soil additive, can improve the water retention capacity, cation exchange capacity and soil nutrient utilization, compensating for the decrease of soil HS content caused by soil overutilization. This paper systematically reviewed the contribution of three different sources of HS in the soil-plant system and explained the mechanisms of N transformation through physiological and biochemical pathways. HS convert the living space and living environment of microorganisms by changing the structure and condition of soil. Generally, HS can fix atmospheric and soil N through biotic and abiotic mechanisms, which improved the availability of N. Besides, HS transform the root structure of plants through physiological and biochemical pathways to promote the absorption of inorganic N by plants. The redox properties of HS participate in soil N transformation by altering the electron gain and loss of microorganisms. Moreover, to alleviate the energy crisis and environmental problems caused by N pollution, we also illustrated the mechanisms reducing soil N2O emissions by HS and the application prospects of artificial-HS. Eventually, a combination of indoor simulation and field test, molecular biology and stable isotope techniques are needed to systematically analyze the potential mechanisms of soil N transformation, representing an important step forward for understanding the relevance between remediation of environmental pollution and improvement of the N utilization in soil-plant system.

Dynamic nitrogen fixation in an aerobic endophyte of Populus

Biological nitrogen fixation by microbial diazotrophs can contribute significantly to nitrogen availability in non-nodulating plant species. In this study of molecular mechanisms and gene expression relating to biological nitrogen fixation, the aerobic nitrogen-fixing endophyte Burkholderia vietnamiensis, strain WPB, isolated from Populus trichocarpa served as a model for endophyte-poplar interactions. Nitrogen-fixing activity was observed to be dynamic on nitrogen-free medium with a subset of colonies growing to form robust, raised globular like structures. Secondary ion mass spectrometry (NanoSIMS) confirmed that N-fixation was uneven within the population. A fluorescent transcriptional reporter (GFP) revealed that the nitrogenase subunit nifH is not uniformly expressed across genetically identical colonies of WPB and that only ~11% of the population was actively expressing the nifH gene. Higher nifH gene expression was observed in clustered cells through monitoring individual bacterial cells using single-molecule fluorescence in situ hybridization. Through 15N2 enrichment, we identified key nitrogenous metabolites and proteins synthesized by WPB and employed targeted metabolomics in active and inactive populations. We cocultivated WPB Pnif-GFP with poplar within a RhizoChip, a synthetic soil habitat, which enabled direct imaging of microbial nifH expression within root epidermal cells. We observed that nifH expression is localized to the root elongation zone where the strain forms a unique physical interaction with the root cells. This work employed comprehensive experimentation to identify novel mechanisms regulating both biological nitrogen fixation and beneficial plant-endophyte interactions.

Surface Plasmon Resonance as a Tool to Elucidate the Molecular Determinants of Key Transcriptional Regulators Controlling Rhizobial Lifestyles

Bacteria must be provided with a battery of tools integrated into regulatory networks, in order to respond and, consequently, adapt their physiology to changing environments. Within these networks, transcription factors finely orchestrate the expression of genes in response to a variety of signals, by recognizing specific DNA sequences at their promoter regions. Rhizobia are host-interacting soil bacteria that face severe changes to adapt their physiology from free-living conditions to the nitrogen-fixing endosymbiotic state inside root nodules associated with leguminous plants. One of these cues is the low partial pressure of oxygen within root nodules.Surface plasmon resonance (SPR) constitutes a technique that allows to measure molecular interactions dynamics at real time by detecting changes in the refractive index of a surface. Here, we implemented the SPR methodology to analyze the discriminatory determinants of transcription factors for specific interaction with their target genes. We focused on FixK2, a CRP/FNR-type protein with a central role in the complex oxygen-responsive regulatory network in the soybean endosymbiont Bradyrhizobium diazoefficiens. Our study unveiled relevant residues for protein-DNA interaction as well as allowed us to monitor kinetics and stability protein-DNA complex. We believe that this approach can be employed for the characterization of other relevant transcription factors which can assist to the better understanding of the adaptation of bacteria with agronomic or human interest to their different modes of life.

Development of a bacterial gene transcription activating strategy based on transcriptional activator positive feedback

INTRODUCTION

Transcription of biological nitrogen fixation (nif) genes is activated by the NifA protein which recognizes specific activating sequences upstream of σ54-dependent nif promoters. The large quantities of nitrogenase which can make up 20% of the total proteins in the cell indicates high transcription activating efficiency of NifA and high transcription level of nifHDK nitrogenase genes.

OBJECTIVES

Development of an efficient gene transcription activating strategy in bacteria based on positive transcription regulatory proteins and their regulating DNA sequences.

METHODS

We designed a highly efficient gene transcription activating strategy in which the nifA gene was placed directly downstream of its regulating sequences. The NifA protein binds its regulating sequences and stimulates transcription of itself and downstream genes. Overexpressed NifA causes transcription activation by positive reinforcement.

RESULTS

When this gene transcription activating strategy was used to overexpress NifA in Pseudomonas stutzeri DSM4166 containing the nif gene cluster, the nitrogenase activity was increased by 368 folds which was 16 times higher than that obtained by nifA driven by the strongest endogenous constitutive promoter. When this strategy was used to activate transcription of exogenous biosynthetic genes for the plant auxin indole-3-acetic acid and the antitumor alkaloid pigment prodigiosin in DSM4166, both of them resulted in better performance than the strongest endogenous constitutive promoter and the highest reported productions in heterologous hosts to date. Finally, we demonstrated the universality of this strategy using the positive transcriptional regulator of the psp operon, PspF, in E. coli and the pathway-specific positive transcription regulator of the polyene antibiotic salinomycin biosynthesis, SlnR, in Streptomyces albus.

CONCLUSION

Many positive transcription regulatory proteins and their regulating DNA sequences have been identified in bacteria. The gene transcription activating strategy developed in this study will have broad applications in molecular biology and biotechnology.

Purification and Biochemical Characterization of the DNA Binding Domain of the Nitrogenase Transcriptional Activator NifA from Gluconacetobacter diazotrophicus

NifA is a σ54 activator that turns on bacterial nitrogen fixation under reducing conditions and when fixed cellular nitrogen levels are low. The redox sensing mechanism in NifA is poorly understood. In α- and β-proteobacteria, redox sensing involves two pairs of Cys residues within and immediately following the protein's central AAA+ domain. In this work, we examine if an additional Cys pair that is part of a C(X)5 C motif and located immediately upstream of the DNA binding domain of NifA from the α-proteobacterium Gluconacetobacter diazotrophicus (Gd) is involved in redox sensing. We hypothesize that the Cys residues' redox state may directly influence the DNA binding domain's DNA binding affinity and/or alter the protein's oligomeric sate. Two DNA binding domain constructs were generated, a longer construct (2C-DBD), consisting of the DNA binding domain with the upstream Cys pair, and a shorter construct (NC-DBD) that lacks the Cys pair. The Kd of NC-DBD for its cognate DNA sequence (nifH-UAS) is equal to 20.0 µM. The Kd of 2C-DBD for nifH-UAS when the Cys pair is oxidized is 34.5 µM. Reduction of the disulfide bond does not change the DNA binding affinity. Additional experiments indicate that the redox state of the Cys residues does not influence the secondary structure or oligomerization state of the NifA DNA binding domain. Together, these results demonstrate that the Cys pair upstream of the DNA binding domain of Gd-NifA does not regulate DNA binding or domain dimerization in a redox dependent manner.

Genome features of a novel hydrocarbonoclastic Chryseobacterium oranimense strain and its comparison to bacterial oil-degraders and to other C. oranimense strains

For the first time, we report the whole genome sequence of a hydrocarbonoclastic Chryseobacterium oranimense strain isolated from Trinidad and Tobago (COTT) and its genes involved in the biotransformation of hydrocarbons and xenobiotics through functional annotation. The assembly consisted of 11 contigs with 2,794 predicted protein-coding genes which included a diverse group of gene families involved in aliphatic and polycyclic hydrocarbon degradation. Comparative genomic analyses with 18 crude-oil degrading bacteria in addition to two C. oranimense strains not associated with oil were carried out. The data revealed important differences in terms of annotated genes involved in the hydrocarbon degradation process that may explain the molecular mechanisms of hydrocarbon and xenobiotic biotransformation. Notably, many gene families were expanded to explain COTT's competitive ability to manage habitat-specific stressors. Gene-based evidence of the metabolic potential of COTT supports the application of indigenous microbes for the remediation of polluted terrestrial environments and provides a genomic resource for improving our understanding of how to optimize these characteristics for more effective bioremediation.

Harnessing biological nitrogen fixation in plant leaves

The importance of biological nitrogen fixation (BNF) in securing food production for the growing world population with minimal environmental cost has been increasingly acknowledged. Leaf surfaces are one of the biggest microbial habitats on Earth, harboring diverse free-living N2-fixers. These microbes inhabit the epiphytic and endophytic phyllosphere and contribute significantly to plant N supply and growth. Here, we summarize the contribution of phyllosphere-BNF to global N cycling, evaluate the diversity of leaf-associated N2-fixers across plant hosts and ecosystems, illustrate the ecological adaptation of N2-fixers to the phyllosphere, and identify the environmental factors driving BNF. Finally, we discuss potential BNF engineering strategies to improve the nitrogen uptake in plant leaves and thus sustainable food production.

Non-cyanobacterial diazotrophs: global diversity, distribution, ecophysiology, and activity in marine waters

Biological dinitrogen (N2) fixation supplies nitrogen to the oceans, supporting primary productivity, and is carried out by some bacteria and archaea referred to as diazotrophs. Cyanobacteria are conventionally considered to be the major contributors to marine N2 fixation, but non-cyanobacterial diazotrophs (NCDs) have been shown to be distributed throughout ocean ecosystems. However, the biogeochemical significance of marine NCDs has not been demonstrated. This review synthesizes multiple datasets, drawing from cultivation-independent molecular techniques and data from extensive oceanic expeditions, to provide a comprehensive view into the diversity, biogeography, ecophysiology, and activity of marine NCDs. A NCD nifH gene catalog was compiled containing sequences from both PCR-based and PCR-free methods, identifying taxa for future studies. NCD abundances from a novel database of NCD nifH-based abundances were colocalized with environmental data, unveiling distinct distributions and environmental drivers of individual taxa. Mechanisms that NCDs may use to fuel and regulate N2 fixation in response to oxygen and fixed nitrogen availability are discussed, based on a metabolic analysis of recently available Tara Oceans expedition data. The integration of multiple datasets provides a new perspective that enhances understanding of the biology, ecology, and biogeography of marine NCDs and provides tools and directions for future research.

Hydrodynamic regimes modulate nitrogen fixation and the mode of diazotrophy in Lake Tanganyika

The factors that govern the geographical distribution of nitrogen fixation are fundamental to providing accurate nitrogen budgets in aquatic environments. Model-based insights have demonstrated that regional hydrodynamics strongly impact nitrogen fixation. However, the mechanisms establishing this physical-biological coupling have yet to be constrained in field surveys. Here, we examine the distribution of nitrogen fixation in Lake Tanganyika - a model system with well-defined hydrodynamic regimes. We report that nitrogen fixation is five times higher under stratified than under upwelling conditions. Under stratified conditions, the limited resupply of inorganic nitrogen to surface waters, combined with greater light penetration, promotes the activity of bloom-forming photoautotrophic diazotrophs. In contrast, upwelling conditions support predominantly heterotrophic diazotrophs, which are uniquely suited to chemotactic foraging in a more dynamic nutrient landscape. We suggest that these hydrodynamic regimes (stratification versus mixing) play an important role in governing both the rates and the mode of nitrogen fixation.

A microbial evolutionary approach for a sustainable future

With the continued population increase, more sustainable use of water, land, air and chemicals is imperative. Microorganisms will need to be called upon to aid in many sustainability efforts. Prokaryotes are the fastest-evolving cellular life, and most manipulatable via synthetic biology. Moreover, their natural diversity in processing organic and inorganic chemicals, and their survivability in extreme niches, make them prime agents to enlist for solving many of society's pressing problems.

Expression of V-nitrogenase and Fe-nitrogenase in Methanosarcina acetivorans is controlled by molybdenum, fixed nitrogen, and the expression of Mo-nitrogenase

All nitrogen-fixing bacteria and archaea (diazotrophs) use molybdenum (Mo) nitrogenase to reduce dinitrogen (N2) to ammonia, with some also containing vanadium (V) and iron-only (Fe) nitrogenases that lack Mo. Among diazotrophs, the regulation and usage of the alternative V-nitrogenase and Fe-nitrogenase in methanogens are largely unknown. Methanosarcina acetivorans contains nif, vnf, and anf gene clusters encoding putative Mo-nitrogenase, V-nitrogenase, and Fe-nitrogenase, respectively. This study investigated nitrogenase expression and growth by M. acetivorans in response to fixed nitrogen, Mo/V availability, and CRISPRi repression of the nif, vnf, and/or anf gene clusters. The availability of Mo and V significantly affected growth of M. acetivorans with N2 but not with NH4Cl. M. acetivorans exhibited the fastest growth rate and highest cell yield during growth with N2 in medium containing Mo, and the slowest growth in medium lacking Mo and V. qPCR analysis revealed the transcription of the nif operon is only moderately affected by depletion of fixed nitrogen and Mo, whereas vnf and anf transcription increased significantly when fixed nitrogen and Mo were depleted, with removal of Mo being key. Immunoblot analysis revealed Mo-nitrogenase is detected when fixed nitrogen is depleted regardless of Mo availability, while V-nitrogenase and Fe-nitrogenase are detected only in the absence of fixed nitrogen and Mo. CRISPRi repression studies revealed that V-nitrogenase and/or Fe-nitrogenase are required for Mo-independent diazotrophy, and unexpectedly that the expression of Mo-nitrogenase is also required. These results reveal that alternative nitrogenase production in M. acetivorans is tightly controlled and dependent on Mo-nitrogenase expression. IMPORTANCE Methanogens and closely related methanotrophs are the only archaea known or predicted to possess nitrogenase. Methanogens play critical roles in both the global biological nitrogen and carbon cycles. Moreover, methanogens are an ancient microbial lineage and nitrogenase likely originated in methanogens. An understanding of the usage and properties of nitrogenases in methanogens can provide new insight into the evolution of nitrogen fixation and aid in the development nitrogenase-based biotechnology. This study provides the first evidence that a methanogen can produce all three forms of nitrogenases, including simultaneously. The results reveal components of Mo-nitrogenase regulate or are needed to produce V-nitrogenase and Fe-nitrogenase in methanogens, a result not seen in bacteria. Overall, this study provides a foundation to understand the assembly, regulation, and activity of the alternative nitrogenases in methanogens.

Nitrogen Journey in Plants: From Uptake to Metabolism, Stress Response, and Microbe Interaction

Plants uptake and assimilate nitrogen from the soil in the form of nitrate, ammonium ions, and available amino acids from organic sources. Plant nitrate and ammonium transporters are responsible for nitrate and ammonium translocation from the soil into the roots. The unique structure of these transporters determines the specificity of each transporter, and structural analyses reveal the mechanisms by which these transporters function. Following absorption, the nitrogen metabolism pathway incorporates the nitrogen into organic compounds via glutamine synthetase and glutamate synthase that convert ammonium ions into glutamine and glutamate. Different isoforms of glutamine synthetase and glutamate synthase exist, enabling plants to fine-tune nitrogen metabolism based on environmental cues. Under stressful conditions, nitric oxide has been found to enhance plant survival under drought stress. Furthermore, the interaction between salinity stress and nitrogen availability in plants has been studied, with nitric oxide identified as a potential mediator of responses to salt stress. Conversely, excessive use of nitrate fertilizers can lead to health and environmental issues. Therefore, alternative strategies, such as establishing nitrogen fixation in plants through diazotrophic microbiota, have been explored to reduce reliance on synthetic fertilizers. Ultimately, genomics can identify new genes related to nitrogen fixation, which could be harnessed to improve plant productivity.

Regulatory response to a hybrid ancestral nitrogenase in Azotobacter vinelandii

Biological nitrogen fixation, the microbial reduction of atmospheric nitrogen to bioavailable ammonia, represents both a major limitation on biological productivity and a highly desirable engineering target for synthetic biology. However, the engineering of nitrogen fixation requires an integrated understanding of how the gene regulatory dynamics of host diazotrophs respond across sequence-function space of its central catalytic metalloenzyme, nitrogenase. Here, we interrogate this relationship by analyzing the transcriptome of Azotobacter vinelandii engineered with a phylogenetically inferred ancestral nitrogenase protein variant. The engineered strain exhibits reduced cellular nitrogenase activity but recovers wild-type growth rates following an extended lag period. We find that expression of genes within the immediate nitrogen fixation network is resilient to the introduced nitrogenase sequence-level perturbations. Rather the sustained physiological compatibility with the ancestral nitrogenase variant is accompanied by reduced expression of genes that support trace metal and electron resource allocation to nitrogenase. Our results spotlight gene expression changes in cellular processes adjacent to nitrogen fixation as productive engineering considerations to improve compatibility between remodeled nitrogenase proteins and engineered host diazotrophs. IMPORTANCE Azotobacter vinelandii is a key model bacterium for the study of biological nitrogen fixation, an important metabolic process catalyzed by nitrogenase enzymes. Here, we demonstrate that compatibilities between engineered A. vinelandii strains and nitrogenase variants can be modulated at the regulatory level. The engineered strain studied here responds by adjusting the expression of proteins involved in cellular processes adjacent to nitrogen fixation, rather than that of nitrogenase proteins themselves. These insights can inform future strategies to transfer nitrogenase variants to non-native hosts.

A Broad Light-Harvesting Conjugated Oligoelectrolyte Enables Photocatalytic Nitrogen Fixation in a Bacterial Biohybrid

We report a rationally designed membrane-intercalating conjugated oligoelectrolyte (COE), namely COE-IC, which endows aerobic N2 -fixing bacteria Azotobacter vinelandii with a light-harvesting ability that enables photosynthetic ammonia production. COE-IC possesses an acceptor-donor-acceptor (A-D-A) type conjugated core, which promotes visible light absorption with a high molar extinction coefficient. Furthermore, COE-IC spontaneously associates with A. vinelandii to form a biohybrid in which the COE is intercalated within the lipid bilayer membrane. In the presence of L-ascorbate as a sacrificial electron donor, the resulting COE-IC/A. vinelandii biohybrid showed a 2.4-fold increase in light-driven ammonia production, as compared to the control. Photoinduced enhancement of bacterial biomass and production of L-amino acids is also observed. Introduction of isotopically enriched 15 N2 atmosphere led to the enrichment of 15 N-containing intracellular metabolites, consistent with the products being generated from atmospheric N2 .

Impact of Fe + 2 ions on structural integrity of A0A6P1CI42_RHITR NifA protein from Rhizobium tropici strain CIAT 899

A0A6P1CI42_RHITR, a protein originating from Rhizobium tropici strain CIAT 899, has emerged as a key player in leguminous plant symbiosis and nitrogen fixation processes. Understanding the intricate details of its structure and function holds immense significance for unraveling the molecular mechanisms underlying its biological activities. In this study, we employed molecular modeling and molecular dynamics (MD) simulations to investigate the A0A6P1CI42_RHITR protein, with a specific emphasis on the influence of Fe-atoms, linker structural integrity, and conformational changes within the GAF domain. Our findings unveiled noteworthy conformational changes in the A0A6P1CI42_RHITR protein, particularly within the GAF domain, when Fe-atoms were present compared to its apo form. Significant conformational rearrangements after an initial 20 ns, accompanied by the opening of the ligand substrate accommodating loop in the GAF domain influenced by Fe-atoms was observed. At the residue level, the investigation revealed substantial activity variations in individual residues, particularly in those contributing to the GAF domain from positions 51 to 223. Intriguingly, the presence of Fe-atoms led to controlled movement of conserved cysteine residues at positions 467 and 472, indicating a correlation between interlinker domain motion and the activity of the GAF domain loop responsible for substrate accommodation. Moreover, in the presence of Fe-atoms, the distance between Cys467 and Cys472 residues was maintained, ensuring the overall structural integrity of the interdomain loop necessary for protein activation. Conversely, in the apo form, a sudden flip motion of cysteine residues' thiol groups was observed, leading to a loss of structural integration. Overall, our study utilizing molecular modeling and MD simulations offers valuable insights into the structural dynamics and functional implications of the A0A6P1CI42_RHITR protein.Communicated by Ramaswamy H. Sarma.

Exploring the cellular surface polysaccharide and root nodule symbiosis characteristics of the rpoN mutants of Bradyrhizobium sp. DOA9 using synchrotron-based Fourier transform infrared microspectroscopy in conjunction with X-ray absorption spectroscopy

The functional significance of rpoN genes that encode two sigma factors in the Bradyrhizobium sp. strain DOA9 has been reported to affect colony formation, root nodulation characteristics, and symbiotic interactions with Aeschynomene americana. rpoN mutant strains are defective in cellular surface polysaccharide (CSP) production compared with the wild-type (WT) strain, and they accordingly exhibit smaller colonies and diminished symbiotic effectiveness. To gain deeper insights into the changes in CSP composition and the nodules of rpoN mutants, we employed synchrotron-based Fourier transform infrared (SR-FTIR) microspectroscopy and X-ray absorption spectroscopy. FTIR analysis of the CSP revealed the absence of specific components in the rpoN mutants, including lipids, carboxylic groups, polysaccharide-pyranose rings, and β-galactopyranosyl residues. Nodules formed by DOA9WT exhibited a uniform distribution of lipids, proteins, and carbohydrates; mutant strains, particularly DOA9∆rpoNp:ΩrpoNc, exhibited decreased distribution uniformity and a lower concentration of C=O groups. Furthermore, Fe K-edge X-ray absorption near-edge structure and extended X-ray absorption fine structure analyses revealed deficiencies in the nitrogenase enzyme in the nodules of DOA9∆rpoNc and DOA9∆rpoNp:ΩrpoNc mutants; nodules from DOA9WT and DOA9∆rpoNp exhibited both leghemoglobin and the nitrogenase enzyme. IMPORTANCE This work provides valuable insights into how two rpoN genes affect the composition of cellular surface polysaccharides (CSPs) in Bradyrhizobium sp., which subsequently dictates root nodule chemical characteristics and nitrogenase production. We used advanced synchrotron methods, including synchrotron-based Fourier transform infrared (SR-FTIR) microspectroscopy and X-ray absorption spectroscopy (XAS), for the first time in this field to analyze CSP components and reveal the biochemical changes occurring within nodules. These cutting-edge techniques confer significant advantages by providing detailed molecular information, enabling the identification of specific functional groups, chemical bonds, and biomolecule changes. This research not only contributes to our understanding of plant-microbe interactions but also establishes a foundation for future investigations and potential applications in this field. The combined use of the synchrotron-based FTIR and XAS techniques represents a significant advancement in facilitating a comprehensive exploration of bacterial CSPs and their implications in plant-microbe interactions.

Electrochemical nitrogen fixation on single metal atom catalysts

The electrochemical reduction of nitrogen (eNRR) offers a promising alternative to the Haber-Bosch (H-B) process for producing ammonia under moderate conditions. However, the inertness of dinitrogen and the competing hydrogen evolution reaction pose significant challenges for eNRR. Thus, developing more efficient electrocatalysts requires a deeper understanding of the underlying mechanistic reactions and electrocatalytic activity. Single atom catalysts, which offer tunable catalytic properties and increased selectivity, have emerged as a promising avenue for eNRR. Carbon and metal-based substrates have proven effective for dispersing highly active single atoms that can enhance eNRR activity. In this review, we explore the use of atomically dispersed single atoms on different substrates for eNRR from both conceptual and experimental perspectives. The review is divided into four sections: the first section describes eNRR mechanistic pathways, the second section focuses on single metal atom catalysts (SMACs) with metal atoms dispersed on carbon substrates for eNRR, the third section covers SMACs with metal atoms dispersed on non-carbon substrates for eNRR, and the final section summarizes the remaining challenges and future scope of eNRR for green ammonia production.

Long-read assembled metagenomic approaches improve our understanding on metabolic potentials of microbial community in mangrove sediments

BACKGROUND

Mangrove wetlands are coastal ecosystems with important ecological features and provide habitats for diverse microorganisms with key roles in nutrient and biogeochemical cycling. However, the overall metabolic potentials and ecological roles of microbial community in mangrove sediment are remained unanswered. In current study, the microbial and metabolic profiles of prokaryotic and fungal communities in mangrove sediments were investigated using metagenomic analysis based on PacBio single-molecule real time (SMRT) and Illumina sequencing techniques.

RESULTS

Comparing to Illumina short reads, the incorporation of PacBio long reads significantly contributed to more contiguous assemblies, yielded more than doubled high-quality metagenome-assembled genomes (MAGs), and improved the novelty of the MAGs. Further metabolic reconstruction for recovered MAGs showed that prokaryotes potentially played an essential role in carbon cycling in mangrove sediment, displaying versatile metabolic potential for degrading organic carbons, fermentation, autotrophy, and carbon fixation. Mangrove fungi also functioned as a player in carbon cycling, potentially involved in the degradation of various carbohydrate and peptide substrates. Notably, a new candidate bacterial phylum named as Candidatus Cosmopoliota with a ubiquitous distribution is proposed. Genomic analysis revealed that this new phylum is capable of utilizing various types of organic substrates, anaerobic fermentation, and carbon fixation with the Wood-Ljungdahl (WL) pathway and the reverse tricarboxylic acid (rTCA) cycle.

CONCLUSIONS

The study not only highlights the advantages of HiSeq-PacBio Hybrid assembly for a more complete profiling of environmental microbiomes but also expands our understanding of the microbial diversity and potential roles of distinct microbial groups in biogeochemical cycling in mangrove sediment. Video Abstract.

Effect of endophytic diazotroph Enterobacter roggenkampii ED5 on nitrogen-metabolism-related microecology in the sugarcane rhizosphere at different nitrogen levels

Sugarcane is an important sugar and energy crop worldwide, requiring a large amount of nitrogen (N). However, excessive application of synthetic N fertilizer causes environmental pollution in farmland. Endophytic nitrogen-fixing bacteria (ENFB) provide N nutrition for plants through biological N fixation, thus reducing the need for chemical fertilizers. The present study investigated the effect of the N-fixing endophytic strain Enterobacter roggenkampii ED5 on phytohormone indole-3-acetic acid (IAA), N-metabolism enzyme activities, microbial community compositions, and N cycle genes in sugarcane rhizosphere soil at different N levels. Three levels of 15N-urea, such as low N (0 kg/ha), medium N (150 kg/ha), and high N (300 kg/ha), were applied. The results showed that, after inoculating strain ED5, the IAA content in sugarcane leaves was significantly increased by 68.82% under low N condition at the seedling stage (60 days). The nitrate reductase (NR) activity showed a downward trend. However, the glutamine synthase (GS) and NADH-glutamate dehydrogenase (NADH-GDH) activities were significantly enhanced compared to the control under the high N condition, and the GS and NR genes had the highest expression at 180 and 120 days, respectively, at the low N level. The total N content in the roots, stems, and leaves of sugarcane was higher than the control. The 15N atom % excess of sugarcane decreased significantly under medium N condition, indicating that the medium N level was conducive to N fixation in strain ED5. Metagenome analysis of sugarcane rhizosphere soil exhibited that the abundance of N-metabolizing microbial richness was increased under low and high N conditions after inoculation of strain ED5 at the genus level, while it was increased at the phylum level only under the low N condition. The LefSe (LDA > 2, p < 0.05) found that the N-metabolism-related differential microorganisms under the high N condition were higher than those under medium and low N conditions. It was also shown that the abundance of nifDHK genes was significantly increased after inoculation of ED5 at the medium N level, and other N cycle genes had high abundance at the high N level after inoculation of strain ED5. The results of this study provided a scientific reference for N fertilization in actual sugarcane production.

Independent Component Analysis Reveals the Transcriptional Regulatory Modules in Bradyrhizobium diazoefficiens USDA110

The dynamic adaptation of bacteria to environmental changes is achieved through the coordinated expression of many genes, which constitutes a transcriptional regulatory network (TRN). Bradyrhizobium diazoefficiens USDA110 is an important model strain for the study of symbiotic nitrogen fixation (SNF), and its SNF ability largely depends on the TRN. In this study, independent component analysis was applied to 226 high-quality gene expression profiles of B. diazoefficiens USDA110 microarray datasets, from which 64 iModulons were identified. Using these iModulons and their condition-specific activity levels, we (1) provided new insights into the connection between the FixLJ-FixK2-FixK1 regulatory cascade and quorum sensing, (2) discovered the independence of the FixLJ-FixK2-FixK1 and NifA/RpoN regulatory cascades in response to oxygen, (3) identified the FixLJ-FixK2 cascade as a mediator connecting the FixK2-2 iModulon and the Phenylalanine iModulon, (4) described the differential activation of iModulons in B. diazoefficiens USDA110 under different environmental conditions, and (5) proposed a notion of active-TRN based on the changes in iModulon activity to better illustrate the relationship between gene regulation and environmental condition. In sum, this research offered an iModulon-based TRN for B. diazoefficiens USDA110, which formed a foundation for comprehensively understanding the intricate transcriptional regulation during SNF.