Postglacial adaptations enabled colonization and quasi-clonal dispersal of ammonia-oxidizing archaea in modern European large lakes

Succession of Bacteria and Archaea Within the Soil Micro-Food Web Shifts Soil Respiration Dynamics

Bacterivorous nematodes are important grazers in the soil micro-food web. Their trophic regulation shapes the composition and ecosystem services of the soil microbiome, but the underlying population dynamics of bacteria and archaea are poorly understood. We followed soil respiration and 221 dominant bacterial and archaeal 16S rRNA gene amplicon sequencing variants (ASVs) in response to top-down control by a common bacterivorous soil nematode, Acrobeloides buetschlii, bottom-up control by maize litter amendment and their combination over 32 days. Maize litter amendment significantly increased soil respiration, while A. buetschlii addition caused an earlier peak in soil respiration. Underlying bacterial and archaeal population dynamics separated into five major response types, differentiating in their temporal abundance maxima and minima. In-depth analysis of these population dynamics identified a broad imprint of A. buetschlii grazing on dominant bacterial (Acidobacteriota, Bacteroidota, Gemmatimonadota, Pseudomonadota) and archaeal (Nitrososphaerota) ASVs. Combined bottom-up control by maize litter and top-down control by A. buetschlii grazing caused a succession of soil microbiota, driven by population changes first in the Bacteroidota, then in the Pseudomonadota and finally in the Acidobacteriota and Nitrososphaerota. Our results are an essential step forward in understanding trophic modulation of soil microbiota and its feedback on soil respiration.

Thaumarchaeota from deep-sea methane seeps provide novel insights into their evolutionary history and ecological implications

BACKGROUND

Ammonia-oxidizing archaea (AOA) of the phylum Thaumarchaeota mediate the rate-limiting step of nitrification and remove the ammonia that inhibits the aerobic metabolism of methanotrophs. However, the AOA that inhabit deep-sea methane-seep surface sediments (DMS) are rarely studied. Here, we used global DMS metagenomics and metagenome-assembled genomes (MAGs) to investigate the metabolic activity, evolutionary history, and ecological contributions of AOA. Expression of AOA-specific ammonia-oxidizing gene (amoA) was examined in the sediments collected from the South China Sea (SCS) to identify their active ammonia metabolism in the DMS.

RESULTS

Our analysis indicated that AOA contribute > 75% to the composition of ammonia-utilization genes within the surface layers (above 30 cm) of global DMS. The AOA-specific ammonia-oxidizing gene was actively expressed in the DMS collected from the SCS. Phylogenomic analysis of medium-/high-quality MAGs from 18 DMS-AOA indicated that they evolved from ancestors in the barren deep-sea sediment and then expanded from the DMS to shallow water forming an amoA-NP-gamma clade-affiliated lineage. Molecular dating suggests that the DMS-AOA origination coincided with the Neoproterozoic oxidation event (NOE), which occurred ~ 800 million years ago (mya), and their expansion to shallow water coincided with the Sturtian glaciation (~ 713 mya). Comparative genomic analysis suggests that DMS-AOA exhibit higher requirement of carbon source for protein synthesis with enhanced genomic capability for osmotic regulation, motility, chemotaxis, and utilization of exogenous organic compounds, suggesting it could be more heterotrophic compared with other lineages.

CONCLUSION

Our findings provide new insights into the evolutionary history of AOA within the Thaumarchaeota, highlighting their critical roles in nitrogen cycling in the global DMS ecosystems. Video Abstract.

Dating Ammonia-Oxidizing Bacteria with Abundant Eukaryotic Fossils

Evolution of a complete nitrogen (N) cycle relies on the onset of ammonia oxidation, which aerobically converts ammonia to nitrogen oxides. However, accurate estimation of the antiquity of ammonia-oxidizing bacteria (AOB) remains challenging because AOB-specific fossils are absent and bacterial fossils amenable to calibrate molecular clocks are rare. Leveraging the ancient endosymbiosis of mitochondria and plastid, as well as using state-of-the-art Bayesian sequential dating approach, we obtained a timeline of AOB evolution calibrated largely by eukaryotic fossils. We show that the first AOB evolved in marine Gammaproteobacteria (Gamma-AOB) and emerged between 2.1 and 1.9 billion years ago (Ga), thus postdating the Great Oxidation Event (GOE; 2.4 to 2.32 Ga). To reconcile the sedimentary N isotopic signatures of ammonia oxidation occurring near the GOE, we propose that ammonia oxidation likely occurred at the common ancestor of Gamma-AOB and Gammaproteobacterial methanotrophs, or the actinobacterial/verrucomicrobial methanotrophs which are known to have ammonia oxidation activities. It is also likely that nitrite was transported from the terrestrial habitats where ammonia oxidation by archaea took place. Further, we show that the Gamma-AOB predated the anaerobic ammonia-oxidizing (anammox) bacteria, implying that the emergence of anammox was constrained by the availability of dedicated ammonia oxidizers which produce nitrite to fuel anammox. Our work supports a new hypothesis that N redox cycle involving nitrogen oxides evolved rather late in the ocean.

Freshwater genome-reduced bacteria exhibit pervasive episodes of adaptive stasis

The emergence of bacterial species is rooted in their inherent potential for continuous evolution and adaptation to an ever-changing ecological landscape. The adaptive capacity of most species frequently resides within the repertoire of genes encoding the secreted proteome (SP), as it serves as a primary interface used to regulate survival/reproduction strategies. Here, by applying evolutionary genomics approaches to metagenomics data, we show that abundant freshwater bacteria exhibit biphasic adaptation states linked to the eco-evolutionary processes governing their genome sizes. While species with average to large genomes adhere to the dominant paradigm of evolution through niche adaptation by reducing the evolutionary pressure on their SPs (via the augmentation of functionally redundant genes that buffer mutational fitness loss) and increasing the phylogenetic distance of recombination events, most of the genome-reduced species exhibit a nonconforming state. In contrast, their SPs reflect a combination of low functional redundancy and high selection pressure, resulting in significantly higher levels of conservation and invariance. Our findings indicate that although niche adaptation is the principal mechanism driving speciation, freshwater genome-reduced bacteria often experience extended periods of adaptive stasis. Understanding the adaptive state of microbial species will lead to a better comprehension of their spatiotemporal dynamics, biogeography, and resilience to global change.

Out of sight, but not out of season: Nitrifier distributions and population dynamics in a large oligotrophic lake

Nitrification is an important control on the form and distribution of nitrogen in freshwater ecosystems. However, the seasonality of nitrogen pools and the diversity of organisms catalyzing this process have not been well documented in oligotrophic lakes. Here, we show that nitrogen pools and nitrifying organisms in Flathead Lake are temporally and vertically dynamic, with nitrifiers displaying specific preferences depending on the season. While the ammonia-oxidizing bacteria (AOB) Nitrosomonadaceae and nitrite-oxidizing bacteria (NOB) Nitrotoga dominate at depth in the summer, the ammonia-oxidizing archaea (AOA) Nitrososphaerota and NOB Nitrospirota become abundant in the winter. Given clear seasonality in ammonium, with higher concentrations during the summer, we hypothesize that the succession between these two nitrifying groups may be due to nitrogen affinity, with AOB more competitive when ammonia concentrations are higher and AOA when they are lower. Nitrifiers in Flathead Lake share more than 99% average nucleotide identity with those reported in other North American lakes but are distinct from those in Europe and Asia, indicating a role for geographic isolation as a factor controlling speciation among nitrifiers. Our study shows there are seasonal shifts in nitrogen pools and nitrifying populations, highlighting the dynamic spatial and temporal nature of nitrogen cycling in freshwater ecosystems.

Gene inversion led to the emergence of brackish archaeal heterotrophs in the aftermath of the Cryogenian Snowball Earth

Land-ocean interactions greatly impact the evolution of coastal life on earth. However, the ancient geological forces and genetic mechanisms that shaped evolutionary adaptations and allowed microorganisms to inhabit coastal brackish waters remain largely unexplored. In this study, we infer the evolutionary trajectory of the ubiquitous heterotrophic archaea Poseidoniales (Marine Group II archaea) presently occurring across global aquatic habitats. Our results show that their brackish subgroups had a single origination, dated to over 600 million years ago, through the inversion of the magnesium transport gene corA that conferred osmotic-stress tolerance. The subsequent loss and gain of corA were followed by genome-wide adjustment, characterized by a general two-step mode of selection in microbial speciation. The coastal family of Poseidoniales showed a rapid increase in the evolutionary rate during and in the aftermath of the Cryogenian Snowball Earth (∼700 million years ago), possibly in response to the enhanced phosphorus supply and the rise of algae. Our study highlights the close interplay between genetic changes and ecosystem evolution that boosted microbial diversification in the Neoproterozoic continental margins, where the Cambrian explosion of animals soon followed.

Novel order-level lineage of ammonia-oxidizing archaea widespread in marine and terrestrial environments

Ammonia-oxidizing archaea (AOA) are among the most ubiquitous and abundant archaea on Earth, widely distributed in marine, terrestrial, and geothermal ecosystems. However, the genomic diversity, biogeography, and evolutionary process of AOA populations in subsurface environments are vastly understudied compared to those in marine and soil systems. Here, we report a novel AOA order Candidatus (Ca.) Nitrosomirales which forms a sister lineage to the thermophilic Ca. Nitrosocaldales. Metagenomic and 16S rRNA gene-read mapping demonstrates the abundant presence of Nitrosomirales AOA in various groundwater environments and their widespread distribution across a range of geothermal, terrestrial, and marine habitats. Terrestrial Nitrosomirales AOA show the genetic capacity of using formate as a source of reductant and using nitrate as an alternative electron acceptor. Nitrosomirales AOA appear to have acquired key metabolic genes and operons from other mesophilic populations via horizontal gene transfer, including genes encoding urease, nitrite reductase, and V-type ATPase. The additional metabolic versatility conferred by acquired functions may have facilitated their radiation into a variety of subsurface, marine, and soil environments. We also provide evidence that each of the four AOA orders spans both marine and terrestrial habitats, which suggests a more complex evolutionary history for major AOA lineages than previously proposed. Together, these findings establish a robust phylogenomic framework of AOA and provide new insights into the ecology and adaptation of this globally abundant functional guild.

Challenges in estimating effective population sizes from metagenome-assembled genomes

Effective population size (Ne) plays a critical role in shaping the relative efficiency between natural selection and genetic drift, thereby serving as a cornerstone for understanding microbial ecological dynamics. Direct Ne estimation relies on neutral genetic diversity within closely related genomes, which is, however, often constrained by the culturing difficulties for the vast majority of prokaryotic lineages. Metagenome-assembled genomes (MAGs) offer a high-throughput alternative for genomic data acquisition, yet their accuracy in Ne estimation has not been fully verified. This study examines the Thermococcus genus, comprising 66 isolated strains and 29 MAGs, to evaluate the reliability of MAGs in Ne estimation. Despite the even distribution across the Thermococcus phylogeny and the comparable internal average nucleotide identity (ANI) between isolate populations and MAG populations, our results reveal consistently lower Ne estimates from MAG populations. This trend of underestimation is also observed in various MAG populations across three other bacterial genera. The underrepresentation of genetic variation in MAGs, including loss of allele frequency data and variable genomic segments, likely contributes to the underestimation of Ne. Our findings underscore the necessity for caution when employing MAGs for evolutionary studies, which often depend on high-quality genome assemblies and nucleotide-level diversity.

Oxygen respiration and polysaccharide degradation by a sulfate-reducing acidobacterium

Sulfate-reducing microorganisms represent a globally important link between the sulfur and carbon cycles. Recent metagenomic surveys expanded the diversity of microorganisms putatively involved in sulfate reduction underscoring our incomplete understanding of this functional guild. Here, we use genome-centric metatranscriptomics to study the energy metabolism of Acidobacteriota that carry genes for dissimilation of sulfur compounds in a long-term continuous culture running under alternating anoxic and oxic conditions. Differential gene expression analysis reveals the unique metabolic flexibility of a pectin-degrading acidobacterium to switch from sulfate to oxygen reduction when shifting from anoxic to oxic conditions. The combination of facultative anaerobiosis and polysaccharide degradation expands the metabolic versatility among sulfate-reducing microorganisms. Our results highlight that sulfate reduction and aerobic respiration are not mutually exclusive in the same organism, sulfate reducers can mineralize organic polymers, and anaerobic mineralization of complex organic matter is not necessarily a multi-step process involving different microbial guilds but can be bypassed by a single microbial species.

Habitat Transition in the Evolution of Bacteria and Archaea

Related groups of microbes are widely distributed across Earth's habitats, implying numerous dispersal and adaptation events over evolutionary time. However, relatively little is known about the characteristics and mechanisms of these habitat transitions, particularly for populations that reside in animal microbiomes. Here, we review the literature concerning habitat transitions among a variety of bacterial and archaeal lineages, considering the frequency of migration events, potential environmental barriers, and mechanisms of adaptation to new physicochemical conditions, including the modification of protein inventories and other genomic characteristics. Cells dependent on microbial hosts, particularly bacteria from the Candidate Phyla Radiation, have undergone repeated habitat transitions from environmental sources into animal microbiomes. We compare their trajectories to those of both free-living cells-including the Melainabacteria, Elusimicrobia, and methanogenic archaea-and cellular endosymbionts and bacteriophages, which have made similar transitions. We conclude by highlighting major related topics that may be worthy of future study.

Metagenomics: An Effective Approach for Exploring Microbial Diversity and Functions

Various fields have been identified in the "omics" era, such as genomics, proteomics, transcriptomics, metabolomics, phenomics, and metagenomics. Among these, metagenomics has enabled a significant increase in discoveries related to the microbial world. Newly discovered microbiomes in different ecologies provide meaningful information on the diversity and functions of microorganisms on the Earth. Therefore, the results of metagenomic studies have enabled new microbe-based applications in human health, agriculture, and the food industry, among others. This review summarizes the fundamental procedures on recent advances in bioinformatic tools. It also explores up-to-date applications of metagenomics in human health, food study, plant research, environmental sciences, and other fields. Finally, metagenomics is a powerful tool for studying the microbial world, and it still has numerous applications that are currently hidden and awaiting discovery. Therefore, this review also discusses the future perspectives of metagenomics.

Adaptive genetic traits in pelagic freshwater microbes

Pelagic microbes have adopted distinct strategies to inhabit the pelagial of lakes and oceans and can be broadly categorized in two groups: free-living, specialized oligotrophs and patch-associated generalists or copiotrophs. In this review, we aim to identify genomic traits that enable pelagic freshwater microbes to thrive in their habitat. To do so, we discuss the main genetic differences of pelagic marine and freshwater microbes that are both dominated by specialized oligotrophs and the difference to freshwater sediment microbes, where copiotrophs are more prevalent. We phylogenomically analysed a collection of >7700 metagenome-assembled genomes, classified habitat preferences on different taxonomic levels, and compared the metabolic traits of pelagic freshwater, marine, and freshwater sediment microbes. Metabolic differences are mainly associated with transport functions, environmental information processing, components of the electron transport chain, osmoregulation and the isoelectric point of proteins. Several lineages with known habitat transitions (Nitrososphaeria, SAR11, Methylophilaceae, Synechococcales, Flavobacteriaceae, Planctomycetota) and the underlying mechanisms in this process are discussed in this review. Additionally, the distribution, ecology and genomic make-up of the most abundant freshwater prokaryotes are described in details in separate chapters for Actinobacteriota, Bacteroidota, Burkholderiales, Verrucomicrobiota, Chloroflexota, and 'Ca. Patescibacteria'.

Decoding geobiological evolution from microbiomes

Genomic records of genetic recombination and mutation rates indicate that freshwater ammonia-oxidizing archaea have evolved through paleoclimate and geohydrological history.