Diversity of D-Amino Acid Utilizing Bacteria From Kongsfjorden, Arctic and the Metabolic Pathways for Seven D-Amino Acids

Evidence for archaeal metabolism of D-amino acids in the deep marine sediments

The deep marine sediments represent a major repository of organic matter whilst hosting a great number of uncultivated microbes. Microbial metabolism plays a key role in the recycling of organic matter in the deep marine sediments. D-amino acids (DAAs) and DAA-containing muropeptides, an important group of organic matter in the deep marine sediments, are primarily derived from bacterial peptidoglycan decomposition. Archaea are abundant in the deep ocean microbiome, yet their role in DAA metabolism remains poorly studied. Here, we report bioinformatic investigation and enzymatic characterization of deep marine sedimentary archaea involved in DAA metabolism. Our analyses suggest that a variety of archaea, particularly the Candidatus Bathyarchaeota and the Candidatus Lokiarchaeaota, can metabolize DAAs. DAAs are converted into L-amino acids via amino acid racemases (Ala racemase, Asp racemase and broad substrate specificity amino acid racemase), and converted into α-keto acid via d-serine ammonia-lyase, whereas DAA-containing di-/tri-muropeptides can be hydrolyzed by peptidases (dipeptidase and D-aminopeptidase). Overall, this study reveals the identity and activity of deep marine sedimentary archaea involved in DAA metabolism, shedding light on the mineralization and biogeochemical cycling of DAAs in the deep marine sediments.

Prophage enhances the ability of deep-sea bacterium Shewanella psychrophila WP2 to utilize D-amino acid

Prophages are prevalent in the marine bacterial genomes and reshape the physiology and metabolism of their hosts. However, whether and how prophages influence the microbial degradation of D-amino acids (D-AAs), which is one of the widely distributed recalcitrant dissolved organic matters (RDOMs) in the ocean, remain to be explored. In this study, we addressed this issue in a representative marine bacterium, Shewanella psychrophila WP2 (WP2), and its integrated prophage SP1. Notably, compared to the WP2 wild-type strain, the SP1 deletion mutant of WP2 (WP2ΔSP1) exhibited a significantly lower D-glutamate (D-Glu) consumption rate and longer lag phase when D-Glu was used as the sole nitrogen source. The subsequent transcriptome analysis identified 1,523 differentially expressed genes involved in diverse cellular processes, especially that multiple genes related to inorganic nitrogen metabolism were highly upregulated. In addition, the dynamic profiles of ammonium, nitrate, and nitrite were distinct between the culture media of WP2 and WP2ΔSP1. Finally, we provide evidence that SP1 conferred a competitive advantage to WP2 when D-Glu was used as the sole nitrogen source and SP1-like phages may be widely distributed in the global ocean. Taken together, these findings offer novel insight into the influences of prophages on host metabolism and RDOM cycling in marine environments.IMPORTANCEThis work represents the first exploration of the impact of prophages on the D-amino acid (D-AA) metabolism of deep-sea bacteria. By using S. psychrophila WP2 and its integrated prophage SP1 as a representative system, we found that SP1 can significantly increase the catabolism rate of WP2 to D-glutamate and produce higher concentrations of ammonium, resulting in faster growth and competitive advantages. Our findings not only deepen our understanding of the interaction between deep-sea prophages and hosts but also provide new insights into the ecological role of prophages in refractory dissolved organic matter and the nitrogen cycle in deep oceans.

The Arctic summer microbiome across Fram Strait: Depth, longitude, and substrate concentrations structure microbial diversity in the euphotic zone

The long-term dynamics of microbial communities across geographic, hydrographic, and biogeochemical gradients in the Arctic Ocean are largely unknown. To address this, we annually sampled polar, mixed, and Atlantic water masses of the Fram Strait (2015-2019; 5-100 m depth) to assess microbiome composition, substrate concentrations, and oceanographic parameters. Longitude and water depth were the major determinants (~30%) of microbial community variability. Bacterial alpha diversity was highest in lower-photic polar waters. Community composition shifted from west to east, with the prevalence of, for example, Dadabacteriales and Thiotrichales in Arctic- and Atlantic-influenced waters, respectively. Concentrations of dissolved organic carbon peaked in the western, compared to carbohydrates in the chlorophyll-maximum of eastern Fram Strait. Interannual differences due to the time of sampling, which varied between early (June 2016/2018) and late (September 2019) phytoplankton bloom stages, illustrated that phytoplankton composition and resulting availability of labile substrates influence bacterial dynamics. We identified 10 species clusters with stable environmental correlations, representing signature populations of distinct ecosystem states. In context with published metagenomic evidence, our microbial-biogeochemical inventory of a key Arctic region establishes a benchmark to assess ecosystem dynamics and the imprint of climate change.

Enantioselective transformation of phytoplankton-derived dihydroxypropanesulfonate by marine bacteria

Chirality, a fundamental property of matter, is often overlooked in the studies of marine organic matter cycles. Dihydroxypropanesulfonate (DHPS), a globally abundant organosulfur compound, serves as an ecologically important currency for nutrient and energy transfer from phytoplankton to bacteria in the ocean. However, the chirality of DHPS in nature and its transformation remain unclear. Here, we developed a novel approach using chiral phosphorus-reagent labeling to separate DHPS enantiomers. Our findings demonstrated that at least one enantiomer of DHPS is present in marine diatoms and coccolithophores, and that both enantiomers are widespread in marine environments. A novel chiral-selective DHPS catabolic pathway was identified in marine Roseobacteraceae strains, where HpsO and HpsP dehydrogenases at the gateway to DHPS catabolism act specifically on R-DHPS and S-DHPS, respectively. R-DHPS is also a substrate for the dehydrogenase HpsN. All three dehydrogenases generate stable hydrogen bonds between the chirality-center hydroxyls of DHPS and highly conserved residues, and HpsP also form coordinate-covalent bonds between the chirality-center hydroxyls and Zn2+, which determines the mechanistic basis of strict stereoselectivity. We further illustrated the role of enzymatic promiscuity in the evolution of DHPS metabolism in Roseobacteraceae and SAR11. This study provides the first evidence of chirality's involvement in phytoplankton-bacteria metabolic currencies, opening a new avenue for understanding the ocean organosulfur cycle.

Characterization of free L- and D-amino acids in size-segregated background aerosols over the Ross Sea, Antarctica

The study of airborne chemical markers is crucial for identifying sources of aerosols, and their atmospheric processes of transport and transformation. The investigation of free amino acids and their differentiation between the L- and D- enantiomers are even more important to understand their sources and atmospheric fate. Aerosol samples were collected with a high-volume sampler with cascade impactor at Mario Zucchelli Station (MZS) on the coast of the Ross Sea (Antarctica) for two summer campaigns (2018/19 and 2019/20). The total mean concentration of free amino acids in PM10 was 4 ± 2 pmol m-3 for both campaigns and most of free amino acids were distributed in fine particles. The coarse mode of airborne D-Alanine and dimethylsufoniopropionate in seawater showed a similar trend during both Antarctic campaigns. Thus, the study of D/L Ala ratio in fine, coarse and PM10 fractions indicated the microlayer as the local source. This paper demonstrated that free amino acids follow the trend of DMS and MSA release occurred in the Ross Sea, confirming their applicability as markers for phytoplankton bloom also in paleoclimatic studies.

Novel D-glutamate catabolic pathway in marine Proteobacteria and halophilic archaea

D-glutamate (D-Glu) is an essential component of bacterial peptidoglycans, representing an important, yet overlooked, pool of organic matter in global oceans. However, little is known on D-Glu catabolism by marine microorganisms. Here, a novel catabolic pathway for D-Glu was identified using the marine bacterium Pseudoalteromonas sp. CF6-2 as the model. Two novel enzymes (DgcN, DgcA), together with a transcriptional regulator DgcR, are crucial for D-Glu catabolism in strain CF6-2. Genetic and biochemical data confirm that DgcN is a N-acetyltransferase which catalyzes the formation of N-acetyl-D-Glu from D-Glu. DgcA is a racemase that converts N-acetyl-D-Glu to N-acetyl-L-Glu, which is further hydrolyzed to L-Glu. DgcR positively regulates the transcription of dgcN and dgcA. Structural and biochemical analyses suggested that DgcN and its homologs, which use D-Glu as the acyl receptor, represent a new group of the general control non-repressible 5 (GCN5)-related N-acetyltransferases (GNAT) superfamily. DgcA and DgcN occur widely in marine bacteria (particularly Rhodobacterales) and halophilic archaea (Halobacteria) and are abundant in marine and hypersaline metagenome datasets. Thus, this study reveals a novel D-Glu catabolic pathway in ecologically important marine bacteria and halophilic archaea and helps better understand the catabolism and recycling of D-Glu in these ecosystems.

Diaminopimelic Acid Metabolism by Pseudomonadota in the Ocean

Diaminopimelic acid (DAP) is a unique component of the cell wall of Gram-negative bacteria. It is also an important component of organic matter and is widely utilized by microbes in the world's oceans. However, neither DAP concentrations nor marine DAP-utilizing microbes have been investigated. Here, DAP concentrations in seawater were measured and the diversity of marine DAP-utilizing bacteria and the mechanisms for their DAP metabolism were investigated. Free DAP concentrations in seawater, from surface to a 5,000 m depth, were found to be between 0.61 μM and 0.96 μM in the western Pacific Ocean. DAP-utilizing bacteria from 20 families in 4 phyla were recovered from the western Pacific seawater and 14 strains were further isolated, in which Pseudomonadota bacteria were dominant. Based on genomic and transcriptomic analyses combined with gene deletion and in vitro activity detection, DAP decarboxylase (LysA), which catalyzes the decarboxylation of DAP to form lysine, was found to be a key and specific enzyme involved in DAP metabolism in the isolated Pseudomonadota strains. Interrogation of the Tara Oceans database found that most LysA-like sequences (92%) are from Pseudomonadota, which are widely distributed in multiple habitats. This study provides an insight into DAP metabolism by marine bacteria in the ocean and contributes to our understanding of the mineralization and recycling of DAP by marine bacteria. IMPORTANCE DAP is a unique component of peptidoglycan in Gram-negative bacterial cell walls. Due to the large number of marine Gram-negative bacteria, DAP is an important component of marine organic matter. However, it remains unclear how DAP is metabolized by marine microbes. This study investigated marine DAP-utilizing bacteria by cultivation and bioinformational analysis and examined the mechanism of DAP metabolism used by marine bacteria. The results demonstrate that Pseudomonadota bacteria are likely to be an important DAP-utilizing group in the ocean and that DAP decarboxylase is a key enzyme involved in DAP metabolism. This study also sheds light on the mineralization and recycling of DAP driven by bacteria.

A pathway for chitin oxidation in marine bacteria

Oxidative degradation of chitin, initiated by lytic polysaccharide monooxygenases (LPMOs), contributes to microbial bioconversion of crystalline chitin, the second most abundant biopolymer in nature. However, our knowledge of oxidative chitin utilization pathways, beyond LPMOs, is very limited. Here, we describe a complete pathway for oxidative chitin degradation and its regulation in a marine bacterium, Pseudoalteromonas prydzensis. The pathway starts with LPMO-mediated extracellular breakdown of chitin into C1-oxidized chitooligosaccharides, which carry a terminal 2-(acetylamino)-2-deoxy-D-gluconic acid (GlcNAc1A). Transmembrane transport of oxidized chitooligosaccharides is followed by their hydrolysis in the periplasm, releasing GlcNAc1A, which is catabolized in the cytoplasm. This pathway differs from the known hydrolytic chitin utilization pathway in enzymes, transporters and regulators. In particular, GlcNAc1A is converted to 2-keto-3-deoxygluconate 6-phosphate, acetate and NH₃ via a series of reactions resembling the degradation of D-amino acids rather than other monosaccharides. Furthermore, genomic and metagenomic analyses suggest that the chitin oxidative utilization pathway may be prevalent in marine Gammaproteobacteria.

New insights into the inhibitory roles and mechanisms of D-amino acids in bacterial biofilms in medicine, industry, and agriculture

Biofilms are complex aggregates of microbes that are tightly protected by an extracellular matrix (ECM) and may attach to a surface or adhere together. A higher persistence of bacteria on biofilms makes them resistant not only to harsh conditions but also to various antibiotics which led to the emergence of problems in different applications. Recently, it has been discovered that many bacteria produce and release various D-amino acids (D-AAs) to inhibit biofilm formation, which made a great deal of interest in research into the control of bacterial biofilms in diverse fields, such as human health, industrial settings, and medical devices. D-AAs have various mechanisms to inhibit bacterial biofilms such as: (i) interfering with protein synthesis (ii) Inhibition of extracellular polymeric materials (EPS) productions (protein, eDNA, and polysaccharide) (iii) Inhibition of quorum sensing (autoinducers), and (iv) interfere with peptidoglycan synthesis, these various modes of action, enables these small molecules to inhibit both Gram-negative and Gram-positive bacterial biofilms. Since most biofilms are multi-species, D-AAs in combination with other antimicrobial agents are good choices to combat a variety of bacterial biofilms without displaying toxicity on human cells. This review article addressed the role of D-AAs in controlling several bacterial biofilms and described the possible or definite mechanisms involved in this process.

Amino Acid Configuration Affects Volatile Fatty Acid Production during Proteinaceous Waste Valorization: Chemotaxis, Quorum Sensing, and Metabolism

During proteinaceous waste valorization to produce volatile fatty acids (VFAs), protein needs to be hydrolyzed to amino acids (AAs), but the effects of the configuration of AAs on their biotransformation and VFA production have not been investigated. In this study, more residual d-AAs than their corresponding l-AAs were observed after VFAs were produced from kitchen waste in a pilot-scale bioreactor. For all AAs investigated, the VFA production from d-AAs was lower than that from corresponding l-AAs. The metagenomics and metaproteomics analyses revealed that the l-AA fermentation system exhibited greater bacterial chemotaxis and quorum sensing (QS) than d-AAs, which benefited the establishment of functional microorganisms (such as Clostridium, Sedimentibacter, and Peptoclostridium) and expression of functional proteins (e.g., substrate transportation cofactors, l-AA dehydrogenase, and acidogenic proteins). In addition, d-AAs need to be racemized to l-AAs before being metabolized, and the difference of VFA production between d-AAs and l-AAs decreased with the increase of racemization activity. The findings of the AA configuration affecting bacterial chemotaxis and QS, which altered microorganism communities and functional protein expression, provided a new insight into the reasons for higher l-AA metabolism than d-AAs and more d-AAs left during VFA production from proteinaceous wastes.

Mechanism of high D-aspartate production in the lactic acid bacterium Latilactobacillus sp. strain WDN19

D-Aspartate (D-Asp) is a useful compound for a semisynthetic antibiotic and has potentially beneficial effects on humans. Several lactic acid bacteria (LAB) species produce D-Asp as a component of cell wall peptidoglycan. We previously isolated a LAB strain (named strain WDN19) that can extracellularly produce a large amount of D-Asp. Here, we show the factors that contribute to high D-Asp production ability. Strain WDN19 was most closely related to Latilactobacillus curvatus. The D-Asp production ability of strain WDN19 in a rich medium was 13.7-fold higher than that of L. curvatus DSM 20019. A major part of D-Asp was synthesized from L-Asp contained in the medium by aspartate racemase (RacD). During their cultivation, the RacD activity in strain WDN19 was higher than in strain DSM 20019, especially much higher in the early exponential growth phase because of the higher racD transcription and the higher activity of RacD itself of strain WDN19. In a synthetic medium, the extracellular production of D,L-Asp was observed in strain WDN19 but not in strain DSM 20019. The addition of L-asparagine (L-Asn) to the medium increased and gave D,L-Asp production in strains WDN19 and DSM 20019, respectively, suggesting L-Asp synthesis by L-asparaginase (AsnA). The L-Asn uptake ability of the strains was similar, but the AsnA activity in the middle exponential and early stationary growth phases and intracellular D,L-Asp was much higher in strain WDN19. In their genome sequences, only an aspartate aminotransferase gene was found among L-Asp-metabolizing enzymes, except for RacD, but was disrupted in strain WDN19 by transposon insertion. These observations indicated that the high D-Asp production ability of strain WDN19 was mainly based on high RacD and AnsA activities and L-Asp supply. KEY POINTS: • Strain WDN19 was suggested to be a strain of Latilactobacillus curvatus. • Extracellular high d-Asp production ability was not a common feature of L. curvatus. • High d-Asp production was due to high RacD and AnsA activities and l-Asp supply.

d-Alanine Metabolism via d-Ala Aminotransferase by a Marine Gammaproteobacterium, Pseudoalteromonas sp. Strain CF6-2

As the most abundant d-amino acid (DAA) in the ocean, d-alanine (d-Ala) is a key component of peptidoglycan in the bacterial cell wall. However, the underlying mechanisms of bacterial metabolization of d-Ala through the microbial food web remain largely unknown. In this study, the metabolism of d-Ala by marine bacterium Pseudoalteromonas sp. strain CF6-2 was investigated. Based on genomic, transcriptional, and biochemical analyses combined with gene knockout, d-Ala aminotransferase was found to be indispensable for the catabolism of d-Ala in strain CF6-2. Investigation on other marine bacteria also showed that d-Ala aminotransferase gene is a reliable indicator for their ability to utilize d-Ala. Bioinformatic investigation revealed that d-Ala aminotransferase sequences are prevalent in genomes of marine bacteria and metagenomes, especially in seawater samples, and Gammaproteobacteria represents the predominant group containing d-Ala aminotransferase. Thus, Gammaproteobacteria is likely the dominant group to utilize d-Ala via d-Ala aminotransferase to drive the recycling and mineralization of d-Ala in the ocean. IMPORTANCE As the most abundant d-amino acid in the ocean, d-Ala is a component of the marine DON (dissolved organic nitrogen) pool. However, the underlying mechanism of bacterial metabolization of d-Ala to drive the recycling and mineralization of d-Ala in the ocean is still largely unknown. The results in this study showed that d-Ala aminotransferase is specific and indispensable for d-Ala catabolism in marine bacteria and that marine bacteria containing d-Ala aminotransferase genes are predominantly Gammaproteobacteria widely distributed in global oceans. This study reveals marine d-Ala-utilizing bacteria and the mechanism of their metabolization of d-Ala. The results shed light on the mechanisms of recycling and mineralization of d-Ala driven by bacteria in the ocean, which are helpful in understanding oceanic microbial-mediated nitrogen cycle.

Definition of the Metagenomic Profile of Ocean Water Samples From the Gulf of Mexico Based on Comparison With Reference Samples From Sites Worldwide

Computational and statistical analysis of shotgun metagenomes can predict gene abundance and is helpful for elucidating the functional and taxonomic compositions of environmental samples. Gene products are compared against physicochemical conditions or perturbations to shed light on the functions performed by the microbial community of an environmental sample; however, this information is not always available. The present study proposes a method for inferring the metabolic potential of metagenome samples by constructing a reference based on determining the probability distribution of the counts of each enzyme annotated. To test the methodology, we used marine water samples distributed worldwide as references. Then, the references were utilized to compare the annotated enzymes of two different water samples extracted from the Gulf of Mexico (GoM) to distinguish those enzymes with atypical behavior. The enzymes whose annotation counts presented frequencies significantly different from those of the reference were used to perform metabolic reconstruction, which naturally identified pathways. We found that several of the enzymes were involved in the biodegradation of petroleum, which is consistent with the impact of human hydrocarbon extraction activity and its ubiquitous presence in the GoM. The examination of other reconstructed pathways revealed significant enzymes indicating the presence of microbial communities characterizing each ocean depth and ocean cycle, providing a fingerprint of each sampled site.

The metabolic core of the prokaryotic community from deep-sea sediments of the southern Gulf of Mexico shows different functional signatures between the continental slope and abyssal plain

Marine sediments harbor an outstanding level of microbial diversity supporting diverse metabolic activities. Sediments in the Gulf of Mexico (GoM) are subjected to anthropic stressors including oil pollution with potential effects on microbial community structure and function that impact biogeochemical cycling. We used metagenomic analyses to provide significant insight into the potential metabolic capacity of the microbial community in Southern GoM deep sediments. We identified genes for hydrocarbon, nitrogen and sulfur metabolism mostly affiliated with Alpha and Betaproteobacteria, Acidobacteria, Chloroflexi and Firmicutes, in relation to the use of alternative carbon and energy sources to thrive under limiting growth conditions, and metabolic strategies to cope with environmental stressors. In addition, results show amino acids metabolism could be associated with sulfur metabolism carried out by Acidobacteria, Chloroflexi and Firmicutes, and may play a crucial role as a central carbon source to favor bacterial growth. We identified the tricarboxylic acid cycle (TCA) and aspartate, glutamate, glyoxylate and leucine degradation pathways, as part of the core carbon metabolism across samples. Further, microbial communities from the continental slope and abyssal plain show differential metabolic capacities to cope with environmental stressors such as oxidative stress and carbon limiting growth conditions, respectively. This research combined taxonomic and functional information of the microbial community from Southern GoM sediments to provide fundamental knowledge that links the prokaryotic structure to its potential function and which can be used as a baseline for future studies to model microbial community responses to environmental perturbations, as well as to develop more accurate mitigation and conservation strategies.