Persistent activity of aerobic methane-oxidizing bacteria in anoxic lake waters due to metabolic versatility.

Lacustrine methane emissions are strongly mitigated by aerobic methane-oxidizing bacteria (MOB) that are typically most active at the oxic-anoxic interface. Although oxygen is required by the MOB for the first step of methane oxidation, their occurrence in anoxic lake waters has raised the possibility that they are capable of oxidizing methane further anaerobically. Here, we investigate the activity and growth of MOB in Lake Zug, a permanently stratified freshwater lake. The rates of anaerobic methane oxidation in the anoxic hypolimnion reached up to 0.2 µM d-1. Single-cell nanoSIMS measurements, together with metagenomic and metatranscriptomic analyses, linked the measured rates to MOB of the order Methylococcales. Interestingly, their methane assimilation activity was similar under hypoxic and anoxic conditions. Our data suggest that these MOB use fermentation-based methanotrophy as well as denitrification under anoxic conditions, thus offering an explanation for their widespread presence in anoxic habitats such as stratified water columns. Thus, the methane sink capacity of anoxic basins may have been underestimated by not accounting for the anaerobic MOB activity.

Methylphosphonate-driven methane formation and its link to primary production in the oligotrophic North Atlantic.

Methylphosphonate is an organic phosphorus compound used by microorganisms when phosphate, a key nutrient limiting growth in most marine surface waters, becomes unavailable. Microbial methylphosphonate use can result in the formation of methane, a potent greenhouse gas, in oxic waters where methane production is traditionally unexpected. The extent and controlling factors of such aerobic methane formation remain underexplored. Here, we show high potential net rates of methylphosphonate-driven methane formation (median 0.4 nmol methane L-1 d-1) in the upper water column of the western tropical North Atlantic. The rates are repressed but still quantifiable in the presence of in-situ or added phosphate, suggesting that some methylphosphonate-driven methane formation persists in phosphate-replete waters. The genetic potential for methylphosphonate utilisation is present in and transcribed by key photo- and heterotrophic microbial taxa, such as Pelagibacterales, SAR116, and Trichodesmium. While the large cyanobacterial nitrogen-fixers dominate in the surface layer, phosphonate utilisation by Alphaproteobacteria appears to become more important in deeper depths. We estimate that at our study site, a substantial part (median 11%) of the measured surface carbon fixation can be sustained by phosphorus liberated from phosphonate utilisation, highlighting the ecological importance of phosphonates in the carbon cycle of the oligotrophic ocean.

Diverse methylotrophic methanogenic archaea cause high methane emissions from seagrass meadows.

Marine coastlines colonized by seagrasses are a net source of methane to the atmosphere. However, methane emissions from these environments are still poorly constrained, and the underlying processes and responsible microorganisms remain largely unknown. Here, we investigated methane turnover in seagrass meadows of Posidonia oceanica in the Mediterranean Sea. The underlying sediments exhibited median net fluxes of methane into the water column of ca. 106 µmol CH4 ⋅ m-2 ⋅ d-1 Our data show that this methane production was sustained by methylated compounds produced by the plant, rather than by fermentation of buried organic carbon. Interestingly, methane production was maintained long after the living plant died off, likely due to the persistence of methylated compounds, such as choline, betaines, and dimethylsulfoniopropionate, in detached plant leaves and rhizomes. We recovered multiple mcrA gene sequences, encoding for methyl-coenzyme M reductase (Mcr), the key methanogenic enzyme, from the seagrass sediments. Most retrieved mcrA gene sequences were affiliated with a clade of divergent Mcr and belonged to the uncultured Candidatus Helarchaeota of the Asgard superphylum, suggesting a possible involvement of these divergent Mcr in methane metabolism. Taken together, our findings identify the mechanisms controlling methane emissions from these important blue carbon ecosystems.

Niche partitioning by photosynthetic plankton as a driver of CO2-fixation across the oligotrophic South Pacific Subtropical Ocean.

Oligotrophic ocean gyre ecosystems may be expanding due to rising global temperatures [1-5]. Models predicting carbon flow through these changing ecosystems require accurate descriptions of phytoplankton communities and their metabolic activities [6]. We therefore measured distributions and activities of cyanobacteria and small photosynthetic eukaryotes throughout the euphotic zone on a zonal transect through the South Pacific Ocean, focusing on the ultraoligotrophic waters of the South Pacific Gyre (SPG). Bulk rates of CO2 fixation were low (0.1 µmol C l-1 d-1) but pervasive throughout both the surface mixed-layer (upper 150 m), as well as the deep chlorophyll a maximum of the core SPG. Chloroplast 16S rRNA metabarcoding, and single-cell 13CO2 uptake experiments demonstrated niche differentiation among the small eukaryotes and picocyanobacteria. Prochlorococcus abundances, activity, and growth were more closely associated with the rims of the gyre. Small, fast-growing, photosynthetic eukaryotes, likely related to the Pelagophyceae, characterized the deep chlorophyll a maximum. In contrast, a slower growing population of photosynthetic eukaryotes, likely comprised of Dictyochophyceae and Chrysophyceae, dominated the mixed layer that contributed 65-88% of the areal CO2 fixation within the core SPG. Small photosynthetic eukaryotes may thus play an underappreciated role in CO2 fixation in the surface mixed-layer waters of ultraoligotrophic ecosystems.

Advection Drives Nitrate Past the Microphytobenthos in Intertidal Sands, Fueling Deeper Denitrification.

Nitrification rates are low in permeable intertidal sand flats such that the water column is the primary source of nitrate to the sediment. During tidal inundation, nitrate is supplied to the pore space by advection rather than diffusion, relieving the microorganisms that reside in the sand from nitrate limitation and supporting higher denitrification rates than those observed under diffusive transport. Sand flats are also home to an abundant community of benthic photosynthetic microorganisms, the microphytobenthos (MPB). Diatoms are an important component of the MPB that can take up and store high concentrations of nitrate within their cells, giving them the potential to alter nitrate availability in the surrounding porewater. We tested whether nitrate uptake by the MPB near the sediment surface decreases its availability to denitrifiers along deeper porewater flow paths. In laboratory experiments, we used NO x (nitrate + nitrite) microbiosensors to confirm that, in the spring, net NO x consumption in the zone of MPB photosynthetic activity was stimulated by light. The maximum potential denitrification rate, measured at high spatial resolution using microsensors with acetylene and nitrate added, occurred below 1.4 cm, much deeper than light-induced NO x uptake (0.13 cm). Therefore, the shallower MPB had the potential to decrease NO x supply to the deeper sediments and limit denitrification. However, when applying a realistic downward advective flow to sediment from our study site, NO x always reached the depths of maximum denitrification potential, regardless of light availability or season. We conclude that during tidal inundation porewater advection overwhelms any influence of shallow NO x uptake by the MPB and drives water column NO x to the depths of maximum denitrification potential.

Small sinking particles control anammox rates in the Peruvian oxygen minimum zone.

Anaerobic oxidation of ammonium (anammox) in oxygen minimum zones (OMZs) is a major pathway of oceanic nitrogen loss. Ammonium released from sinking particles has been suggested to fuel this process. During cruises to the Peruvian OMZ in April-June 2017 we found that anammox rates are strongly correlated with the volume of small particles (128-512 µm), even though anammox bacteria were not directly associated with particles. This suggests that the relationship between anammox rates and particles is related to the ammonium released from particles by remineralization. To investigate this, ammonium release from particles was modelled and theoretical encounters of free-living anammox bacteria with ammonium in the particle boundary layer were calculated. These results indicated that small sinking particles could be responsible for ~75% of ammonium release in anoxic waters and that free-living anammox bacteria frequently encounter ammonium in the vicinity of smaller particles. This indicates a so far underestimated role of abundant, slow-sinking small particles in controlling oceanic nutrient budgets, and furthermore implies that observations of the volume of small particles could be used to estimate N-loss across large areas.

Nitrate respiration and diel migration patterns of diatoms are linked in sediments underneath a microbial mat.

Diatoms are among the few eukaryotes known to store nitrate (NO3 - ) and to use it as an electron acceptor for respiration in the absence of light and O2 . Using microscopy and 15 N stable isotope incubations, we studied the relationship between dissimilatory nitrate/nitrite reduction to ammonium (DNRA) and diel vertical migration of diatoms in phototrophic microbial mats and the underlying sediment of a sinkhole in Lake Huron (USA). We found that the diatoms rapidly accumulated NO3 - at the mat-water interface in the afternoon and 40% of the population migrated deep into the sediment, where they were exposed to dark and anoxic conditions for ~75% of the day. The vertical distribution of DNRA rates and diatom abundance maxima coincided, suggesting that DNRA was the main energy generating metabolism of the diatom population. We conclude that the illuminated redox-dynamic ecosystem selects for migratory diatoms that can store nitrate for respiration in the absence of light. A major implication of this study is that the dominance of DNRA over denitrification is not explained by kinetics or thermodynamics. Rather, the dynamic conditions select for migratory diatoms that perform DNRA and can outcompete sessile denitrifiers.

Rapid microbial diversification of dissolved organic matter in oceanic surface waters leads to carbon sequestration.

The pool of dissolved organic matter (DOM) in the deep ocean represents one of the largest carbon sinks on the planet. In recent years, studies have shown that most of this pool is recalcitrant, because individual compounds are present at low concentrations and because certain compounds seem resistant to microbial degradation. The formation of the diverse and recalcitrant deep ocean DOM pool has been attributed to repeated and successive processing of DOM by microorganisms over time scales of weeks to years. Little is known however, about the transformation and cycling that labile DOM undergoes in the first hours upon its release from phytoplankton. Here we provide direct experimental evidence showing that within hours of labile DOM release, its breakdown and recombination with ambient DOM leads to the formation of a diverse array of new molecules in oligotrophic North Atlantic surface waters. Furthermore, our results reveal a preferential breakdown of N and P containing molecules versus those containing only carbon. Hence, we show the preferential breakdown and molecular diversification are the crucial first steps in the eventual formation of carbon rich DOM that is resistant to microbial remineralization.

Versatile cyanobacteria control the timing and extent of sulfide production in a Proterozoic analog microbial mat.

Cyanobacterial mats were hotspots of biogeochemical cycling during the Precambrian. However, mechanisms that controlled O2 release by these ecosystems are poorly understood. In an analog to Proterozoic coastal ecosystems, the Frasassi sulfidic springs mats, we studied the regulation of oxygenic and sulfide-driven anoxygenic photosynthesis (OP and AP) in versatile cyanobacteria, and interactions with sulfur reducing bacteria (SRB). Using microsensors and stable isotope probing we found that dissolved organic carbon (DOC) released by OP fuels sulfide production, likely by a specialized SRB population. Increased sulfide fluxes were only stimulated after the cyanobacteria switched from AP to OP. O2 production triggered migration of large sulfur-oxidizing bacteria from the surface to underneath the cyanobacterial layer. The resultant sulfide shield tempered AP and allowed OP to occur for a longer duration over a diel cycle. The lack of cyanobacterial DOC supply to SRB during AP therefore maximized O2 export. This mechanism is unique to benthic ecosystems because transitions between metabolisms occur on the same time scale as solute transport to functionally distinct layers, with the rearrangement of the system by migration of microorganisms exaggerating the effect. Overall, cyanobacterial versatility disrupts the synergistic relationship between sulfide production and AP, and thus enhances diel O2 production.

Author Correction: Metabolic activity analyses demonstrate that Lokiarchaeon exhibits homoacetogenesis in sulfidic marine sediments.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Anaerobic ammonium oxidation is a major N-sink in aquifer systems around the world.

Global-scale N-oxide contamination of groundwater within aquifers occurs due to the widespread use of N-bearing fertilizers and chemicals, threatening both human and environmental health. However, the conversion of these pollutants in active nitrogen (N) cycling processes in the subsurface biosphere still remains unclear. This study investigates the global occurrence of anaerobic ammonium oxidation (anammox) in aquifers, where anammox was found to be turned on and off between saturated and unsaturated soil horizons, and contributed 36.8-79.5% to N loss in saturated soil horizons, the remainder being due to denitrification which has traditionally been considered the main pathway for removal of N-pollutants from aquifers. Although anammox activity was undetectable in the unsaturated soil horizons, it could potentially be activated by contact with ascending groundwater. High-throughput pyrosequencing analysis identified Candidatus Brocadia anammoxidans as being the most abundant anammox bacterium in the saturated soils investigated. However, the anammox bacterial abundance was determined by the relative richness of Candidatus Jettenia asiatica. Isotopic pairing experiments revealed that coupling anammox with ammonium oxidation and respiratory ammonification enabled the formation of a revised N cycle in aquifer systems, in which respiratory ammonification acted as an important coordinator. Anammox can therefore contribute substantially to aquifer N cycling and its role in remediation of aquifers contaminated with N-oxides may be of global importance.

Metabolic activity analyses demonstrate that Lokiarchaeon exhibits homoacetogenesis in sulfidic marine sediments.

The genomes of the Asgard superphylum of Archaea hold clues pertaining to the nature of the host cell that acquired the mitochondrion at the origin of eukaryotes1-4. Representatives of the Asgard candidate phylum Candidatus Lokiarchaeota (Lokiarchaeon) have the capacity for acetogenesis and fermentation5-7, but how their metabolic activity responds to environmental conditions is poorly understood. Here, we show that in anoxic Namibian shelf sediments, Lokiarchaeon gene expression levels are higher than those of bacterial phyla and increase with depth below the seafloor. Lokiarchaeon gene expression was significantly different across a hypoxic-sulfidic redox gradient, whereby genes involved in growth, fermentation and H2-dependent carbon fixation had the highest expression under the most reducing (sulfidic) conditions. Quantitative stable isotope probing revealed that anaerobic utilization of CO2 and diatomaceous extracellular polymeric substances by Lokiarchaeon was higher than the bacterial average, consistent with higher expression of Lokiarchaeon genes, including those involved in transport and fermentation of sugars and amino acids. The quantitative stable isotope probing and gene expression data demonstrate homoacetogenic activity of Candidatus Lokiarchaeota, whereby fermentative H2 production from organic substrates is coupled with the Wood-Ljungdahl carbon fixation pathway8. The high energetic efficiency provided by homoacetogenesis8 helps to explain the elevated metabolic activity of Lokiarchaeon in this anoxic, energy-limited setting.

Arcobacter peruensis sp. nov., a Chemolithoheterotroph Isolated from Sulfide- and Organic-Rich Coastal Waters off Peru.

Members of the epsilonproteobacterial genus Arcobacter have been identified to be potentially important sulfide oxidizers in marine coastal, seep, and stratified basin environments. In the highly productive upwelling waters off the coast of Peru, Arcobacter cells comprised 3 to 25% of the total microbial community at a near-shore station where sulfide concentrations exceeded 20 µM in bottom waters. From the chemocline where the Arcobacter population exceeded 106 cells ml-1 and where high rates of denitrification (up to 6.5 ± 0.4 µM N day-1) and dark carbon fixation (2.8 ± 0.2 µM C day-1) were measured, we isolated a previously uncultivated Arcobacter species, Arcobacter peruensis sp. nov. (BCCM LMG-31510). Genomic analysis showed that A. peruensis possesses genes encoding sulfide oxidation and denitrification pathways but lacks the ability to fix CO2 via autotrophic carbon fixation pathways. Genes encoding transporters for organic carbon compounds, however, were present in the A. peruensis genome. Physiological experiments demonstrated that A. peruensis grew best on a mix of sulfide, nitrate, and acetate. Isotope labeling experiments further verified that A. peruensis completely reduced nitrate to N2 and assimilated acetate but did not fix CO2, thus coupling heterotrophic growth to sulfide oxidation and denitrification. Single-cell nanoscale secondary ion mass spectrometry analysis of samples taken from shipboard isotope labeling experiments also confirmed that the Arcobacter population in situ did not substantially fix CO2 The efficient growth yield associated with the chemolithoheterotrophic metabolism of A. peruensis may allow this Arcobacter species to rapidly bloom in eutrophic and sulfide-rich waters off the coast of Peru.IMPORTANCE Our multidisciplinary approach provides new insights into the ecophysiology of a newly isolated environmental Arcobacter species, as well as the physiological flexibility within the Arcobacter genus and sulfide-oxidizing, denitrifying microbial communities within oceanic oxygen minimum zones (OMZs). The chemolithoheterotrophic species Arcobacter peruensis may play a substantial role in the diverse consortium of bacteria that is capable of coupling denitrification and fixed nitrogen loss to sulfide oxidation in eutrophic, sulfidic coastal waters. With increasing anthropogenic pressures on coastal regions, e.g., eutrophication and deoxygenation (D. Breitburg, L. A. Levin, A. Oschlies, M. Grégoire, et al., Science 359:eaam7240, 2018, https://doi.org/10.1126/science.aam7240), niches where sulfide-oxidizing, denitrifying heterotrophs such as A. peruensis thrive are likely to expand.

Turbulence simultaneously stimulates small- and large-scale CO2 sequestration by chain-forming diatoms in the sea.

Chain-forming diatoms are key CO2-fixing organisms in the ocean. Under turbulent conditions they form fast-sinking aggregates that are exported from the upper sunlit ocean to the ocean interior. A decade-old paradigm states that primary production in chain-forming diatoms is stimulated by turbulence. Yet, direct measurements of cell-specific primary production in individual field populations of chain-forming diatoms are poorly documented. Here we measured cell-specific carbon, nitrate and ammonium assimilation in two field populations of chain-forming diatoms (Skeletonema and Chaetoceros) at low-nutrient concentrations under still conditions and turbulent shear using secondary ion mass spectrometry combined with stable isotopic tracers and compared our data with those predicted by mass transfer theory. Turbulent shear significantly increases cell-specific C assimilation compared to still conditions in the cells/chains that also form fast-sinking, aggregates rich in carbon and ammonium. Thus, turbulence simultaneously stimulates small-scale biological CO2 assimilation and large-scale biogeochemical C and N cycles in the ocean.

H2S events in the Peruvian oxygen minimum zone facilitate enhanced dissolved Fe concentrations.

Dissolved iron (DFe) concentrations in oxygen minimum zones (OMZs) of Eastern Boundary Upwelling Systems are enhanced as a result of high supply rates from anoxic sediments. However, pronounced variations in DFe concentrations in anoxic coastal waters of the Peruvian OMZ indicate that there are factors in addition to dissolved oxygen concentrations (O2) that control Fe cycling. Our study demonstrates that sediment-derived reduced Fe (Fe(II)) forms the main DFe fraction in the anoxic/euxinic water column off Peru, which is responsible for DFe accumulations of up to 200 nmol L-1. Lowest DFe values were observed in anoxic shelf waters in the presence of nitrate and nitrite. This reflects oxidation of sediment-sourced Fe(II) associated with nitrate/nitrite reduction and subsequent removal as particulate Fe(III) oxyhydroxides. Unexpectedly, the highest DFe levels were observed in waters with elevated concentrations of hydrogen sulfide (up to 4 µmol L-1) and correspondingly depleted nitrate/nitrite concentrations (<0.18 µmol L-1). Under these conditions, Fe removal was reduced through stabilization of Fe(II) as aqueous iron sulfide (FeSaqu) which comprises complexes (e.g., FeSH+) and clusters (e.g., Fe2S2|4H2O). Sulfidic events on the Peruvian shelf consequently enhance Fe availability, and may increase in frequency in future due to projected expansion and intensification of OMZs.

Metabolic specialization of denitrifiers in permeable sediments controls N2 O emissions.

Coastal oceans receive large amounts of anthropogenic fixed nitrogen (N), most of which is denitrified in the sediment before reaching the open ocean. Sandy sediments, which are common in coastal regions, seem to play an important role in catalysing this N-loss. Permeable sediments are characterized by advective porewater transport, which supplies high fluxes of organic matter into the sediment, but also leads to fluctuations in oxygen and nitrate concentrations. Little is known about how the denitrifying communities in these sediments are adapted to such fluctuations. Our combined results indicate that denitrification in eutrophied sandy sediments from the world's largest tidal flat system, the Wadden Sea, is carried out by different groups of microorganisms. This segregation leads to the formation of N2 O which is advectively transported to the overlying waters and thereby emitted to the atmosphere. At the same time, the production of N2 O within the sediment supports a subset of Flavobacteriia which appear to be specialized on N2 O reduction. If the mechanisms shown here are active in other coastal zones, then denitrification in eutrophied sandy sediments may substantially contribute to current marine N2 O emissions.

Filamentous Giant Beggiatoaceae from the Guaymas Basin Are Capable of both Denitrification and Dissimilatory Nitrate Reduction to Ammonium.

Filamentous large sulfur-oxidizing bacteria (FLSB) of the family Beggiatoaceae are globally distributed aquatic bacteria that can control geochemical fluxes from the sediment to the water column through their metabolic activity. FLSB mats from hydrothermal sediments of Guaymas Basin, Mexico, typically have a "fried-egg" appearance, with orange filaments dominating near the center and wider white filaments at the periphery, likely reflecting areas of higher and lower sulfide fluxes, respectively. These FLSB store large quantities of intracellular nitrate that they use to oxidize sulfide. By applying a combination of 15N-labeling techniques and genome sequence analysis, we demonstrate that the white FLSB filaments were capable of reducing their intracellular nitrate stores to both nitrogen gas and ammonium by denitrification and dissimilatory nitrate reduction to ammonium (DNRA), respectively. On the other hand, our combined results show that the orange filaments were primarily capable of DNRA. Microsensor profiles through a laboratory-incubated white FLSB mat revealed a 2- to 3-mm vertical separation between the oxic and sulfidic zones. Denitrification was most intense just below the oxic zone, as shown by the production of nitrous oxide following exposure to acetylene, which blocks nitrous oxide reduction to nitrogen gas. Below this zone, a local pH maximum coincided with sulfide oxidation, consistent with nitrate reduction by DNRA. The balance between internally and externally available electron acceptors (nitrate) and electron donors (reduced sulfur) likely controlled the end product of nitrate reduction both between orange and white FLSB mats and between different spatial and geochemical niches within the white FLSB mat.IMPORTANCE Whether large sulfur bacteria of the family Beggiatoaceae reduce NO3- to N2 via denitrification or to NH4+ via DNRA has been debated in the literature for more than 25 years. We resolve this debate by showing that certain members of the Beggiatoaceae use both metabolic pathways. This is important for the ecological role of these bacteria, as N2 production removes bioavailable nitrogen from the ecosystem, whereas NH4+ production retains it. For this reason, the topic of environmental controls on the competition for NO3- between N2-producing and NH4+-producing bacteria is of great scientific interest. Recent experiments on the competition between these two types of microorganisms have demonstrated that the balance between electron donor and electron acceptor availability strongly influences the end product of NO3- reduction. Our results suggest that this is also the case at the even more fundamental level of enzyme system regulation within a single organism.

Oxygen minimum zone cryptic sulfur cycling sustained by offshore transport of key sulfur oxidizing bacteria.

Members of the gammaproteobacterial clade SUP05 couple water column sulfide oxidation to nitrate reduction in sulfidic oxygen minimum zones (OMZs). Their abundance in offshore OMZ waters devoid of detectable sulfide has led to the suggestion that local sulfate reduction fuels SUP05-mediated sulfide oxidation in a so-called "cryptic sulfur cycle". We examined the distribution and metabolic capacity of SUP05 in Peru Upwelling waters, using a combination of oceanographic, molecular, biogeochemical and single-cell techniques. A single SUP05 species, U Thioglobus perditus, was found to be abundant and active in both sulfidic shelf and sulfide-free offshore OMZ waters. Our combined data indicated that mesoscale eddy-driven transport led to the dispersal of U T. perditus and elemental sulfur from the sulfidic shelf waters into the offshore OMZ region. This offshore transport of shelf waters provides an alternative explanation for the abundance and activity of sulfide-oxidizing denitrifying bacteria in sulfide-poor offshore OMZ waters.

Methane stimulates massive nitrogen loss from freshwater reservoirs in India.

The fate of the enormous amount of reactive nitrogen released to the environment by human activities in India is unknown. Here we show occurrence of seasonal stratification and generally low concentrations of dissolved inorganic combined nitrogen, and high molecular nitrogen (N2) to argon ratio, thus suggesting seasonal loss to N2 in anoxic hypolimnia of several dam-reservoirs. However, 15N-experiments yielded low rates of denitrification, anaerobic ammonium oxidation and dissimilatory nitrate reduction to ammonium-except in the presence of methane (CH4) that caused ~12-fold increase in denitrification. While nitrite-dependent anaerobic methanotrophs belonging to the NC10 phylum were present, previously considered aerobic methanotrophs were far more abundant (up to 13.9%) in anoxic hypolimnion. Methane accumulation in anoxic freshwater systems seems to facilitate rapid loss of reactive nitrogen, with generally low production of nitrous oxide (N2O), through widespread coupling between methanotrophy and denitrification, potentially mitigating eutrophication and emissions of CH4 and N2O to the atmosphere.

Metabolic versatility of a novel N2 -fixing Alphaproteobacterium isolated from a marine oxygen minimum zone.

The N2 -fixing (diazotrophic) community in marine ecosystems is dominated by non-cyanobacterial microorganisms. Yet, very little is known about their identity, function and ecological relevance due to a lack of cultured representatives. Here we report a novel heterotrophic diazotroph isolated from the oxygen minimum zone (OMZ) off Peru. The new species belongs to the genus Sagittula (Rhodobacteraceae, Alphaproteobacteria) and its capability to fix N2 was confirmed in laboratory experiments. Genome sequencing revealed that it is a strict heterotroph with a high versatility in substrate utilization and energy acquisition mechanisms. Pathways for sulfide oxidation and nitrite reduction to nitrous oxide are encoded in the genome and might explain the presence throughout the Peruvian OMZ. The genome further indicates that this novel organism could be in direct interaction with other microbes or particles. NanoSIMS analyses were used to compare the metabolic potential of S. castanea with single-cell activity in situ; however, N2 fixation by this diazotroph could not be detected at the isolation site. While the biogeochemical impact of S. castanea is yet to be resolved, its abundance and widespread distribution suggests that its potential to contribute to the marine N input could be significant at a larger geographical scale.