Spatial and temporal distribution of nitrite-dependent anaerobic methane-oxidizing bacteria in an intertidal zone of the East China Sea

Nitrite-dependent anaerobic methane oxidation (N-DAMO), which couples anaerobic methane oxidation and nitrite reduction, is a recently discovered bioprocess coupling microbial nitrogen and carbon cycles. The discovery of this microbial process challenges the traditional knowledge of global methane sinks and nitrogen losses. In this study, the abundance and activity of N-DAMO bacteria were investigated and their contributions to methane sink and nitrogen loss were estimated in different seasons and different partitions of an intertidal zone of the East China Sea. The results showed that N-DAMO bacteria were extensively and continuously present in the intertidal zone, with the number of cells ranging from 5.5 × 10⁴ to 2.8 × 10⁵ copy g⁻¹ soil and the potential activity ranging from 0.52 to 5.7 nmol CO₂ g⁻¹ soil day⁻¹, contributing 5.0–36.6% of nitrite- and sulfate-dependent anaerobic methane oxidation in the intertidal zone. The N-DAMO activity and its contribution to the methane consumption were highest in the spring and in the low intertidal zone. These findings showed that the N-DAMO process is an important methane and nitrogen sink in the intertidal zone and varies with the seasons and the partitions of the intertidal zone.

     

Carbon pump dynamics and limited organic carbon burial during OAE1a

Oceanic Anoxic Events (OAEs) are conspicuous intervals in the geologic record that are associated with the deposition of organic carbon (OC)-rich marine sediment, linked to extreme biogeochemical perturbations, and characterized by widespread ocean deoxygenation. Mechanistic links between the marine biological carbon pump (BCP), redox conditions, and organic carbon burial during OAEs, however, remain poorly constrained. In this work we reconstructed the BCP in the western Tethys Ocean across OAE1a (~120 Mya) using sediment geochemistry and OC mass accumulation rates (OC_Acc). We find that OC_Acc were between 0.006 and 3.3 gC m⁻² yr⁻¹, with a mean value of 0.79 ± 0.78 SD gC m⁻² yr⁻¹—these rates are low and comparable to oligotrophic regions in the modern oceans. This challenges longstanding assumptions that oceanic anoxic events are intervals of strongly elevated organic carbon burial. Numerical modelling of the BCP, furthermore, reveals that such low OC fluxes are only possible with either or both low to moderate OC export fluxes from ocean surface waters, with rates similar to oligotrophic (nutrient-poor, <30 gC m⁻² yr⁻¹) and mesotrophic (moderate-nutrients, ~50–100 gC m⁻² yr⁻¹) regions in the modern ocean, and stronger than modern vertical OC attenuation. The low OC fluxes thus reflect a relatively weak BCP. Low to moderate productivity is further supported by palaeoecological and geochemical evidence and was likely maintained through nutrient limitation that developed in response to the burial and sequestration of phosphorus in association with iron minerals under ferruginous (anoxic iron-rich) ocean conditions. Without persistently high productivity, ocean deoxygenation during OAE1a was more likely driven by other physicochemical and biological factors including ocean warming, changes in marine primary producer community composition, and fundamental shifts in the efficiency of the BCP with associated effects and feedbacks.     

Amsterdam urban canals contain novel niches for methane-cycling microorganisms

Urbanised environments have been identified as hotspots of anthropogenic methane emissions. Especially urban aquatic ecosystems are increasingly recognised as important sources of methane. However, the microbiology behind these emissions remains unexplored. Here, we applied microcosm incubations and molecular analyses to investigate the methane-cycling community of the Amsterdam canal system in the Netherlands. The sediment methanogenic communities were dominated by Methanoregulaceae and Methanosaetaceae, with co-occurring methanotrophic Methanoperedenaceae and Methylomirabilaceae indicating the potential for anaerobic methane oxidation. Methane was readily produced after substrate amendment, suggesting an active but substrate-limited methanogenic community. Bacterial 16S rRNA gene amplicon sequencing of the sediment revealed a high relative abundance of Thermodesulfovibrionia. Canal wall biofilms showed the highest initial methanotrophic potential under oxic conditions compared to the sediment. During prolonged incubations the maximum methanotrophic rate increased to 8.08 mmol g_DW⁻¹ d⁻¹ that was concomitant with an enrichment of Methylomonadaceae bacteria. Metagenomic analysis of the canal wall biofilm lead to the recovery of a single methanotroph metagenome-assembled genome. Taxonomic analysis showed that this methanotroph belongs to the genus Methyloglobulus. Our results underline the importance of previously unidentified and specialised environmental niches at the nexus of the natural and human-impacted carbon cycle.     

土壤中甲烷厌氧氧化菌多样性的分子检测

甲烷厌氧氧化作用是减少海洋底泥甲烷释放的重要生物地球化学过程,然而在陆地生态系统中甲烷厌氧氧化作用及其功能菌群的生态功能仍然不确定。对甲烷厌氧氧化菌多样性的研究可为减少甲烷排放提供重要科学依据。与传统的分离培养方法比较,分子检测方法是一种更为快速和高效的研究手段,可直接和全面的反映参与甲烷厌氧氧化作用的功能微生物。以DNA分子标记物为研究对象,重点探讨三类主要的分子标记基因,即16S rRNA,mcr A和pmo A,所采用的相关探针和引物信息,同时从定性和定量两个角度综述土壤甲烷厌氧氧化菌的多样性研究的主要进展,最后提出厌氧甲烷氧化菌多样性研究中存在的一些问题和相应的解决思路。     

Methane oxidation coupled to oxygenic photosynthesis in anoxic waters

Freshwater lakes represent large methane sources that, in contrast to the Ocean, significantly contribute to non-anthropogenic methane emissions to the atmosphere. Particularly mixed lakes are major methane emitters, while permanently and seasonally stratified lakes with anoxic bottom waters are often characterized by strongly reduced methane emissions. The causes for this reduced methane flux from anoxic lake waters are not fully understood. Here we identified the microorganisms and processes responsible for the near complete consumption of methane in the anoxic waters of a permanently stratified lake, Lago di Cadagno. Interestingly, known anaerobic methanotrophs could not be detected in these waters. Instead, we found abundant gamma-proteobacterial aerobic methane-oxidizing bacteria active in the anoxic waters. In vitro incubations revealed that, among all the tested potential electron acceptors, only the addition of oxygen enhanced the rates of methane oxidation. An equally pronounced stimulation was also observed when the anoxic water samples were incubated in the light. Our combined results from molecular, biogeochemical and single-cell analyses indicate that methane removal at the anoxic chemocline of Lago di Cadagno is due to true aerobic oxidation of methane fuelled by in situ oxygen production by photosynthetic algae. A similar mechanism could be active in seasonally stratified lakes and marine basins such as the Black Sea, where light penetrates to the anoxic chemocline. Given the widespread occurrence of seasonally stratified anoxic lakes, aerobic methane oxidation coupled to oxygenic photosynthesis might have an important but so far neglected role in methane emissions from lakes.     

Contemporary carbon balance and late Holocene carbon accumulation in a northern peatland

Northern peatlands contain up to 25% of the world's soil carbon (C) and have an estimated annual exchange of CO₂‐C with the atmosphere of 0.1–0.5 Pg yr⁻¹ and of CH₄‐C of 10–25 Tg yr⁻¹. Despite this overall importance to the global C cycle, there have been few, if any, complete multiyear annual C balances for these ecosystems. We report a 6‐year balance computed from continuous net ecosystem CO₂ exchange (NEE), regular instantaneous measurements of methane (CH₄) emissions, and export of dissolved organic C (DOC) from a northern ombrotrophic bog. From these observations, we have constructed complete seasonal and annual C balances, examined their seasonal and interannual variability, and compared the mean 6‐year contemporary C exchange with the apparent C accumulation for the last 3000 years obtained from C density and age‐depth profiles from two peat cores. The 6‐year mean NEE‐C and CH₄‐C exchange, and net DOC loss are −40.2±40.5 (±1 SD), 3.7±0.5, and 14.9±3.1 g m⁻² yr⁻¹, giving a 6‐year mean balance of −21.5±39.0 g m⁻² yr⁻¹ (where positive exchange is a loss of C from the ecosystem). NEE had the largest magnitude and variability of the components of the C balance, but DOC and CH₄ had similar proportional variabilities and their inclusion is essential to resolve the C balance. There are large interseasonal and interannual ranges to the exchanges due to variations in climatic conditions. We estimate from the largest and smallest seasonal exchanges, quasi‐maximum limits of the annual C balance between 50 and −105 g m⁻² yr⁻¹. The net C accumulation rate obtained from the two peatland cores for the interval 400–3000 bp (samples from the anoxic layer only) were 21.9±2.8 and 14.0±37.6 g m⁻² yr⁻¹, which are not significantly different from the 6‐year mean contemporary exchange.     

Microbial processes of the carbon and sulfur cycles in an ice‐covered,iron‐rich meromictic lake Svetloe (Arkhangelsk region,Russia)

Biogeochemical, isotope geochemical and microbiological investigation of Lake Svetloe (White Sea basin), a meromictic freshwater was carried out in April 2014, when ice thickness was ～0.5 m, and the ice‐covered water column contained oxygen to 23 m depth. Below, the anoxic water column contained ferrous iron (up to 240 μμM), manganese (60 μM), sulfide (up to 2 μM) and dissolved methane (960 μM). The highest abundance of microbial cells revealed by epifluorescence microscopy was found in the chemocline (redox zone) at 23–24.5 m. Oxygenic photosynthesis exhibited two peaks: the major one (0.43 μmol C L^?1 day^?1) below the ice and the minor one in the chemocline zone, where cyanobacteria related to Synechococcus rubescens were detected. The maximum of anoxygenic photosynthesis (0.69 μmol C L^?1 day^?1) at the oxic/anoxic interface, for which green sulfur bacteria Chlorobium phaeoclathratiforme were probably responsible, exceeded the value for oxygenic photosynthesis. Bacterial sulfate reduction peaked (1.5 μmol S L^?1 day^?1) below the chemocline zone. The rates of methane oxidation were as high as 1.8 μmol CH₄ L^?1 day^?1 at the oxi/anoxic interface and much lower in the oxic zone. Small phycoerythrin‐containing Synechococcus‐related cyanobacteria were probably involved in accumulation of metal oxides in the redox zone.     

NOB suppression and adaptation strategies in the partial nitrification–Anammox process for a poultry manure anaerobic digester

Poultry manure contains high levels of ammonia, which result in a suboptimal bioconversion to methane in anaerobic digesters (AD). A simultaneous process of nitrification, Anammox and denitrification (SNAD) in a continuous granular bubble column reactor to treat the anaerobically digested poultry manure was implemented. Thus, two strategies to achieve high efficiencies were proposed in this study: (1) ammonia overload to suppress nitrite oxidizing bacteria (NOB) and (2) gradual adaptation of the partial nitrification–Anammox (PN–A) biomass to organic matter. During the NOB-suppression stage, microbial and physical biomass characterizations were performed and the NOB abundance decreased from 31.3% to 3.3%. During the adaptation stage, with a nitrogen loading rate of 0.34 g L⁻¹ d⁻¹, a hydraulic retention time of 1.24 d and an influent COD/N ratio of 2.63 ± 0.02, a maximum ammonia and total nitrogen removal of 100% and 91.68% were achieved, respectively. The relative abundances of the aerobic and the anaerobic ammonia-oxidizing bacteria were greater than 35% and 40% respectively, during the study. These strategies provided useful design tools for the efficient removal of nitrogen species in the presence of organic matter.     

Aerobic and anaerobic methanotrophs in the Black Sea water column

Inputs of CH(4) from sediments, including methane seeps on the continental margin and methane-rich mud volcanoes on the abyssal plain, make the Black Sea the world's largest surface water reservoir of dissolved methane and drive a high rate of aerobic and anaerobic oxidation of methane in the water column. Here we present the first combined organic geochemical and molecular ecology data on a water column profile of the western Black Sea. We show that aerobic methanotrophs type I are responsible for methane oxidation in the oxic water column and ANME-1- and ANME-2-related organisms for anaerobic methane oxidation. The occurrence of methanotrophs type I cells in the anoxic zone suggests that inactive cells settle to deeper waters. Molecular and biomarker results suggest that a clear distinction between the occurrence of ANME-1- and ANME-2-related lineages exists, i.e. ANME-1-related organisms are responsible for anaerobic methane oxidation below 600 m water depth, whereas ANME-2-related organisms are responsible for this process in the anoxic water column above approximately 600 m water depth.     

No general soil carbon sequestration under Central European short rotation coppices

Wood from short rotation coppices (SRCs) is discussed as bioenergy feedstock with good climate mitigation potential inter alia because soil organic carbon (SOC) might be sequestered by a land-use change (LUC) from cropland to SRC. To test if SOC is generally enhanced by SRC over the long term, we selected the oldest Central European SRC plantations for this study. Following the paired plot approach soils of the 21 SRCs were sampled to 80 cm depth and SOC stocks, C/N ratios, pH and bulk densities were compared to those of adjacent croplands or grasslands. There was no general trend to SOC stock change by SRC establishment on cropland or grassland, but differences were very site specific. The depth distribution of SOC did change. Compared to cropland soils, the SOC density in 0–10 cm was significantly higher under SRC (17 ± 2 in cropland and 21 ± 2 kg C m⁻³ in SRC). Under SRC established on grassland SOC density in 0–10 cm was significantly lower than under grassland. The change rates of total SOC stocks by LUC from cropland to SRC ranged from −1.3 to 1.4 Mg C ha⁻¹ yr⁻¹ and −0.6 Mg C ha⁻¹ yr⁻¹ to +0.1 Mg C ha⁻¹ yr⁻¹ for LUC from grassland to SRC, respectively. The accumulation of organic carbon in the litter layer was low (0.14 ± 0.08 Mg C ha⁻¹ yr⁻¹). SOC stocks of both cropland and SRC soils were correlated with the clay content. No correlation could be detected between SOC stock change and soil texture or other abiotic factors. In summary, we found no evidence of any general SOC stock change when cropland is converted to SRC and the identification of the factors determining whether carbon may be sequestered under SRC remains a major challenge.     

Anaerobic oxidation of methane by sulfate in hypersaline groundwater of the Dead Sea aquifer

Geochemical and microbial evidence points to anaerobic oxidation of methane (AOM) likely coupled with bacterial sulfate reduction in the hypersaline groundwater of the Dead Sea (DS) alluvial aquifer. Groundwater was sampled from nine boreholes drilled along the Arugot alluvial fan next to the DS. The groundwater samples were highly saline (up to 6300 mm chlorine), anoxic, and contained methane. A mass balance calculation demonstrates that the very low δ¹³C_DIC in this groundwater is due to anaerobic methane oxidation. Sulfate depletion coincident with isotope enrichment of sulfur and oxygen isotopes in the sulfate suggests that sulfate reduction is associated with this AOM. DNA extraction and 16S amplicon sequencing were used to explore the microbial community present and were found to be microbial composition indicative of bacterial sulfate reducers associated with anaerobic methanotrophic archaea (ANME) driving AOM. The net sulfate reduction seems to be primarily controlled by the salinity and the available methane and is substantially lower as salinity increases (2.5 mm sulfate removal at 3000 mm chlorine but only 0.5 mm sulfate removal at 6300 mm chlorine). Low overall sulfur isotope fractionation observed (³⁴ε = 17 ± 3.5‰) hints at high rates of sulfate reduction, as has been previously suggested for sulfate reduction coupled with methane oxidation. The new results demonstrate the presence of sulfate‐driven AOM in terrestrial hypersaline systems and expand our understanding of how microbial life is sustained under the challenging conditions of an extremely hypersaline environment.     

Methane emissions from a full-scale A/A/O wastewater treatment plant

Methane (CH₄) emissions from a full-scale anaerobic/anoxic/oxic (A/A/O) wastewater treatment plant (WWTP) (Jinan, China) were investigated during spring and summer of 2010. Results showed that the major emission sources of CH₄ performed the following descending order: anaerobic tanks, oxic tanks, aerated grit chambers and sludge concentration tanks. The total annual fluxes of CH₄ emissions from the Jinan WWTP were 1.69 × 10⁴ kg yr⁻¹, with the emission factors of per capita emissions of 11.3 g CH₄ person⁻¹ yr⁻¹ and flow-based emissions of 1.55 × 10⁻⁴ g CH₄ (L of wastewater)⁻¹. The estimated source strength of methane for all WWTPs in China was 6.2 Gg yr⁻¹ (1 Gg = 10⁹ g). The most significant factors influencing methane emissions were dissolved oxygen concentration in aerated grit chamber and oxic tank and water temperature in high density settler tanks.     

Microbial niche differentiation explains nitrite oxidation in marine oxygen minimum zones

Nitrite is a pivotal component of the marine nitrogen cycle. The fate of nitrite determines the loss or retention of fixed nitrogen, an essential nutrient for all organisms. Loss occurs via anaerobic nitrite reduction to gases during denitrification and anammox, while retention occurs via nitrite oxidation to nitrate. Nitrite oxidation is usually represented in biogeochemical models by one kinetic parameter and one oxygen threshold, below which nitrite oxidation is set to zero. Here we find that the responses of nitrite oxidation to nitrite and oxygen concentrations vary along a redox gradient in a Pacific Ocean oxygen minimum zone, indicating niche differentiation of nitrite-oxidizing assemblages. Notably, we observe the full inhibition of nitrite oxidation by oxygen addition and nitrite oxidation coupled with nitrogen loss in the absence of oxygen consumption in samples collected from anoxic waters. Nitrite-oxidizing bacteria, including novel clades with high relative abundance in anoxic depths, were also detected in the same samples. Mechanisms corresponding to niche differentiation of nitrite-oxidizing bacteria across the redox gradient are considered. Implementing these mechanisms in biogeochemical models has a significant effect on the estimated fixed nitrogen budget.Subject terms: Biogeochemistry, Water microbiology, Microbial ecology     

Modeling the carbon cycle in Lake Matano

Lake Matano, Indonesia, is a stratified anoxic lake with iron‐rich waters that has been used as an analogue for the Archean and early Proterozoic oceans. Past studies of Lake Matano report large amounts of methane production, with as much as 80% of primary production degraded via methanogenesis. Low δ¹³C values of DIC in the lake are difficult to reconcile with this notion, as fractionation during methanogenesis produces isotopically heavy CO₂. To help reconcile these observations, we develop a box model of the carbon cycle in ferruginous Lake Matano, Indonesia, that satisfies the constraints of CH₄ and DIC isotopic profiles, sediment composition, and alkalinity. We estimate methane fluxes smaller than originally proposed, with about 9% of organic carbon export to the deep waters degraded via methanogenesis. In addition, despite the abundance of Fe within the waters, anoxic ferric iron respiration of organic matter degrades <3% of organic carbon export, leaving methanogenesis as the largest contributor to anaerobic organic matter remineralization, while indicating a relatively minor role for iron as an electron acceptor. As the majority of carbon exported is buried in the sediments, we suggest that the role of methane in the Archean and early Proterozoic oceans is less significant than presumed in other studies.     

Nitrogen source effects on the denitrifying anaerobic methane oxidation culture and anaerobic ammonium oxidation bacteria enrichment process

The co-culture system of denitrifying anaerobic methane oxidation (DAMO) and anaerobic ammonium oxidation (Anammox) has a potential application in wastewater treatment plant. This study explored the effects of permutation and combination of nitrate, nitrite, and ammonium on the culture enrichment from freshwater sediments. The co-existence of NO₃ ⁻, NO₂ ⁻, and NH₄ ⁺ shortened the enrichment time from 75 to 30 days and achieved a total nitrogen removal rate of 106.5 mg/L/day on day 132. Even though ammonium addition led to Anammox bacteria increase and a higher nitrogen removal rate, DAMO bacteria still dominated in different reactors with the highest proportion of 64.7% and the maximum abundance was 3.07 ± 0.25 × 10⁸ copies/L (increased by five orders of magnitude) in the nitrite reactor. DAMO bacteria showed greater diversity in the nitrate reactor, and one was similar to M. oxyfera; DAMO bacteria in the nitrite reactor were relatively unified and similar to M. sinica. Interestingly, no DAMO archaea were found in the nitrate reactor. This study will improve the understanding of the impact of nitrogen source on DAMO and Anammox co-culture enrichment.

     

Contemporary carbon accumulation in a boreal oligotrophic minerogenic mire – a significant sink after accounting for all C-fluxes

Based on theories of mire development and responses to a changing climate, the current role of mires as a net carbon sink has been questioned. A rigorous evaluation of the current net C-exchange in mires requires measurements of all relevant fluxes. Estimates of annual total carbon budgets in mires are still very limited. Here, we present a full carbon budget over 2 years for a boreal minerogenic oligotrophic mire in northern Sweden (64°11′N, 19°33′E). Data on the following fluxes were collected: land–atmosphere CO₂ exchange (continuous Eddy covariance measurements) and CH₄ exchange (static chambers during the snow free period); TOC (total organic carbon) in precipitation; loss of TOC, dissolved inorganic carbon (DIC) and CH₄ through stream water runoff (continuous discharge measurements and regular C-concentration measurements). The mire constituted a net sink of 27±3.4 (±SD) g C m⁻² yr⁻¹ during 2004 and 20±3.4 g C m⁻² yr⁻¹ during 2005. This could be partitioned into an annual surface–atmosphere CO₂ net uptake of 55±1.9 g C m⁻² yr⁻¹ during 2004 and 48±1.6 g C m⁻² yr⁻¹ during 2005. The annual NEE was further separated into a net uptake season, with an uptake of 92 g C m⁻² yr⁻¹ during 2004 and 86 g C m⁻² yr⁻¹ during 2005, and a net loss season with a loss of 37 g C m⁻² yr⁻¹ during 2004 and 38 g C m⁻² yr⁻¹ during 2005. Of the annual net CO₂-C uptake, 37% and 31% was lost through runoff (with runoff TOC>DIC≫CH₄) and 16% and 29% through methane emission during 2004 and 2005, respectively. This mire is still a significant C-sink, with carbon accumulation rates comparable to the long-term Holocene C-accumulation, and higher than the C-accumulation during the late Holocene in the region.     

Biogeochemistry and microbial ecology of methane oxidation in anoxic environments: a review

Evidence supporting a key role for anaerobic methane oxidation in the global methane cycle is reviewed. Emphasis is on recent microbiological advances. The driving force for research on this process continues to be the fact that microbial communities intercept and consume methane from anoxic environments, methane that would otherwise enter the atmosphere. Anaerobic methane oxidation is biogeochemically important because methane is a potent greenhouse gas in the atmosphere and is abundant in anoxic environments. Geochemical evidence for this process has been observed in numerous marine sediments along the continental margins, in methane seeps and vents, around methane hydrate deposits, and in anoxic waters. The anaerobic oxidation of methane is performed by at least two phylogenetically distinct groups of archaea, the ANME-1 and ANME-2. These archaea are frequently observed as consortia with sulfate-reducing bacteria, and the metabolism of these consortia presumably involves a syntrophic association based on interspecies electron transfer. The archaeal member of a consortium apparently oxidizes methane and shuttles reduced compounds to the sulfate-reducing bacteria. Despite recent advances in understanding anaerobic methane oxidation, uncertainties still remain regarding the nature and necessity of the syntrophic association, the biochemical pathway of methane oxidation, and the interaction of the process with the local chemical and physical environment. This review will consider the microbial ecology and biogeochemistry of anaerobic methane oxidation with a special emphasis on the interactions between the responsible organisms and their environment. This revised version was published online in August 2006 with corrections to the Cover Date.     

Methane oxidation in anoxic lake water stimulated by nitrate and sulfate addition

Methanotrophic bacteria play a key role in limiting methane emissions from lakes. It is generally assumed that methanotrophic bacteria are mostly active at the oxic-anoxic transition zone in stratified lakes, where they use oxygen to oxidize methane. Here, we describe a methanotroph of the genera Methylobacter that is performing high-rate (up to 72 μM day⁻¹) methane oxidation in the anoxic hypolimnion of the temperate Lacamas Lake (Washington, USA), stimulated by both nitrate and sulfate addition. Oxic and anoxic incubations both showed active methane oxidation by a Methylobacter species, with anoxic rates being threefold higher. In anoxic incubations, Methylobacter cell numbers increased almost two orders of magnitude within 3 days, suggesting that this specific Methylobacter species is a facultative anaerobe with a rapid response capability. Genomic analysis revealed adaptations to oxygen-limitation as well as pathways for mixed-acid fermentation and H₂ production. The denitrification pathway was incomplete, lacking the genes narG/napA and nosZ, allowing only for methane oxidation coupled to nitrite-reduction. Our data suggest that Methylobacter can be an important driver of the conversion of methane in oxygen-limited lake systems and potentially use alternative electron acceptors or fermentation to remain active under oxygen-depleted conditions.     

Facets of diazotrophy in the oxygen minimum zone waters off Peru

Nitrogen fixation, the biological reduction of dinitrogen gas (N₂) to ammonium (NH₄⁺), is quantitatively the most important external source of new nitrogen (N) to the open ocean. Classically, the ecological niche of oceanic N₂ fixers (diazotrophs) is ascribed to tropical oligotrophic surface waters, often depleted in fixed N, with a diazotrophic community dominated by cyanobacteria. Although this applies for large areas of the ocean, biogeochemical models and phylogenetic studies suggest that the oceanic diazotrophic niche may be much broader than previously considered, resulting in major implications for the global N-budget. Here, we report on the composition, distribution and abundance of nifH, the functional gene marker for N₂ fixation. Our results show the presence of eight clades of diazotrophs in the oxygen minimum zone (OMZ) off Peru. Although proteobacterial clades dominated overall, two clusters affiliated to spirochaeta and archaea were identified. N₂ fixation was detected within OMZ waters and was stimulated by the addition of organic carbon sources supporting the view that non-phototrophic diazotrophs were actively fixing dinitrogen. The observed co-occurrence of key functional genes for N₂ fixation, nitrification, anammox and denitrification suggests that a close spatial coupling of N-input and N-loss processes exists in the OMZ off Peru. The wide distribution of diazotrophs throughout the water column adds to the emerging view that the habitat of marine diazotrophs can be extended to low oxygen/high nitrate areas. Furthermore, our statistical analysis suggests that NO₂⁻ and PO₄³⁻ are the major factors affecting diazotrophic distribution throughout the OMZ. In view of the predicted increase in ocean deoxygenation resulting from global warming, our findings indicate that the importance of OMZs as niches for N₂ fixation may increase in the future.     

Cooccurrence of Aerobic and Anaerobic Methane Oxidation in the Water Column of Lake Plußsee

Dissolved methane was investigated in the water column of eutrophic Lake Plußsee and compared to temperature, oxygen, and sulfide profiles. Methane concentrations and δ-¹³C signatures indicated a zone of aerobic methane oxidation and additionally a zone of anaerobic methane oxidation in the anoxic water body. The latter coincided with a peak in hydrogen sulfide concentration. High cell numbers of aerobic and anaerobic methane-oxidizing microorganisms were detected by fluorescence in situ hybridization (FISH) or the more sensitive catalyst-amplified reporter deposition-FISH, respectively, in these layers.