Thermal effects on microbial composition and microbiologically induced corrosion and mineral precipitation affecting operation of a geothermal plant in a deep saline aquifer

The microbial diversity of a deep saline aquifer used for geothermal heat storage in the North German Basin was investigated. Genetic fingerprinting analyses revealed distinct microbial communities in fluids produced from the cold and warm side of the aquifer. Direct cell counting and quantification of 16S rRNA genes and dissimilatory sulfite reductase (dsrA) genes by real-time PCR proved different population sizes in fluids, showing higher abundance of bacteria and sulfate reducing bacteria (SRB) in cold fluids compared with warm fluids. The operation-dependent temperature increase at the warm well probably enhanced organic matter availability, favoring the growth of fermentative bacteria and SRB in the topside facility after the reduction of fluid temperature. In the cold well, SRB predominated and probably accounted for corrosion damage to the submersible well pump and iron sulfide precipitates in the near wellbore area and topside facility filters. This corresponded to lower sulfate content in fluids produced from the cold well as well as higher content of hydrogen gas that was probably released from corrosion, and maybe favored growth of hydrogenotrophic SRB. This study reflects the high influence of microbial populations for geothermal plant operation, because microbiologically induced precipitative and corrosive processes adversely affect plant reliability.     

The search for sulfate-reducing bacteria in mat samples from the lost city hydrothermal field by molecular cloning

The work is dedicated to searching for microorganisms of the domain Bacteria capable of dissimilatory sulfate reduction in the samples of microbial mats from a carbonate chimney in the Lost City hydro-thermal field. Cloning of 16S rRNA genes, the universal phylogenetic marker, and dsrAB, the functional marker for sulfate reduction, revealed phylotypes related to spore-forming Desulfotomaculum. No members of the Deltaproteobacteria, comprising the most numerous bacterial group with demonstrated capacity for dissimilatory sulfate reduction, were found. The phylogenetic position of 16S rRNA clones from the mats suggests that this microbial community is a unique consortium, where the energy flow is related to hydrogen of hydrothermal origin, while mass growth of primary produces results from utilization of sulfide formed by sulfate-and sulfur-reducing microorganisms.     

An inorganic geochemical argument for coupled anaerobic oxidation of methane and iron reduction in marine sediments

Here, we present results from sediments collected in the Argentine Basin, a non‐steady state depositional marine system characterized by abundant oxidized iron within methane‐rich layers due to sediment reworking followed by rapid deposition. Our comprehensive inorganic data set shows that iron reduction in these sulfate and sulfide‐depleted sediments is best explained by a microbially mediated process—implicating anaerobic oxidation of methane coupled to iron reduction (Fe‐AOM) as the most likely major mechanism. Although important in many modern marine environments, iron‐driven AOM may not consume similar amounts of methane compared with sulfate‐dependent AOM. Nevertheless, it may have broad impact on the deep biosphere and dominate both iron and methane cycling in sulfate‐lean marine settings. Fe‐AOM might have been particularly relevant in the Archean ocean, >2.5 billion years ago, known for its production and accumulation of iron oxides (in iron formations) in a biosphere likely replete with methane but low in sulfate. Methane at that time was a critical greenhouse gas capable of sustaining a habitable climate under relatively low solar luminosity, and relationships to iron cycling may have impacted if not dominated methane loss from the biosphere.     

Spatial scales of bacterial community diversity at cold seeps (Eastern Mediterranean Sea)

Cold seeps are highly productive, fragmented marine ecosystems that form at the seafloor around hydrocarbon emission pathways. The products of microbial utilization of methane and other hydrocarbons fuel rich chemosynthetic communities at these sites, with much higher respiration rates compared with the surrounding deep-sea floor. Yet little is known as to the richness, composition and spatial scaling of bacterial communities of cold seeps compared with non-seep communities. Here we assessed the bacterial diversity across nine different cold seeps in the Eastern Mediterranean deep-sea and surrounding seafloor areas. Community similarity analyses were carried out based on automated ribosomal intergenic spacer analysis (ARISA) fingerprinting and high-throughput 454 tag sequencing and were combined with in situ and ex situ geochemical analyses across spatial scales of a few tens of meters to hundreds of kilometers. Seep communities were dominated by Deltaproteobacteria, Epsilonproteobacteria and Gammaproteobacteria and shared, on average, 36% of bacterial types (ARISA OTUs (operational taxonomic units)) with communities from nearby non-seep deep-sea sediments. Bacterial communities of seeps were significantly different from those of non-seep sediments. Within cold seep regions on spatial scales of only tens to hundreds of meters, the bacterial communities differed considerably, sharing <50% of types at the ARISA OTU level. Their variations reflected differences in porewater sulfide concentrations from anaerobic degradation of hydrocarbons. This study shows that cold seep ecosystems contribute substantially to the microbial diversity of the deep-sea.     

Observations on the diagenetic behavior of arsenic in a deep coastal sediment

Vertical profiles of total dissolved arsenic, manganese and iron, pH, Eh and rates of sulfate reduction were determined in a freshly-collected box core from a 335m depth station in the Laurentian Trough. The relationships observed between the profiles were further examined in the laboratory by measuring these same parameters with time in surficial sediment slurries as the Eh decreased in response to biological activity or chemical alteration.Both field and laboratory observations have shown that arsenic is released predominantly as As(III) into reducing sediment porewaters. This occurs after the dissolution of manganese oxides and at the same time as the dissolution of iron oxyhydroxides and the onset of sulfate reduction. Laboratory experiments indicated that sulfate reduction and the production of sulfide ions are not solely responsible for the release of arsenic to the porewaters, although this process is necessary to create and maintain a highly reducing environment conducive to rapid iron dissolution.The diagenesis of arsenic in Laurentain Trough sediments involves the simultaneous release of arsenic and iron at a subsurface depth, followed by its removal from porewaters by precipitation and adsorption reactions after migration by diffusion along concentration gradients. A qualitative model is presented to describe the behavior of arsenic in coastal marine sediments.Present address: Department of Geological Sciences, McGill University, 3450 UniversityStreet, Montreal, Quebec H3A 2A7, Canada     

Effects of added manganic and ferric oxides on sulfate reduction and sulfide oxidation in intertidal sediments

Abstract Freshly precipitated iron or manganese oxides were added to surface sediments from a salt marsh and from the intertidal region of Lowes Cove, Maine. In the presence of added manganese, sulfate was formed under anoxic conditions, suggesting a manganese dependent sulfide oxidation. Sulfate formation was not observed with iron additions. Sulfate reduction was substantially but not completely inhibited by either metal oxide, even though both were added at levels well in excess though both were added at levels well in excess of natural concentrations. Manganese-catalysed sulfide oxidation was further documented using a combination of radiolabel, metal oxide, and inhibitor additions, Results from this study suggested that losses of radiolabelled sulfide could result in underestimates of gross sulfate reduction rates in the presence of significant manganic oxide concentrations. In addition, manganic oxides may facilitate the anaerobic regeneration of sulfate from sulfides.     

Seasonal oscillation of microbial iron and sulfate reduction in saltmarsh sediments (Sapelo Island, GA, USA)

Seasonal variations in anaerobic respiration pathways were investigated at three saltmarsh sites using chemical data, sulfate reduction rate measurements, enumerations of culturable populations of anaerobic iron-reducing bacteria (FeRB), and quantification of in situ 16S rRNA hybridization signals targeted for sulfate-reducing bacteria (SRB). Bacterial sulfate reduction in the sediments followed seasonal changes in temperature and primary production of the saltmarsh, with activity levels lowest in winter and highest in summer. In contrast, a dramatic decrease in the FeRB population size was observed during summer at all sites. The collapse of FeRB populations during summer was ascribed to high rates of sulfide production by SRB, resulting in abiotic reduction of bioavailable Fe(III) (hydr)oxides. To test this hypothesis, sediment slurry incubations at 10, 20 and 30 °C were carried out. Increases in temperature and labile organic carbon availability (acetate or lactate additions) increased rates of sulfate reduction while decreasing the abundance of culturable anaerobic FeRB. These trends were not reversed by the addition of amorphous Fe(III) (hydr)oxides to the slurries. However, when sulfate reduction was inhibited by molybdate, no decline in FeRB growth was observed with increasing temperature. Addition of dissolved sulfide adversely impacted propagation of FeRB whether molybdate was added or not. Both field and laboratory data therefore support a sulfide-mediated limitation of microbial iron respiration by SRB. When total sediment respiration rates reach their highest levels during summer, SRB force a decline in the FeRB populations. As sulfate reduction activity slows down after the summer, the FeRB are able to recover.     

Integrated metagenomic and metaproteomic analyses of an ANME-1-dominated community in marine cold seep sediments

Sulfate‐reducing methanotrophy by anaerobic methanotrophic archaea (ANME) and sulfate‐reducing bacteria (SRB) is a major biological sink of methane in anoxic methane‐enriched marine sediments. The physiology of a microbial community dominated by free‐living ANME‐1 at 14–16 cm below the seafloor in the G11 pockmark at Nyegga was investigated by integrated metagenomic and metaproteomic approaches. Total DNA was subjected to 454‐pyrosequencing (829 527 reads), and 16.6 Mbp of sequence information was assembled into 27352 contigs. Taxonomic analysis supported a high abundance of Euryarchaea (70%) with 66% of the assembled metagenome belonging to ANME‐1. Extracted sediment proteins were separated in two dimensions and subjected to mass spectrometry (LTQ‐Orbitrap XL). Of 356 identified proteins, 245 were expressed by ANME‐1. These included proteins for cold‐adaptation and production of gas vesicles, reflecting both the adaptation of the ANME‐1 community to a permanently cold environment and its potential for positioning in specific sediment depths respectively. In addition, key metabolic enzymes including the enzymes in the reverse methanogenesis pathway (except N⁵,N¹⁰‐methylene‐tetrahydromethanopterin reductase), heterodisulfide reductases and the F₄₂₀H₂:quinone oxidoreductase (Fqo) complex were identified. A complete dissimilatory sulfate reduction pathway was expressed by sulfate‐reducing Deltaproteobacteria. Interestingly, an APS‐reductase comprising Gram‐positive SRB and related sequences were identified in the proteome. Overall, the results demonstrated that our approach was effective in assessing in situ metabolic processes in cold seep sediments.     

Microbiological and Geochemical Heterogeneity in an In Situ Uranium Bioremediation Field Site

The geochemistry and microbiology of a uranium-contaminated subsurface environment that had undergone two seasons of acetate addition to stimulate microbial U(VI) reduction was examined. There were distinct horizontal and vertical geochemical gradients that could be attributed in large part to the manner in which acetate was distributed in the aquifer, with more reduction of Fe(III) and sulfate occurring at greater depths and closer to the point of acetate injection. Clone libraries of 16S rRNA genes derived from sediments and groundwater indicated an enrichment of sulfate-reducing bacteria in the order Desulfobacterales in sediment and groundwater samples. These samples were collected nearest the injection gallery where microbially reducible Fe(III) oxides were highly depleted, groundwater sulfate concentrations were low, and increases in acid volatile sulfide were observed in the sediment. Further down-gradient, metal-reducing conditions were present as indicated by intermediate Fe(II)/Fe(total) ratios, lower acid volatile sulfide values, and increased abundance of 16S rRNA gene sequences belonging to the dissimilatory Fe(III)- and U(VI)-reducing family Geobacteraceae. Maximal Fe(III) and U(VI) reduction correlated with maximal recovery of Geobacteraceae 16S rRNA gene sequences in both groundwater and sediment; however, the sites at which these maxima occurred were spatially separated within the aquifer. The substantial microbial and geochemical heterogeneity at this site demonstrates that attempts should be made to deliver acetate in a more uniform manner and that closely spaced sampling intervals, horizontally and vertically, in both sediment and groundwater are necessary in order to obtain a more in-depth understanding of microbial processes and the relative contribution of attached and planktonic populations to in situ uranium bioremediation.     

Biogeochemistry of an Iron-Rich Hypersaline Microbial Mat (Camargue, France)

In situ microsensor measurements were combined with biogeochemical methods to determine oxygen, sulfur, and carbon cycling in microbial mats growing in a solar saltern (Salin-de-Giraud, France). Sulfate reduction rates closely followed the daily temperature changes and were highest during the day at 25°C and lowest during the night at 11°C, most probably fueled by direct substrate interactions between cyanobacteria and sulfate-reducing bacteria. Sulfate reduction was the major mineralization process during the night and the contribution of aerobic respiration to nighttime DIC production decreased. This decrease of aerobic respiration led to an increasing contribution of sulfide (and iron) oxidation to nighttime O₂ consumption. A peak of elemental sulfur in a layer of high sulfate reduction at low sulfide concentration underneath the oxic zone indicated anoxygenic photosynthesis and/or sulfide oxidation by iron, which strongly contributed to sulfide consumption. We found a significant internal carbon cycling in the mat, and sulfate reduction directly supplied DIC for photosynthesis. The mats were characterized by a high iron content of 56 mol Fe cm^–3, and iron cycling strongly controlled the sulfur cycle in the mat. This included sulfide precipitation resulting in high FeS contents with depth, and reactions of iron oxides with sulfide, especially after sunset, leading to a pronounced gap between oxygen and sulfide gradients and an unusual persistence of a pH peak in the uppermost mat layer until midnight.     

Effects of Iron and Nitrogen Limitation on Sulfur Isotope Fractionation during Microbial Sulfate Reduction

Sulfate-reducing microbes utilize sulfate as an electron acceptor and produce sulfide that is depleted in heavy isotopes of sulfur relative to sulfate. Thus, the distribution of sulfur isotopes in sediments can trace microbial sulfate reduction (MSR), and it also has the potential to reflect the physiology of sulfate-reducing microbes. This study investigates the relationship between the availability of iron and reduced nitrogen and the magnitude of S-isotope fractionation during MSR by a marine sulfate-reducing bacterium, DMSS-1, a Desulfovibrio species, isolated from salt marsh in Cape Cod, MA. Submicromolar levels of iron increase sulfur isotope fractionation by about 50% relative to iron-replete cultures of DMSS-1. Iron-limited cultures also exhibit decreased cytochrome c-to-total protein ratios and cell-specific sulfate reduction rates (csSRR), implying changes in the electron transport chain that couples carbon and sulfur metabolisms. When DMSS-1 fixes nitrogen in ammonium-deficient medium, it also produces larger fractionation, but it occurs at faster csSRRs than in the ammonium-replete control cultures. The energy and reducing power required for nitrogen fixation may be responsible for the reverse trend between S-isotope fractionation and csSRR in this case. Iron deficiency and nitrogen fixation by sulfate-reducing microbes may lead to the large observed S-isotope effects in some euxinic basins and various anoxic sediments.     

Potential for biogeochemical cycling of sulfur,iron and carbon within massive sulfide deposits below the seafloor

Seafloor massive sulfides are a potential energy source for the support of chemosynthetic ecosystems in dark, deep‐sea environments; however, little is known about microbial communities in these ecosystems, especially below the seafloor. In the present study, we performed culture‐independent molecular analyses of sub‐seafloor sulfide samples collected in the Southern Mariana Trough by drilling. The depth for the samples ranged from 0.52 m to 2.67 m below the seafloor. A combination of 16S rRNA and functional gene analyses suggested the presence of chemoautotrophs, sulfur‐oxidizers, sulfate‐reducers, iron‐oxidizers and iron‐reducers. In addition, mineralogical and thermodynamic analyses are consistent with chemosynthetic microbial communities sustained by sulfide minerals below the seafloor. Although distinct bacterial community compositions were found among the sub‐seafloor sulfide samples and hydrothermally inactive sulfide chimneys on the seafloor collected from various areas, we also found common bacterial members at species level including the sulfur‐oxidizers and sulfate‐reducers, suggesting that the common members are widely distributed within massive sulfide deposits on and below the seafloor and play a key role in the ecosystem function.     

Hydrodynamic disturbance controls microbial community assembly and biogeochemical processes in coastal sediments

The microbial community composition and biogeochemical dynamics of coastal permeable (sand) sediments differs from cohesive (mud) sediments. Tide- and wave-driven hydrodynamic disturbance causes spatiotemporal variations in oxygen levels, which select for microbial generalists and disrupt redox cascades. In this work, we profiled microbial communities and biogeochemical dynamics in sediment profiles from three sites varying in their exposure to hydrodynamic disturbance. Strong variations in sediment geochemistry, biogeochemical activities, and microbial abundance, composition, and capabilities were observed between the sites. Most of these variations, except for microbial abundance and diversity, significantly correlated with the relative disturbance level of each sample. In line with previous findings, metabolically flexible habitat generalists (e.g., Flavobacteriaceae, Woeseaiceae, Rhodobacteraceae) dominated in all samples. However, we present evidence that aerobic specialists such as ammonia-oxidizing archaea (Nitrosopumilaceae) were more abundant and active in more disturbed samples, whereas bacteria capable of sulfate reduction (e.g., uncultured Desulfobacterales), dissimilatory nitrate reduction to ammonium (DNRA; e.g., Ignavibacteriaceae), and sulfide-dependent chemolithoautotrophy (e.g., Sulfurovaceae) were enriched and active in less disturbed samples. These findings are supported by insights from nine deeply sequenced metagenomes and 169 derived metagenome-assembled genomes. Altogether, these findings suggest that hydrodynamic disturbance is a critical factor controlling microbial community assembly and biogeochemical processes in coastal sediments. Moreover, they strengthen our understanding of the relationships between microbial composition and biogeochemical processes in these unique environments.Subject terms: Water microbiology, Biogeochemistry     

Comparison of the microbial diversity in cold-seep sediments from different depths in the Nankai Trough

We have investigated the molecular phylogeny of cold-seep sediments obtained from the Nankai Trough, at depths of about 600, 2,000, and 3,300 m, and compared the microbial diversity profiles of those sediments samples. The gamma-Proteobacteria that might function as sulfide oxidizers and the symbiotically related delta-Proteobacteria which might function as sulfate reducers were identified amongst the bacteria from all depths of the sediments. However, anoxic methane oxidizing archaea (ANME) and methanogens were only found in the 600 m deep sediments. These results indicated that the cold-seep microbial sulfur circulation system could be functioning in the shallow seep sediment at a depth of 600 m and the microbial activities at these sites might be more dynamic than at other deeper cold-seep sites.     

Bacterial Disproportionation of Elemental Sulfur Coupled to Chemical Reduction of Iron or Manganese

A new chemolithotrophic bacterial metabolism was discovered in anaerobic marine enrichment cultures. Cultures in defined medium with elemental sulfur (S⁰) and amorphous ferric hydroxide (FeOOH) as sole substrates showed intense formation of sulfate. Furthermore, precipitation of ferrous sulfide and pyrite was observed. The transformations were accompanied by growth of slightly curved, rod-shaped bacteria. The quantification of the products revealed that S⁰ was microbially disproportionated to sulfate and sulfide, as follows: 4S⁰ + 4H₂O → SO₄^2- + 3H₂S + 2H⁺. Subsequent chemical reactions between the formed sulfide and the added FeOOH led to the observed precipitation of iron sulfides. Sulfate and iron sulfides were also produced when FeOOH was replaced by FeCO₃. Further enrichment with manganese oxide, MnO₂, instead of FeOOH yielded stable cultures which formed sulfate during concomitant reduction of MnO₂ to Mn²⁺. Growth of small rod-shaped bacteria was observed. When incubated without MnO₂, the culture did not grow but produced small amounts of SO₄^2- and H₂S at a ratio of 1:3, indicating again a disproportionation of S⁰. The observed microbial disproportionation of S⁰ only proceeds significantly in the presence of sulfide-scavenging agents such as iron and manganese compounds. The population density of bacteria capable of S⁰ disproportionation in the presence of FeOOH or MnO₂ was high, > 10⁴ cm^-3 in coastal sediments. The metabolism offers an explanation for recent observations of anaerobic sulfide oxidation to sulfate in anoxic sediments.     

Dominance of sulfur-fueled iron oxide reduction in low-sulfate freshwater sediments

A central tenant in microbial biogeochemistry is that microbial metabolisms follow a predictable sequence of terminal electron acceptors based on the energetic yield for the reaction. It is thereby oftentimes assumed that microbial respiration of ferric iron outcompetes sulfate in all but high-sulfate systems, and thus sulfide has little influence on freshwater or terrestrial iron cycling. Observations of sulfate reduction in low-sulfate environments have been attributed to the presumed presence of highly crystalline iron oxides allowing sulfate reduction to be more energetically favored. Here we identified the iron-reducing processes under low-sulfate conditions within columns containing freshwater sediments amended with structurally diverse iron oxides and fermentation products that fuel anaerobic respiration. We show that despite low sulfate concentrations and regardless of iron oxide substrate (ferrihydrite, Al-ferrihydrite, goethite, hematite), sulfidization was a dominant pathway in iron reduction. This process was mediated by (re)cycling of sulfur upon reaction of sulfide and iron oxides to support continued sulfur-based respiration—a cryptic sulfur cycle involving generation and consumption of sulfur intermediates. Although canonical iron respiration was not observed in the sediments amended with the more crystalline iron oxides, iron respiration did become dominant in the presence of ferrihydrite once sulfate was consumed. Thus, despite more favorable energetics, ferrihydrite reduction did not precede sulfate reduction and instead an inverse redox zonation was observed. These findings indicate that sulfur (re)cycling is a dominant force in iron cycling even in low-sulfate systems and in a manner difficult to predict using the classical thermodynamic ladder.     

Three manganese oxide-rich marine sediments harbor similar communities of acetate-oxidizing manganese-reducing bacteria

Dissimilatory manganese reduction dominates anaerobic carbon oxidation in marine sediments with high manganese oxide concentrations, but the microorganisms responsible for this process are largely unknown. In this study, the acetate-utilizing manganese-reducing microbiota in geographically well-separated, manganese oxide-rich sediments from Gullmar Fjord (Sweden), Skagerrak (Norway) and Ulleung Basin (Korea) were analyzed by 16S rRNA-stable isotope probing (SIP). Manganese reduction was the prevailing terminal electron-accepting process in anoxic incubations of surface sediments, and even the addition of acetate stimulated neither iron nor sulfate reduction. The three geographically distinct sediments harbored surprisingly similar communities of acetate-utilizing manganese-reducing bacteria: 16S rRNA of members of the genera Colwellia and Arcobacter and of novel genera within the Oceanospirillaceae and Alteromonadales were detected in heavy RNA-SIP fractions from these three sediments. Most probable number (MPN) analysis yielded up to 10⁶ acetate-utilizing manganese-reducing cells cm⁻³ in Gullmar Fjord sediment. A 16S rRNA gene clone library that was established from the highest MPN dilutions was dominated by sequences of Colwellia and Arcobacter species and members of the Oceanospirillaceae, supporting the obtained RNA-SIP results. In conclusion, these findings strongly suggest that (i) acetate-dependent manganese reduction in manganese oxide-rich sediments is catalyzed by members of taxa (Arcobacter, Colwellia and Oceanospirillaceae) previously not known to possess this physiological function, (ii) similar acetate-utilizing manganese reducers thrive in geographically distinct regions and (iii) the identified manganese reducers differ greatly from the extensively explored iron reducers in marine sediments.     

Sulfhydrolase Activity in Sediments of Wintergreen Lake, Kalamazoo County, Michigan

The hydrolysis of p-nitrophenyl sulfate, p-nitrocatechol sulfate, and [³⁵S]sodium dodecyl sulfate was examined in anoxic sediments of Wintergreen Lake, Michigan. Significant levels of sulfhydrolase activity were observed in littoral, transition, and profundal sediment samples. Rates of sulfate formation suggest that the sulfhydrolase system would represent a major source of sulfate within these sediments. Sulfate formed by ester sulfate hydrolysis can support dissimilatory sulfate reduction as shown by the incorporation of ³⁵S from labeled sodium dodecyl sulfate into H₂³⁵S. Sulfhydrolase activity varied with sediment depth, was greatest in the littoral zone, and was sensitive to the presence of oxygen. Estimations of ester sulfate concentrations in sediments revealed large quantities of ester sulfate (~30% of total sulfur). Both total sulfur and ester sulfate concentrations varied with the sediment type and were two to three orders of magnitude greater than the inorganic sulfur concentration.     

Thermophilic anaerobes in Arctic marine sediments induced to mineralize complex organic matter at high temperature

Marine sediments harbour diverse populations of dormant thermophilic bacterial spores that become active in sediment incubation experiments at much higher than in situ temperature. This response was investigated in the presence of natural complex organic matter in sediments of two Arctic fjords, as well as with the addition of freeze‐dried Spirulina or individual high‐molecular‐weight polysaccharides. During 50°C incubation experiments, Arctic thermophiles catalysed extensive mineralization of the organic matter via extracellular enzymatic hydrolysis, fermentation and sulfate reduction. This high temperature‐induced food chain mirrors sediment microbial processes occurring at cold in situ temperatures (near 0°C), yet it is catalysed by a completely different set of microorganisms. Using sulfate reduction rates (SRR) as a proxy for organic matter mineralization showed that differences in organic matter reactivity determined the extent of the thermophilic response. Fjord sediments with higher in situ SRR also supported higher SRR at 50°C. Amendment with Spirulina significantly increased volatile fatty acids production and SRR relative to unamended sediment in 50°C incubations. Spirulina amendment also revealed temporally distinct sulfate reduction phases, consistent with 16S rRNA clone library detection of multiple thermophilic Desulfotomaculum spp. enriched at 50°C. Incubations with four different fluorescently labelled polysaccharides at 4°C and 50°C showed that the thermophilic population in Arctic sediments produce a different suite of polymer‐hydrolysing enzymes than those used in situ by the cold‐adapted microbial community. Over time, dormant marine microorganisms like these are buried in marine sediments and might eventually encounter warmer conditions that favour their activation. Distinct enzymatic capacities for organic polymer degradation could allow specific heterotrophic populations like these to play a role in sustaining microbial metabolism in the deep, warm, marine biosphere.     

Microbial Community Stratification Controlled by the Subseafloor Fluid Flow and Geothermal Gradient at the Iheya North Hydrothermal Field in the Mid-Okinawa Trough (Integrated Ocean Drilling Program Expedition 331)

The impacts of lithologic structure and geothermal gradient on subseafloor microbial communities were investigated at a marginal site of the Iheya North hydrothermal field in the Mid-Okinawa Trough. Subsurface marine sediments composed of hemipelagic muds and volcaniclastic deposits were recovered through a depth of 151 m below the seafloor at site C0017 during Integrated Ocean Drilling Program Expedition 331. Microbial communities inferred from 16S rRNA gene clone sequencing in low-temperature hemipelagic sediments were mainly composed of members of the Chloroflexi and deep-sea archaeal group. In contrast, 16S rRNA gene sequences of marine group I Thaumarchaeota dominated the microbial phylotype communities in the coarse-grained pumiceous gravels interbedded between the hemipelagic sediments. Based on the physical properties of sediments such as temperature and permeability, the porewater chemistry, and the microbial phylotype compositions, the shift in the physical properties of the sediments is suggested to induce a potential subseafloor recharging flow of oxygenated seawater in the permeable zone, leading to the generation of variable chemical environments and microbial communities in the subseafloor habitats. In addition, the deepest section of sediments under high-temperature conditions (∼90°C) harbored the sequences of an uncultivated archaeal lineage of hot water crenarchaeotic group IV that may be associated with the high-temperature hydrothermal fluid flow. These results indicate that the subseafloor microbial community compositions and functions at the marginal site of the hydrothermal field are highly affected by the complex fluid flow structure, such as recharging seawater and underlying hydrothermal fluids, coupled with the lithologic transition of sediments.