Diffuse Fluids Research Articles

Diffuse Venting and Near Seafloor Hydrothermal Circulation at the Lucky Strike Vent Field, Mid‐Atlantic Ridge

AbstractWe report on a 3 years monitoring experiment of low to medium temperature diffuse venting at two vent sites (Tour Eiffel and White Castle) of the Lucky Strike, black smoker‐type hydrothermal field, Mid‐Atlantic Ridge. Diffuse vents account for a large part of the energy flux of mid‐ocean ridges hydrothermal fields and provide key habitats for the hydrothermal fauna. We document the time and space variability of diffuse venting temperature and chemistry, describe the effect of tidal loading and currents and discuss the extent of mixing, cooling of black smoker fluids, heating of entrained seawater and anhydrite precipitation/dissolution in the substratum. We emphasize the role of a thin (<2 m) volcaniclastic formation capping the brecciated basalt substratum. This formation is porous, but becomes impermeable when indurated by hydrothermal precipitates. It forms an intermediate layer between the vents at the seabed and the fluids as they discharge out of the brecciated basalts. Diffuse fluids inferred to discharge out of meter‐spaced cracks in the brecciated basalts beneath this volcaniclastic layer are hot (>80°C) and contain >10% of the hot endmember fluid component, over distances of up to 25 m from the black smokers. These results provide a geologically integrated framework in which to study site‐scale, near seafloor hydrothermal circulation and associated vent habitats at Lucky Strike and other black smoker‐type hydrothermal fields. They suggest diffuse heat fluxes in the upper range of previously published estimates at the two studied Lucky Strike hydrothermal vent sites.

Microbial eukaryotic predation pressure and biomass at deep-sea hydrothermal vents.

Deep-sea hydrothermal vent geochemistry shapes the foundation of the microbial food web by fueling chemolithoautotrophic microbial activity. Microbial eukaryotes (or protists) play a critical role in hydrothermal vent food webs as consumers and hosts of symbiotic bacteria, and as a nutritional source to higher trophic levels. We measured microbial eukaryotic cell abundance and predation pressure in low-temperature diffuse hydrothermal fluids at the Von Damm and Piccard vent fields along the Mid-Cayman Rise in the Western Caribbean Sea. We present findings from experiments performed under in situ pressure that show cell abundances and grazing rates higher than those done at 1 atmosphere (shipboard ambient pressure); this trend was attributed to the impact of depressurization on cell integrity. A relationship between the protistan grazing rate, prey cell abundance, and temperature of end-member hydrothermal vent fluid was observed at both vent fields, regardless of experimental approach. Our results show substantial protistan biomass at hydrothermally fueled microbial food webs, and when coupled with improved grazing estimates, suggest an important contribution of grazers to the local carbon export and supply of nutrient resources to the deep ocean.

Microbial ecosystem assessment and hydrogen oxidation potential of newly discovered vent systems from the Central and South-East Indian Ridge.

In order to expand the knowledge of microbial ecosystems from deep-sea hydrothermal vent systems located on the Central and South-East Indian Ridge, we sampled hydrothermal fluids, massive sulfides, ambient water and sediments of six distinct vent fields. Most of these vent sites were only recently discovered in the course of the German exploration program for massive sulfide deposits and no previous studies of the respective microbial communities exist. Apart from typically vent-associated chemosynthetic members of the orders Campylobacterales, Mariprofundales, and Thiomicrospirales, high numbers of uncultured and unspecified Bacteria were identified via 16S rRNA gene analyses in hydrothermal fluid and massive sulfide samples. The sampled sediments however, were characterized by an overall lack of chemosynthetic Bacteria and the presence of high proportions of low abundant bacterial groups. The archaeal communities were generally less diverse and mostly dominated by members of Nitrosopumilales and Woesearchaeales, partly exhibiting high proportions of unassigned Archaea. Correlations with environmental parameters were primarily observed for sediment communities and for microbial species (associated with the nitrogen cycle) in samples from a recently identified vent field, which was geochemically distinct from all other sampled sites. Enrichment cultures of diffuse fluids demonstrated a great potential for hydrogen oxidation coupled to the reduction of various electron-acceptors with high abundances of Hydrogenovibrio and Sulfurimonas species. Overall, given the large number of currently uncultured and unspecified microorganisms identified in the vent communities, their respective metabolic traits, ecosystem functions and mediated biogeochemical processes have still to be resolved for estimating consequences of potential environmental disturbances by future mining activities.

Hydrothermal alteration of basalts in the ultramafic-associated Tianxiu Vent Field, Carlsberg Ridge

The Tianxiu vent field (TVF) is the first active ultramafic-associated hydrothermal field to be discovered at the termination of a detachment fault on the slow-spreading Carlsberg Ridge. Most hydrothermal chimneys in the TVF are developed on the basaltic apron. This paper presents mineralogical, petrological, and geochemical data for altered basalts in and surrounding the TVF. The basalt samples can be classified into seafloor-weathered and hydrothermally altered (i.e., slightly saponite-altered, highly saponite-altered, and silicified) types. At the periphery of the TVF, the basalts have experienced seafloor weathering, which has resulted in slight mineralogical and geochemical changes. In the TVF, most basalts have interacted with diffuse low-temperature hydrothermal fluids, leading to variable degrees of saponite alteration. As the distance from the vents decreases, basaltic glass, olivine, and pyroxene are progressively replaced by saponite; Zn, Cu, Co, and U are enriched; and Ca and Mn are depleted. Magnesium, Fe, and Li have been lost due to olivine replacement by saponite, and have been gained due to pyroxene replacement by saponite. In the fluid flow conduits, the basalts have interacted with focused medium- to high-temperature fluids, which has led to the replacement of saponite by opal; depletion in Mg, Fe, Li, and rare earth elements; and enrichment in Zn, Cu, Co, and U. We propose a new hydrothermal alteration model for basalts in an ultramafic-associated vent field. Based on a comparison with mafic-hosted vent fields along mid-ocean ridges, we propose that: (1) the permeable host rocks promote lateral fluid flow, widespread mixing with cold seawater, and pervasive low-temperature hydrothermal alteration; and (2) H2S-poor reducing fluids reduce the solubility of U and inhibit the precipitation of Fe, leading to high Cu/Fe and Zn/Fe ratios, and high U concentrations. The hydrothermal alteration model for the TVF is applicable to similar vent fields and can guide exploration for seafloor massive sulfide deposits along slow-spreading ridges.

Spatial comparison and temporal evolution of two marine iron-rich microbial mats from the Lucky Strike Hydrothermal Field, related to environmental variations

In hydrothermal environments, diffuse fluids emanations provide optimal conditions for the development of iron-rich microbial mats. Here, we present a unique spatial and temporal study of phylogenetic and chemical data from this type of mats and their associated hydrothermal fluids from two sites of the Lucky Strike Hydrothermal Field (EMSO-Azores deep-sea observatory), collected annually from 2016 to 2020. Our metabarcoding analyses reveal a completely different microbial community at each site, linked to the distinctive chemical composition of the diffuse fluids nourishing the mats. Capelinhos site is dominated by microorganisms with metabolisms related to iron, methane, and reduced sulphur compounds, coming from hydrothermal fluids, while North Tour Eiffel site presents higher abundances of microorganisms with metabolisms related to nitrogen, organic and oxidized sulphur compounds, coming from seawater. We present for the first time the yearly evolution of these mats over a five-year period. This analysis reveals similar variations of the microbial communities over time at both sites, indicating a regional Lucky Strike influence on the temporal scale. We also highlight more diversified microbial communities at both sites in 2016, pointing out the occurrence of a geological event that could have affected them during this specific year. Except for this year, our study shows that the communities of iron-rich microbial mats remain stable over time at both sites.

Virus diversity and interactions with hosts in deep-sea hydrothermal vents

BackgroundThe deep sea harbors many viruses, yet their diversity and interactions with hosts in hydrothermal ecosystems are largely unknown. Here, we analyzed the viral composition, distribution, host preference, and metabolic potential in different habitats of global hydrothermal vents, including vent plumes, background seawater, diffuse fluids, and sediments.ResultsFrom 34 samples collected at eight vent sites, a total of 4662 viral populations (vOTUs) were recovered from the metagenome assemblies, encompassing diverse phylogenetic groups and defining many novel lineages. Apart from the abundant unclassified viruses, tailed phages are most predominant across the global hydrothermal vents, while single-stranded DNA viruses, including Microviridae and small eukaryotic viruses, also constitute a significant part of the viromes. As revealed by protein-sharing network analysis, hydrothermal vent viruses formed many novel genus-level viral clusters and are highly endemic to specific vent sites and habitat types. Only 11% of the vOTUs can be linked to hosts, which are the key microbial taxa of hydrothermal habitats, such as Gammaproteobacteria and Campylobacterota. Intriguingly, vent viromes share some common metabolic features in that they encode auxiliary genes that are extensively involved in the metabolism of carbohydrates, amino acids, cofactors, and vitamins. Specifically, in plume viruses, various auxiliary genes related to methane, nitrogen, and sulfur metabolism were observed, indicating their contribution to host energy conservation. Moreover, the prevalence of sulfur-relay pathway genes indicated the significant role of vent viruses in stabilizing the tRNA structure, which promotes host adaptation to steep environmental gradients.ConclusionsThe deep-sea hydrothermal systems hold untapped viral diversity with novelty. They may affect both vent prokaryotic and eukaryotic communities and modulate host metabolism related to vent adaptability. More explorations are needed to depict global vent virus diversity and its roles in this unique ecosystem.3aGZ9ofcs6zPBgoYBabc9bVideo

Globally-distributed microbial eukaryotes exhibit endemism at deep-sea hydrothermal vents.

Single-celled microbial eukaryotes inhabit deep-sea hydrothermal vent environments and play critical ecological roles in the vent-associated microbial food web. 18S rRNA amplicon sequencing of diffuse venting fluids from four geographically- and geochemically-distinct hydrothermal vent fields was applied to investigate community diversity patterns among protistan assemblages. The four vent fields include Axial Seamount at the Juan de Fuca Ridge, Sea Cliff and Apollo at the Gorda Ridge, all in the NE Pacific Ocean, and Piccard and Von Damm at the Mid-Cayman Rise in the Caribbean Sea. We describe species diversity patterns with respect to hydrothermal vent field and sample type, identify putative vent endemic microbial eukaryotes, and test how vent fluid geochemistry may influence microbial community diversity. At a semi-global scale, microbial eukaryotic communities at deep-sea vents were composed of similar proportions of dinoflagellates, ciliates, Rhizaria, and stramenopiles. Individual vent fields supported distinct and highly diverse assemblages of protists that included potentially endemic or novel vent-associated strains. These findings represent a census of deep-sea hydrothermal vent protistan communities. Protistan diversity, which is shaped by the hydrothermal vent environment at a local scale, ultimately influences the vent-associated microbial food web and the broader deep-sea carbon cycle.

Insights from modern diffuse-flow hydrothermal systems into the origin of post-GOE deep-water Fe-Si precipitates

Post-GOE deep-water Fe-Si precipitates associated with volcanogenic massive sulfide deposits are an important feature of the Proterozoic rock record. Although it is clear that these enigmatic deposits formed in oxygen-deficient and hydrothermally influenced deep-water settings, the oxidation mechanism(s) resulting in their precipitation remain(s) unclear. Whilst existing genetic models typically couple direct and/or bacterially-mediated iron oxidation with abiotic silica precipitation, low temperature diffuse hydrothermal fluids offer a potential mechanistic alternative to explain the observed layering. Herein, via combination of petrographic observations with elemental and isotopic data (Fe, Si, and O), we explore the genesis of the primary mineral phases present within recent Fe-Si precipitates obtained from the Southwest Indian Ridge. Formation of ferrihydrite and opal-A in these precipitates provides insight into the genesis of widely invoked precursor minerals to post-GOE deep-water Fe-Si precipitates. Specifically, we find that the mineralogical layers that typify these Fe-Si precipitates may have originated via biologically mediated ferrihydrite precipitation and abiotic precipitation of opal-A from oversaturated fluids during diffuse flow. By analogy, we propose that diffuse hydrothermal fluid flow played an important role in the formation of post-GOE deep-water Fe-Si precipitates.

Protistan grazing impacts microbial communities and carbon cycling at deep-sea hydrothermal vents

Microbial eukaryotes (or protists) in marine ecosystems are a link between primary producers and all higher trophic levels, and the rate at which heterotrophic protistan grazers consume microbial prey is a key mechanism for carbon transport and recycling in microbial food webs. At deep-sea hydrothermal vents, chemosynthetic bacteria and archaea form the base of a food web that functions in the absence of sunlight, but the role of protistan grazers in these highly productive ecosystems is largely unexplored. Here, we pair grazing experiments with a molecular survey to quantify protistan grazing and to characterize the composition of vent-associated protists in low-temperature diffuse venting fluids from Gorda Ridge in the northeast Pacific Ocean. Results reveal protists exert higher predation pressure at vents compared to the surrounding deep seawater environment and may account for consuming 28 to 62% of the daily stock of prokaryotic biomass within discharging hydrothermal vent fluids. The vent-associated protistan community was more species rich relative to the background deep sea, and patterns in the distribution and co-occurrence of vent microbes provide additional insights into potential predator-prey interactions. Ciliates, followed by dinoflagellates, Syndiniales, rhizaria, and stramenopiles, dominated the vent protistan community and included bacterivorous species, species known to host symbionts, and parasites. Our findings provide an estimate of protistan grazing pressure within hydrothermal vent food webs, highlighting the important role that diverse protistan communities play in deep-sea carbon cycling.

Organic Biogeochemistry in West Mata, NE Lau Hydrothermal Vent Fields

AbstractThe impact of submarine hydrothermal systems on organic carbon in the ocean—one of the largest fixed carbon reservoirs on Earth—could be profound. Yet, different vent sites show diverse fluid chemical compositions and the subsequent biological responses. Observations from various vent sites are to evaluate hydrothermal systems' impact on the ocean carbon cycle. A response cruise in May 2009 to an on‐going submarine eruption at West Mata Volcano, northeast Lau Basin, provided an opportunity to quantify the organic matter production in a back‐arc spreading hydrothermal system. Hydrothermal vent fluids contained elevated dissolved organic carbon, particulate organic carbon (POC), and particulate nitrogen (PN) relative to background seawater. The δ13C‐POC values for suspended particles in the diffuse vent fluids (−15.5‰ and −12.3‰) are distinct from those in background seawater (−23 ± 1‰), indicative of unique carbon synthesis pathways of the vent microbes from the seawater counterparts. The first dissolved organic nitrogen concentrations reported for diffuse vents were similar to or higher than those for background seawater. Enhanced nitrogen fixation and denitrification removed 37%–89% of the total dissolved nitrogen in the recharging background seawater in the hydrothermal vent flow paths. The hydrothermal plume samples were enriched in POC and PN, indicating enhanced biological production. The total “dark” organic carbon production within the plume matches the thermodynamic prediction based on available reducing chemical substances supplied to the plume. This research combines the measured organic carbon contents with thermodynamic modeled results and demonstrates the importance of hydrothermal activities on the water column carbon production in the deep ocean.

Imprint of Kairei and Pelagia deep-sea hydrothermal systems (Indian Ocean) on marine dissolved organic matter

Only few studies exist that investigate the dynamics of deep-sea dissolved organic matter (DOM) derived from hydrothermal vents. In this study, we provide first insight into the molecular composition of DOM associated with Indian Ocean hydrothermal systems covering the full range from hot focussed endmember fluids over diffuse fluids to open ocean hydrothermal plumes and deep seawater. We combined geochemical analyses with molecular characterization of DOM using ultra-high resolution mass spectrometry (FT-ICR-MS). We studied two vent systems with fluids venting >330 °C and up to 97% of hydrothermal endmember: the Kairei vents (Central Indian Ridge) with brine phase separation, and the newly discovered Pelagia vents (South-East Indian Ridge). The hot fluids in both systems were highly enriched in dissolved Fe, Si, K, Li, Mn and Zn compared to seawater. The molecular composition of DOM from hot fluids differed substantially from that of diffuse fluids and plumes, in which the composition was highly dominated by the seawater DOM signature. Low O/C ratio average in hot fluids (<0.38) indicated potential input of more reduced (O-poor) DOM compounds from the vents into the surrounding seawater, independently of the vent location and of whether the fluid has undergone phase separation. To test the importance of pure thermal degradation, we compared our samples to fluids subjected to abiotic thermal degradation under laboratory conditions and we observed that our natural samples largely differed (>94% Bray Curtis dissimilarity) from the experimental ones, suggesting additional degradation processes of organic compounds at the Indian Ocean hydrothermal systems.

The characteristics of Fe speciation and Fe-binding ligands in the Mariana back-arc hydrothermal plumes

We investigated the speciation of Fe and distributions of Fe-binding ligands in the hydrothermal plumes over high- and low-temperature vents and over diffuse venting fields above the Mariana back-arc spreading center. The concentrations of ligands ([L]) and conditional stability constants (K’FeL) of the natural Fe ligand pool were measured by a reverse titration-competitive ligand exchange-adsorption cathodic stripping voltammetry (RT-CLE-ACSV). The results showed that the buoyant and non-buoyant plume samples over the low-temperature Burke had the highest dissolved Fe/total Fe (DFe/TFe) of 82.2 ± 8.8% and labile Fe/total Fe (FeLab/TFe) of 43.3 ± 5.6%. In contrast, in the plume samples above the high-temperature Perseverance field, TFe had the lowest proportions of DFe (48.8 ± 12.2%) and FeLab (19.4 ± 8.8%). The linear relationships between Fe species and total Mn (TMn) in the buoyant plume over Burke suggest a simple conservative mixing with ambient seawater, which resulted in constant values of DFe/TFe. However, in the non-buoyant plume over Burke, DFe/TFe decreased with plume dilution. The plume samples sourced from the diffuse flow over newly erupted lava had the highest proportion of DFe present as FeLab (74.6 ± 4.0%). The [L] in the buoyant plumes over low-temperature Burke vent were up to 113.6 nM, and the logK’FeL decreased with increasing DFe and [L], suggesting the importance of weaker ligands in stabilizing Fe. For all the plume samples, the organically complexed Fe (FeL) constituted significant proportions of the DFe (29.0 ± 9.3%) and FeLab (57.5 ± 15.6%) fractions. These Fe-binding ligands were likely sourced from diffuse venting fluids adjacent to the venting sites, and may be also produced by microbes within the hydrothermal plumes.

Abiotic redox reactions in hydrothermal mixing zones: Decreased energy availability for the subsurface biosphere

Subseafloor mixing of high-temperature hot-spring fluids with cold seawater creates intermediate-temperature diffuse fluids that are replete with potential chemical energy. This energy can be harnessed by a chemosynthetic biosphere that permeates hydrothermal regions on Earth. Shifts in the abundance of redox-reactive species in diffuse fluids are often interpreted to reflect the direct influence of subseafloor microbial activity on fluid geochemical budgets. Here, we examine hydrothermal fluids venting at 44 to 149 °C at the Piccard hydrothermal field that span the canonical 122 °C limit to life, and thus provide a rare opportunity to study the transition between habitable and uninhabitable environments. In contrast with previous studies, we show that hydrocarbons are contributed by biomass pyrolysis, while abiotic sulfate (SO42-) reduction produces large depletions in H2 The latter process consumes energy that could otherwise support key metabolic strategies employed by the subseafloor biosphere. Available Gibbs free energy is reduced by 71 to 86% across the habitable temperature range for both hydrogenotrophic SO42- reduction to hydrogen sulfide (H2S) and carbon dioxide (CO2) reduction to methane (CH4). The abiotic H2 sink we identify has implications for the productivity of subseafloor microbial ecosystems and is an important process to consider within models of H2 production and consumption in young oceanic crust.

The influence of fluids on the species composition and preservation of microfossils in biogenic carbonate sediments by hydrothermal cluster “Pobeda” (Mid-Atlantic ridge)

The species composition, distribution and preservation of carbonate shells of nanno and microfossils of biogenic bottom sediments within the hydrothermal cluster Pobeda (1707.451708.7 n) were studied on the basis of materials obtained in flight No. 37 of NIS Professor Logachev in the Russian exploration area (Mid-Atlantic ridge). Physical-chemical parameters and the total carbonate content of sediments were measured, their changes in column sections were revealed. For complexes of coccoliths studied precipitation is attributed to the top (acme) biostratigraphic zone of Emiliania huxleyi. It is established that within the zone of hydrothermal activity (ore cluster Pobeda) the total number of microorganisms decreases and their species diversity decreases, as a rule, in the lower part of the columns. Down the column section also decreases the Eh, pH and carbonate precipitation. These changes are explained by the influence of diffuse hydrothermal fluids.

Influence of Fluids on the Species Composition and Preservation of Microfossils in Biogenic Carbonate Sediments in the Pobeda Hydrothermal Cluster (Mid-Atlantic Ridge)

Based on materials obtained during Cruise 37 of the R/V Professor Logatchev in the Russian exploration area, the species composition, distribution, and preservation of nanno- and microfossil carbonate shells in biogenic bottom sediments within the Pobeda hydrothermal cluster (17°07.45′–17°08.7′ N MAR) were studied. Physicochemical parameters and total carbonate content were measured and their changes along column sections were revealed. Based on the coccolith assemblages, the studied sediments are assigned to the upper part (acme) of the biostratigraphic zone Emiliania huxleyi. It has been established that the hydrothermal activity zone (Pobeda hydrothermal cluster) is marked usually by a decrease of both total population and species diversity of microrganisms in the lower part of columns. The column is also marked by downsection decrease of Eh, pH, and carbonate content. These changes are attributed to the influence of diffuse hydrothermal fluids.

A Symmetric Particle-Based Simulation Scheme towards Large Scale Diffuse Fluids

We present a symmetric particle simulation scheme for diffuse fluids based on the Lagrangian Smoothed Particle Hydrodynamics (SPH) model. In our method, the generation of diffuse particles is determined by the entropy of fluid particles, and it is calculated by the velocity difference and kinetic energy. Diffuse particles are generated near the qualified diffuse particle emitters whose diffuse material generation rate is greater than zero. Our method fits the laws of physics better, as it abandons the common practice of adding diffuse materials at the crest empirically. The coupling between diffuse materials and fluid is a post-processing step achieved by the velocity field, which enables the avoiding of the time-consuming process of cross finding neighbors. The influence weights of the fluid particles are assigned based on the degree of coupling. Therefore, it improved the accuracy of the diffuse particle position and made the simulation results more realistic. The approach is appropriate for large scale diffuse fluid, as it can be easily integrated in existing SPH simulation methods and the computational overhead is negligible.

Boiling-induced formation of colloidal gold in black smoker hydrothermal fluids

Gold colloids occur in black smoker fluids from the Niua South hydrothermal vent field, Lau Basin (South Pacific Ocean), confirming the long-standing hypothesis that gold may undergo colloidal transport in hydrothermal fluids. Six black smoker vents, varying in temperature from 250 °C to 325 °C, were sampled; the 325 °C vent was boiling at the time of sampling and the 250 °C fluids were diffusely venting. Native gold particles ranging from <50 nm to 2 μm were identified in 4 of the fluid samples and were also observed to precipitate on the sampler during collection from the boiling vent. Total gold concentrations (dissolved and particulate) in the fluid samples range from 1.6 to 5.4 nM in the high-temperature, focused flow vents. Although the gold concentrations in the focused flow fluids are relatively high, they are lower than potential solubilities prior to boiling and indicate that precipitation was boiling induced, with sulfide lost upon boiling to exsolution and metal sulfide formation. Gold concentrations reach 26.7 nM in the 250 °C diffuse flow sample, and abundant native gold particles were also found in the fluids and associated sulfide chimney and are interpreted to be a product of colloid accumulation and growth following initial precipitation upon boiling. These results indicate that colloid-driven precipitation as a result of boiling, the persistence of colloids after boiling, and the accumulation of colloids in diffuse flow fluids are important mechanisms for the enrichment of gold in seafloor hydrothermal systems.

Detection of diffuse seafloor venting using a structured light laser sensor: 2. Evaluation of detection sensitivity and limitations

Algorithms to detect diffuse hydrothermal vent fluids using the structured light laser sensor have proven to be successful. However, potential limitations due to survey parameters, including altitude and vehicle heading, have not been explored and are the focus of this study. Data from 12 surveys conducted over a single hydrothermal vent at three different altitudes (2.7 m, 4.2 m, and 6.2 m) and four different headings are analyzed. Increasing survey altitude decreases the resolution and intensity of the imaged laser line; therefore, range‐dependent processing considerations to maintain detection sensitivity are presented. Analysis of these surveys suggests minimal degradation of vent detection capabilities from survey altitudes up to 6 m. Additionally, as the shape and location of the detected distortion patterns vary with survey heading and altitude they are presented as potential indications of plume shape and rise height. The effects of range and survey heading are important for future applications of this sensor which may include surveying from higher, faster flying autonomous underwater vehicles.

Hydrogen Limitation and Syntrophic Growth among Natural Assemblages of Thermophilic Methanogens at Deep-sea Hydrothermal Vents.

Thermophilic methanogens are common autotrophs at hydrothermal vents, but their growth constraints and dependence on H2 syntrophy in situ are poorly understood. Between 2012 and 2015, methanogens and H2-producing heterotrophs were detected by growth at 80°C and 55°C at most diffuse (7–40°C) hydrothermal vent sites at Axial Seamount. Microcosm incubations of diffuse hydrothermal fluids at 80°C and 55°C demonstrated that growth of thermophilic and hyperthermophilic methanogens is primarily limited by H2 availability. Amendment of microcosms with NH4+ generally had no effect on CH4 production. However, annual variations in abundance and CH4 production were observed in relation to the eruption cycle of the seamount. Microcosm incubations of hydrothermal fluids at 80°C and 55°C supplemented with tryptone and no added H2 showed CH4 production indicating the capacity in situ for methanogenic H2 syntrophy. 16S rRNA genes were found in 80°C microcosms from H2-producing archaea and H2-consuming methanogens, but not for any bacteria. In 55°C microcosms, sequences were found from H2-producing bacteria and H2-consuming methanogens and sulfate-reducing bacteria. A co-culture of representative organisms showed that Thermococcus paralvinellae supported the syntrophic growth of Methanocaldococcus bathoardescens at 82°C and Methanothermococcus sp. strain BW11 at 60°C. The results demonstrate that modeling of subseafloor methanogenesis should focus primarily on H2 availability and temperature, and that thermophilic H2 syntrophy can support methanogenesis within natural microbial assemblages and may be an important energy source for thermophilic autotrophs in marine geothermal environments.

Thiosulfate oxidation by Thiomicrospira thermophila: metabolic flexibility in response to ambient geochemistry.

Previous studies of the stoichiometry of thiosulfate oxidation by colorless sulfur bacteria have failed to demonstrate mass balance of sulfur, indicating that unidentified oxidized products must be present. Here the reaction stoichiometry and kinetics under variable pH conditions during the growth of Thiomicrospira thermophila strain EPR85, isolated from diffuse hydrothermal fluids at the East Pacific Rise, is presented. At pH 8.0, thiosulfate was stoichiometrically converted to sulfate. At lower pH, the products of thiosulfate oxidation were extracellular elemental sulfur and sulfate. We were able to replicate previous experiments and identify the missing sulfur as tetrathionate, consistent with previous reports of the activity of thiosulfate dehydrogenase. Tetrathionate was formed under slightly acidic conditions. Genomic DNA from T. thermophila strain EPR85 contains genes homologous to those in the Sox pathway (soxAXYZBCDL), as well as rhodanese and thiosulfate dehydrogenase. No other sulfur oxidizing bacteria containing sox(CD)2 genes have been reported to produce extracellular elemental sulfur. If the apparent modified Sox pathway we observed in T. thermophila is present in marine Thiobacillus and Thiomicrospira species, production of extracellular elemental sulfur may be biogeochemically important in marine sulfur cycling.