Low-temperature Hydrothermal Vents Research Articles

Non-Deterministic Factors Affect Competition Between Thermophilic Autotrophs from Deep-Sea Hydrothermal Vents

Hydrothermal vents provide windows into the rocky subseafloor on Earth and serve as terrestrial analog sites for extraterrestrial environments. By studying patterns of community assembly in hydrothermal vents and using geochemical models, we can better understand how the deep-sea biosphere contributes to local and global biogeochemical cycling and gather valuable information about how similar communities may arise on Earth and beyond Earth. One prevailing thought is that vent microbial community assembly is driven by deterministic factors such as the thermodynamic favorability of redox reactions. We hypothesized that subsurface microbial communities may also be significantly influenced by other factors, such as differential cell yields, varying optimal growth temperatures, and stochasticity. At Axial Seamount in the Pacific Ocean, H2-consuming methanogens of the genera Methanocaldococcus (Topt 82°C) and Methanothermococcus (Topt 65°C) and H2-consuming sulfur reducers of the genus Desulfurobacterium (Topt 72°C) are the most abundant autotrophs that grow optimally at or above 65°C (Fortunato et al. 2017). At one low-temperature hydrothermal vent site, Marker 113, methanogens are the predominant thermophilic autotrophs while at another site, Marker 33, thermophilic autotrophic sulfur reducers predominate. There is no apparent geochemical or thermodynamic explanation for the differences in community composition. In this study, we performed a series of co-culture competition experiments using Methanocaldococcus jannaschii, Methanothermococcus thermolithotrophicus, and Desulfurobacterium thermolithotrophum HR11 as representative methanogens and sulfur reducers common to hydrothermal vents to explain the variations in community composition between thermophilic autotrophs. M. jannaschii increases its cell yield (cells produced per mole of CH4 produced) when grown on very low H2 concentrations as part of a growth rate-growth yield tradeoff (Topçuoğlu et al. 2019). This increase in cell yield could provide methanogens with a competitive growth advantage over H2-consuming sulfur reducers, who otherwise catalyze a more thermodynamically favorable growth reaction. Competition co-culture experiments were conducted between M. jannaschii and D. thermolithotrophum at 72°C and between M. thermolithotrophicus and D. thermolithotrophum at 65°C, both at 1:1 ratios and initial aqueous H2 concentrations of 1.2 mM (high H2) and 85 μM (low H2) to determine the effects of temperature and H2 availability on autotroph competition. For both methanogens, the growth rate, maximum cell concentration, and total CH4 produced decreased when they were grown in co-culture, at low H2, or both relative to monocultures grown with high H2. The methanogen cell yields generally increased in co-culture and at low H2. At both experimental temperatures, the growth rate of D. thermolithotrophum remained unchanged in co-culture and at low H2 relative to monocultures but the maximum cell concentration decreased in co-culture relative to monocultures at both H2 concentrations. However, at low H2, both in mono- and co-culture, there was no detectable H2S produced by the sulfur reducer suggesting a significant shift in growth yield. At both temperatures and H2 concentrations, the sulfur reducer reached higher cell concentrations than the methanogens. Stochasticity or vent fluid chemistry could lead to early colonization of a vent by methanogens followed by niche exclusion of autotrophic sulfur reducers due to a numerical advantage of the methanogens. Therefore, competitive co-culture experiments were run as before at high H2 with varying initial methanogen:sulfur reducer ratios. At 72°C, D. thermolithotrophum reached the same maximum cell concentration and produced the same amount of H2S in monoculture and co-culture even when the methanogens initially outnumbered the sulfur reducer up to 10,000-fold. M. jannaschii reached a lower maximum cell concentration and produced less CH4 in all co-cultures relative to growth in monoculture. At 65°C, D. thermolithotrophum reached the same maximum cell concentrations and produced the same amount of H2S in monoculture and co-culture when the methanogens initially outnumbered the sulfur reducers up to 100-fold. However, when the methanogens initially outnumbered the sulfur reducers 1,000-fold, M. thermolithotrophicus grew as well as in monoculture and the maximum cell concentration and amount of H2S produced by D. thermolithotrophum was significantly lower than in monoculture and the other co-culture conditions. In conclusion, both methanogens and sulfur reducers shift their redox reactions away from CH4 and H2S production, respectively, and towards biomass production when H2 is limiting. This should be accounted for in thermodynamic predictive models. Furthermore, a combination of growth temperatures lower than the optimum of sulfur reducers and high initial methanogen cell concentrations relative to sulfur reducers can lead to a long-term predominance of methanogens over autotrophic sulfur reducers in vent environments through niche exclusion.

White and green rust chimneys accumulate RNA in a ferruginous chemical garden.

Mechanisms of nucleic acid accumulation were likely critical to life's emergence in the ferruginous oceans of the early Earth. How exactly prebiotic geological settings accumulated nucleic acids from dilute aqueous solutions, is poorly understood. As a possible solution to this concentration problem, we simulated the conditions of prebiotic low-temperature alkaline hydrothermal vents in co-precipitation experiments to investigate the potential of ferruginous chemical gardens to accumulate nucleic acids via sorption. The injection of an alkaline solution into an artificial ferruginous solution under anoxic conditions (O2 < 0.01% of present atmospheric levels) and at ambient temperatures, caused the precipitation of amakinite ("white rust"), which quickly converted to chloride-containing fougerite ("green rust"). RNA was only extractable from the ferruginous solution in the presence of a phosphate buffer, suggesting RNA in solution was bound to Fe2+ ions. During chimney formation, this iron-bound RNA rapidly accumulated in the white and green rust chimney structure from the surrounding ferruginous solution at the fastest rates in the initial white rust phase and correspondingly slower rates in the following green rust phase. This represents a new mechanism for nucleic acid accumulation in the ferruginous oceans of the early Earth, in addition to wet-dry cycles and may have helped to concentrate RNA in a dilute prebiotic ocean.

Linking Methanogenesis in Low-Temperature Hydrothermal Vent Systems to Planetary Spectra: Methane Biosignatures on an Archean-Earth-like Exoplanet.

In this work, the viability of the detection of methane produced by microbial activity in low-temperature hydrothermal vents on an Archean-Earth-like exoplanet in the habitable zone is explored via a simplified bottom-up approach using a toy model. By simulating methanogens at hydrothermal vent sites in the deep ocean, biological methane production for a range of substrate inflow rates was determined and compared to literature values. These production rates were then used, along with a range of ocean floor vent coverage fractions, to determine likely methane concentrations in the simplified atmosphere. At maximum production rates, a vent coverage of 4-15 × 10-4 % (roughly 2000-6500 times that of modern Earth) is required to achieve 0.25% atmospheric methane. At minimum production rates, 100% vent coverage is not enough to produce 0.25% atmospheric methane. NASA's Planetary Spectrum Generator was then used to assess the detectability of methane features at various atmospheric concentrations. Even with future space-based observatory concepts (such as LUVOIR and HabEx), our results show the importance of both mirror size and distance to the observed planet. Planets with a substantial biomass of methanogens in hydrothermal vents can still lack a detectable, convincingly biological methane signature if they are beyond the scope of the chosen instrument. This work shows the value of coupling microbial ecological modeling with exoplanet science to better understand the constraints on biosignature gas production and its detectability.

Iron isotope constraints on the metal source and depositional environment of the Neoproterozoic banded iron- and manganese deposits in Urucum, Brazil

The Urucum area of Brazil hosts a series of Cryogenian ironstones intercalated by oxide-dominated manganese layers. The Urucum iron and manganese formations (IF-MnF) are among the largest sedimentary iron and associated manganese deposits of the Neoproterozoic, however, the depositional model and the source of metals for the IF-MnF in this area are highly controversial. In this study, we performed systematic Fe isotope analysis on fresh and geochemically characterized drill core samples of the Urucum iron and manganese formation deposited in the center of the ancient Urucum graben system. The samples have a large variation in Fe isotope composition, with a δ56Fe range of −2.04‰ to +0.75‰, and exhibit a general trend of decreasing δ56Fe values with increasing manganese contents. The low δ56Fe values of the IF and MnF samples reflect Rayleigh fractionation processes of contineous partial oxidation of aqueous Fe(II) prior to deposition at the sampling site. Using a mixing model and previously published Nd isotope data on the same samples, we estimated that benthic (i.e., porewaters released from submarine sediments in the Urucum basin) Fe fluxes provided 7–50% of total Fe in the Urucum IF-MnF, and the rest of Fe source was from low-temperature hydrothermal vents. Based on combined Fe and Nd isotope data of the Urucum IF-MnF, we propose that low-temperature hydrothermal fluids and benthic fluxes of pore waters were mixed and transported by an upwelling current. The fluid subsequently experienced partial oxidation during the transportation process and became enriched in light Fe isotopes. In the Urucum graben basin, the iron- and manganese-rich oxides deposition occurred progressively under increasingly oxidizing conditions, and such process could have operated repeatedly to produce the alternation of iron and manganese formations. The chemical sediments of the Urucum IF-MnF deposits thus reflect the existence of a sharp redox gradient in the marine environment during the late Cryogenian period.

Low-temperature, shallow-water hydrothermal vent mineralization following the recent submarine eruption of Tagoro volcano (El Hierro, Canary Islands)

Hydrothermal iron (Fe)-rich sediments were recovered from the Tagoro underwater volcano (Central Atlantic) that formed during the 2011–2012 volcanic event. Cruises in 2012 and 2014 enabled the monitoring and sampling of the early-stage establishment of a hydrothermal system. Degassing vents produced acoustic flares imaged on echo-sounders in June 2012, four months after the eruption. A novel hydrothermal vent system was discovered and sampled in 2014 during a ROV dive. The system is characterized by hornito-like structures and chimneys showing active CO2 degassing and anomalous temperatures at 120–89 m water depth, and along the SE flank at 215-185 m water depth associated with secondary cones. Iron- and silica-rich gelatinous deposits pooled over and between basanite in the hornitos, brecciated lavas, and lapilli. The low temperature, shallow-water hydrothermal system was discovered by the venting of Fe-rich fluids that produced a seafloor draped by extensive Fe-flocculate deposits precipitated from the neutrally buoyant plumes located along the oxic/photic zone at 50-70 m water depths. The basanite is capped by mm- to cm-thick hydrothermally derived Fe-oxyhydroxide sediment, and contains micro-cracks and degasification vesicles filled by sulfides (mostly pyrite). Mineralogically, the Fe-oxyhydroxide sediment consists of proto-ferrihydrite and ferrihydrite with scarce pyrite at their base. The Fe-rich endmember contains low concentrations of most trace elements in comparison with hydrogenetic ferromanganese deposits, and the sediments show some dilution of the Fe oxyhydroxide by volcanic ash. The Fe-oxyhydroxide phase, with a mean particle size of 3–4 nm, low average La/Fe ratios of the mineralized deposits from the various sampling sites, and the positive Eu anomalies indicate rapid deposition of the Fe oxyhydroxide near the hydrothermal vents. Electron microprobe studies show the presence of various organomineral structures, mainly twisted stalks and sheaths covered by iron-silica deposits within the mineralized samples, reflecting microbial iron-oxidation from the hydrothermal fluids. Sequencing of 16 s rRNA genes also reveals the presence of other microorganisms involved in sulfur and methane cycles. Samples collected from hornito chimneys contain silicified microorganisms coated by Fe-rich precipitates. The rapid silicification may have been indirectly promoted by microorganisms acting as nucleation sites. We suggest that this type of hydrothermal deposit might be more frequent than presently reported to occur in submarine volcanoes. On a geological scale, these volcanic eruptions and low-temperature hydrothermal vents might contribute to increased dissolved metals in seawater, and generate considerable Fe-oxyhydroxide deposits as identified in older hot-spot seamounts.

A seawater throttle on H2 production in Precambrian serpentinizing systems

Since the initial discovery of low-temperature alkaline hydrothermal vents off the Mid-Atlantic Ridge axis nearly 20 y ago, the observation that serpentinizing systems produce abundant H2 has strongly influenced models of atmospheric evolution and geological scenarios for the origin of life. Nevertheless, the principal mechanisms that generate H2 in these systems, and how secular changes in seawater composition may have modified serpentinization-driven H2 fluxes, remain poorly constrained. Here, we demonstrate that the dominant mechanism for H2 production during low-temperature serpentinization is directly related to a Si deficiency in the serpentine structure, which itself is caused by low SiO2(aq) concentrations in serpentinizing fluids derived from modern seawater. Geochemical calculations explicitly incorporating this mechanism illustrate that H2 production is directly proportional to both the SiO2(aq) concentration and temperature of serpentinization. These results imply that, before the emergence of silica-secreting organisms, elevated SiO2(aq) concentrations in Precambrian seawater would have generated serpentinites that produced up to two orders of magnitude less H2 than their modern counterparts, consistent with Fe-oxidation states measured on ancient igneous rocks. A mechanistic link between the marine Si cycle and off-axis H2 production requires a reevaluation of the processes that supplied H2 to prebiotic and early microbial systems, as well as those that balanced ocean-atmosphere redox through time.

Precipitation of Mn Oxides in Quaternary Microbially Induced Sedimentary Structures (MISS), Cape Vani Paleo-Hydrothermal Vent Field, Milos, Greece

Understanding microbial mediation in sediment-hosted Mn deposition has gained importance in low-temperature ore genesis research. Here we report Mn oxide ores dominated by todorokite, vernadite, hollandite, and manjiroite, which cement Quaternary microbially induced sedimentary structures (MISS) developed along bedding planes of shallow-marine to tidal-flat volcaniclastic sandstones/sandy tuffs, Cape Vani paleo-hydrothermal vent field, Milos, Greece. This work aims to decipher the link between biological Mn oxide formation, low-T hydrothermalism, and, growth and preservation of Mn-bearing MISS (MnMISS). Geobiological processes, identified by microtexture petrography, scanning and transmission electron microscopy, lipid biomarkers, bulk- and lipid-specific δ13Corganic composition, and field data, and, low-temperature hydrothermal venting of aqueous Mn2+ in sunlit shallow waters, cooperatively enabled microbially-mediated Mn (II) oxidation and biomineralization. The MnMISS biomarker content and δ13Corg signatures strongly resemble those of modern Mn-rich hydrothermal sediments, Milos coast. Biogenic and syngenetic Mn oxide precipitation established by electron paramagnetic resonance (EPR) spectroscopy and petrography, combined with hydrothermal fluid flow-induced pre-burial curing/diagenesis, may account for today’s crystalline Mn oxide resource. Our data suggests that MISS are not unique to cyanobacteria mats. Furthermore, microbial mats inhabited by aerobic methanotrophs may have contributed significantly to the formation of the MnMISS, thus widening the spectrum of environments responsible for marine Mn biometallogenesis.

Abiotic synthesis of graphite in hydrothermal vents

Deciphering the origin, age, and composition of deep marine organic carbon remains a challenge in understanding the dynamics of the marine carbon cycle. In particular, the composition of aged organic carbon and what allows its persistence in the deep ocean and in sediment is unresolved. Here, we observe that both high and low temperature hydrothermal vents at the 9° 50′ N; 104° 17.5 W East Pacific Rise (EPR) vent field are a source for (sub)micron-sized graphite particles. We demonstrate that commonly applied analytical techniques for quantification of organic carbon detect graphite. These analyses thereby classify graphite as either dissolved or particulate organic carbon, depending on the particle size and filtration method, and overlook its relevance as a carbon source to the deep ocean. Settling velocity calculations indicate the potential for these (sub)micron particles to become entrained in the buoyant plume and distributed far from the vent fields. Thus, our observations provide direct evidence for hydrothermal vents acting as a source of old carbon to the deep ocean.

Single Cell Genomics-Based Analysis of Gene Content and Expression of Prophages in a Diffuse-Flow Deep-Sea Hydrothermal System.

Phage–host interactions likely play a major role in the composition and functioning of many microbiomes, yet remain poorly understood. Here, we employed single cell genomics to investigate phage–host interactions in a diffuse-flow, low-temperature hydrothermal vent that may be reflective of a broadly distributed biosphere in the subseafloor. We identified putative prophages in 13 of 126 sequenced single amplified genomes (SAGs), with no evidence for lytic infections, which is in stark contrast to findings in the surface ocean. Most were distantly related to known prophages, while their hosts included bacterial phyla Campylobacterota, Bacteroidetes, Chlorobi, Proteobacteria, Lentisphaerae, Spirochaetes, and Thermotogae. Our results suggest the predominance of lysogeny over lytic interaction in diffuse-flow, deep-sea hydrothermal vents, despite the high activity of the dominant Campylobacteria that would favor lytic infections. We show that some of the identified lysogens have co-evolved with their host over geological time scales and that their genes are transcribed in the environment. Functional annotations of lysogeny-related genes suggest involvement in horizontal gene transfer enabling host’s protection against toxic metals and antibacterial compounds.

Fluid geochemistry, local hydrology, and metabolic activity define methanogen community size and composition in deep-sea hydrothermal vents

The size and biogeochemical impact of the subseafloor biosphere in oceanic crust remain largely unknown due to sampling limitations. We used reactive transport modeling to estimate the size of the subseafloor methanogen population, volume of crust occupied, fluid residence time, and nature of the subsurface mixing zone for two low-temperature hydrothermal vents at Axial Seamount. Monod CH4 production kinetics based on chemostat H2 availability and batch-culture Arrhenius growth kinetics for the hyperthermophile Methanocaldococcus jannaschii and thermophile Methanothermococcus thermolithotrophicus were used to develop and parameterize a reactive transport model, which was constrained by field measurements of H2, CH4, and metagenome methanogen concentration estimates in 20–40 °C hydrothermal fluids. Model results showed that hyperthermophilic methanogens dominate in systems where a narrow flow path geometry is maintained, while thermophilic methanogens dominate in systems where the flow geometry expands. At Axial Seamount, the residence time of fluid below the surface was 29–33 h. Only 1011 methanogenic cells occupying 1.8–18 m3 of ocean crust per m2 of vent seafloor area were needed to produce the observed CH4 anomalies. We show that variations in local geology at diffuse vents can create fluid flow paths that are stable over space and time, harboring persistent and distinct microbial communities.

Dissolved organic carbon compounds in deep-sea hydrothermal vent fluids from the East Pacific Rise at 9°50′N

Deep-sea hydrothermal vents are unique ecosystems that may release chemically distinct dissolved organic matter to the deep ocean. Here, we describe the composition and concentrations of polar dissolved organic compounds observed in low and high temperature hydrothermal vent fluids at 9°50′N on the East Pacific Rise. The concentration of dissolved organic carbon was 46 µM in the low temperature hydrothermal fluids and 14 µM in the high temperature hydrothermal fluids. In the low temperature vent fluids, quantifiable dissolved organic compounds were dominated by water-soluble vitamins and amino acids. Derivatives of benzoic acid and the organic sulfur compound 2,3-dihydroxypropane-1-sulfonate (DHPS) were also present in low and high temperature hydrothermal fluids. The low temperature vent fluids contain organic compounds that are central to biological processes, suggesting that they are a by-product of biological activity in the subseafloor. These compounds may fuel heterotrophic and other metabolic processes at deep-sea hydrothermal vents and beyond.

Metagenomic investigation of vestimentiferan tubeworm endosymbionts from Mid-Cayman Rise reveals new insights into metabolism and diversity

BackgroundThe microbial endosymbionts of two species of vestimentiferan tubeworms (Escarpia sp. and Lamellibrachia sp.2) collected from an area of low-temperature hydrothermal diffuse vent flow at the Mid-Cayman Rise (MCR) in the Caribbean Sea were characterized using microscopy, phylogenetic analyses, and a metagenomic approach.ResultsBacteria, with a typical Gram negative cell envelope contained within membrane-bound vacuoles, were observed within the trophosome of both tubeworm species. Phylogenetic analysis of the 16S rRNA gene and ITS region suggested MCR individuals harbored highly similar endosymbionts that were > 98% identical, with the exception of two symbionts that showed a 60 bp insertion within the ITS region. All sequences from MCR endosymbionts formed a separate well-supported clade that diverged from those of symbionts of seep and vent vestimentiferans from the Pacific, Gulf of Mexico, and Mediterranean Sea. The metagenomes of the symbionts of two specimens of each tubeworm species were sequenced, and two distinct Gammaproteobacteria metagenome-assembled genomes (MAGs) of more than 4 Mbp assembled. An Average Nucleotide Identity (ANI) of 86.5% between these MAGs, together with distinct 16S rRNA gene and ITS sequences, indicate the presence of multiple endosymbiont phylotypes at the MCR, with one MAG shared between one Escarpia and two Lamellibrachia individuals, indicating these endosymbionts are not specific to either host species. Genes for sulfur and hydrogen oxidation, nitrate reduction (assimilatory and dissimilatory), glycolysis and the Krebs cycle, peptide, sugar, and lipid transporters, and both rTCA and CBB carbon fixation cycles were detected in the MAGs, highlighting key and shared functions with symbiont metagenomes of the vestimentiferans Riftia, Tevnia, and Ridgeia from the Pacific. The potential for a second hydrogen oxidation pathway (via a bidirectional hydrogenase), formate dehydrogenase, a catalase, and several additional peptide transporters were found exclusively in the MCR endosymbiont MAGs.ConclusionsThe present study adds new evidence that tubeworm endosymbionts can potentially switch from autotrophic to heterotrophic metabolism, or may be mixotrophic, presumably while free-living, and also suggests their versatile metabolic potential may enable both the host and symbionts to exploit a wide range of environmental conditions. Together, the marked gene content and sequence dissimilarity at the rRNA operon and whole genome level between vent and seep symbionts suggest these newly described endosymbionts from the MCR belong to a novel tubeworm endosymbiont genera, introduced as Candidatus Vondammii.

Geochemical and iron isotopic insights into hydrothermal iron oxyhydroxide deposit formation at Loihi Seamount

Low-temperature hydrothermal vents, such as those encountered at Loihi Seamount, harbor abundant microbial communities and provide ideal systems to test hypotheses on biotic versus abiotic formation of hydrous ferric oxide (FeOx) deposits at the seafloor. Hydrothermal activity at Loihi Seamount produces abundant microbial mats associated with rust-colored FeOx deposits and variably encrusted with Mn-oxyhydroxides. Here, we applied Fe isotope systematics together with major and trace element geochemistry to study the formation mechanisms and preservation of such mineralized microbial mats. Iron isotope composition of warm (<60 °C), Fe-rich and H2S-depleted hydrothermal fluids yielded δ56Fe values near +0.1‰, indistinguishable from basalt values. Suspended particles in the vent fluids and FeOx deposits recovered nearby active vents yielded systematically positive δ56Fe values. The enrichment in heavy Fe isotopes between +1.05‰ and +1.43‰ relative to Fe(II) in vent fluids suggest partial oxidation of Fe(II) during mixing of the hydrothermal fluid with seawater. By comparing the results with experimentally determined Fe isotope fractionation factors, we determined that less than 20% of Fe(II) is oxidized within active microbial mats, although this number may reach 80% in aged or less active deposits. These results are consistent with Fe(II) oxidation mediated by microbial processes considering the expected slow kinetics of abiotic Fe oxidation in low oxygen bottom water at Loihi Seamount. In contrast, FeOx deposits recovered at extinct sites have distinctly negative Fe-isotope values down to −1.77‰ together with significant enrichment in Mn and occurrence of negative Ce anomalies. These results are best explained by the near-complete oxidation of an isotopically light Fe(II) source produced during the waning stage of hydrothermal activity under more oxidizing conditions. Light Fe isotope values of FeOx are therefore generated by subsurface precipitation of isotopically heavy Fe-oxides rather than by the activity of dissimilatory Fe reduction in the subsurface. Overall, Fe-isotope compositions of microbial mats at Loihi Seamount display a remarkable range between −1.2‰ and +1.6‰ which indicate that Fe isotope compositions of hydrothermal Fe-oxide precipitates are particularly sensitive to local environmental conditions where they form, and are less sensitive to abiotic versus biotic origins. It follows that FeOx deposits at Loihi Seamount provides important modern analogues for ancient seafloor Fe-rich deposits allowing for testing hypotheses about the biogeochemical cycling of Fe isotopes on early Earth.

Bringing microbial diversity into focus: high-resolution analysis of iron mats from the Lō'ihi Seamount.

Thirty kilometers south of the island of Hawai'i lies the Lō'ihi Seamount, an active submarine volcano that hosts a network of low-temperature hydrothermal vents enriched in ferrous iron that supports extensive microbial mats. These mats, which can be a half a meter deep, are composed of ferric iron bound to organic polymers - the metabolic byproduct of iron-oxidizing Zetaproteobacteria. Though the role of Zetaproteobacteria in mat formation is well established, we have a limited understanding of how differences in diversity are related to mat morphology. We used Minimum Entropy Decomposition and ZetaOtu classification to demonstrate cryptic diversity between closely related Zetaproteobacteria while showing habitat and geographic specificity. Veiled mats, common structures at Lō'ihi, exhibit distinct community composition and contain diversity not detected in other mat types, including specific Zetaproteobacteria and an unclassified Gammaproteobacteria. Our analyses also indicate that diversity can change dramatically across small spatial transects from points of active venting, yet we found comparatively few differences between major sampling sites. This study provides a better picture of the microbiome responsible for iron mat production at Lō'ihi and has broad implications for our understanding of these globally distributed communities.

Environmental controls on biomineralization and Fe-mound formation in a low-temperature hydrothermal system at the Jan Mayen Vent Fields

Diffuse low-temperature hydrothermal vents on the seafloor host neutrophilic microaerophilic Fe-oxidizing bacteria that utilize the Fe(II) supplied by hydrothermal fluids and produce intricate twisted and branching extracellular stalks. The growth behavior of Fe-oxidizing bacteria in strongly opposing gradients of Fe(II) and O2 have been thoroughly investigated in laboratory settings to assess whether extracellular stalks and aligned biomineralized fabrics may serve as biosignatures of Fe-oxidizing bacteria and indications of palaeo-redox conditions in the rock record. However, the processes controlling the growth of biogenic Fe-oxyhydroxide deposits in natural, modern hydrothermal systems are still not well constrained. In this study, we aimed to establish how variations in the texture of stratified hydrothermal Fe-oxyhydroxide deposits are linked to the physicochemical conditions of the hydrothermal environment. We conducted 16S rRNA gene analyses, microscopy and geochemical analyses of laminated siliceous Fe-mounds from the Jan Mayen Vent Fields at the Arctic Mid-Ocean Ridge. Chemical analyses of low- and high-temperature hydrothermal fluids were performed to characterize the hydrothermal system in which the Fe-deposits form. Our results reveal synchronous inter-laminar variations in texture and major and trace element geochemistry. The Fe-deposits are composed of alternating porous laminae of mineralized twisted stalks and branching tubes, Mn-rich horizons with abundant detrital sediment, domal internal cavities and thin P- and REE-enriched lamina characterized by networks of ≪1μm wide fibers. Zetaproteobacteria constitute one third of the microbial community in the surface layer of actively forming mounds, indicating that microbial Fe-oxidation is contributing to mound accretion. We suggest that Mn-oxide precipitation and detrital sediment accumulation take place during periodically low hydrothermal fluid discharge conditions. The elevated concentrations of P and REE in distinct laminae suggest Fe-cycling and accumulation of diagenetic species at depth in the deposits during hydrothermal quiescence and co-precipitation of these species with Fe-oxyhydroxides at the mound surface with reinitiated hydrothermal discharge. The origin of the low-temperature hydrothermal source fluid and the Fe-deposits are evident by low LREE/HREE ratios and negative Eu-anomalies, which clearly differ from the LREE and Eu enrichment of nearby high-temperature white smoker venting fluids. Our study demonstrates that hydrothermal fluctuations exert the primary control on the formation of laminae and the activity of Fe-oxidizing bacteria in marine hydrothermal Fe-deposits and indicates that REE-patterns may be used to distinguish high-temperature plume fallout and biomineralized low-temperature Fe-deposits in the rock record.

Multiple metal sources in the glaciomarine facies of the Neoproterozoic Jacadigo iron formation in the “Santa Cruz deposit”, Corumbá, Brazil

The Rapitan-type banded iron formation (BIF) in the Banda Alta Formation (Fm) of the Neoproterozoic Jacadigo Group in Brazil was deposited in a redox-stratified, marine sub-basin, which was strongly influenced by glacial advance/retraction cycles with temporary influx of continental freshwater and upwelling metal-enriched seawater from deeper anoxic parts. These new finding are based on new stratigraphic, whole-rock geochemical, and stable Fe and C isotope data from the “Santa Cruz” hematite deposit near Corumbá, Mato Grosso do Sul, where a stratigraphy of lower and upper dolomite-rich and intermediate chert-rich BIF facies with up to three intercalated diamictites is revealed. The Ca-Mg-Fe-Mn-carbonate-chert-hematite and chert-hematite BIF (∼30–45wt% Fe) of the lower dolomite-rich facies shows chemical signatures consistent with well-oxidized seawater, i.e. low (Pr/Yb)PAAS, strong negative CePAAS-, and positive GdPAAS- and YPAAS-anomalies, as well as negative-δ13C carbonate typical for Neoproterozoic glaciogenic carbonates. Sedimentation in a rather shallow water depth during relatively warm interglacial periods was likely influenced by abundant freshwater from fluviatile runoff and melting icebergs. In such conditions abundant microbial activity accommodated CO2 sequestration in carbonates as spheroids and mats and fractionated δ57Fe (−2.6 and −1.2‰) in primary Fe-hydroxides. In contrast, the intermediate chert-rich facies, characterized by chert-hematite BIF (∼35–55wt% Fe) and isolated hematite chert and hematite mud, recorded trace element signatures of non-oxidized (absence of Ce anomalies and variable YPAAS-anomalies), more metal-enriched (Fe, Mn, Si, Ni, Zn, Pb, U) seawater, thus deposition was within and below the shallow-level redoxcline during ice cover. Colder water and isolation from sunlight reduced microbial activity, and thus an almost non-fractionated δ57Fe (−0.7 to 0.0‰) fluid signature reveals that low-temperature (based on almost absence of Eu anomaly in BIF) hydrothermal (MOR) vents or (sub-) seafloor alteration of mafic or felsic rocks, or shales fertilized seawater with metals. All results lead to the model that the Jacadigo Group formed during one major marine transgression-regression cycle, and BIF facies in the Banda Alta Fm were a response to first- and second-order periodic variations of the depth of the redoxcline, induced by the juxtaposition of glacial advance/retraction cycles, active graben tectonics, and glacial isostatic adjustment or eustatic water level changes. The chert-rich BIF facies marked the maximum of transgression. Age and tectonic setting of the Jacadigo basin remains contentious: it may represent a continental back arc basin of the Brasiliano collision zone (∼590Ma), in which submarine alteration was possibly related to low-temperature hydrothermal fluids moving through the active graben system (as an alternative to basin-wide benthic pore water flux). This would place the Jacadigo group to the suite of Ediacarian “Gaskier” glaciations, although a relationship to the Marinoan glaciation (660–635Ma), mainly based on the published sedimentation age-bracket of 706–587Ma, is also possible.

Microbial Communities on Seafloor Basalts at Dorado Outcrop Reflect Level of Alteration and Highlight Global Lithic Clades.

Areas of exposed basalt along mid-ocean ridges and at seafloor outcrops serve as conduits of fluid flux into and out of a subsurface ocean, and microbe–mineral interactions can influence alteration reactions at the rock–water interface. Located on the eastern flank of the East Pacific Rise, Dorado Outcrop is a site of low-temperature (<20°C) hydrothermal venting and represents a new end-member in the current survey of seafloor basalt biomes. Consistent with prior studies, a survey of 16S rRNA gene sequence diversity using universal primers targeting the V4 hypervariable region revealed much greater richness and diversity on the seafloor rocks than in surrounding seawater. Overall, Gamma-, Alpha-, and Deltaproteobacteria, and Thaumarchaeota dominated the sequenced communities, together making up over half of the observed diversity, though bacterial sequences were more abundant than archaeal in all samples. The most abundant bacterial reads were closely related to the obligate chemolithoautotrophic, sulfur-oxidizing Thioprofundum lithotrophicum, suggesting carbon and sulfur cycling as dominant metabolic pathways in this system. Representatives of Thaumarchaeota were detected in relatively high abundance on the basalts in comparison to bottom water, possibly indicating ammonia oxidation. In comparison to other sequence datasets from globally distributed seafloor basalts, this study reveals many overlapping and cosmopolitan phylogenetic groups and also suggests that substrate age correlates with community structure.

Phylogenetic diversity of nitrogen-utilizing genes in hydrothermal chimneys from 3 middle ocean ridges.

Nitrogen-metabolizing genes, including nitrogenase (nifH), periplasmic nitrate reductase (napA), and cytochrome cd 1-type nitrite reductase (nirS), were collected from hydrothermal chimney sulfides on 3 middle ocean ridges and compared for the first time. There was a clear phylogenetic distinction of these nifH genes between different hydrothermal ecosystems, which supported the colonization and potential adaptation by different nitrogen fixing microbes in those sulfides. In particular, in sulfides from low-temperature hydrothermal vents of the Southwest Indian Ocean Ridge, the prevalence of nifH genes appears to be attributed to sulfate-reducing bacteria, suggesting their ecological significance. Phylogenetic analysis of nitrate/nitrite reductase genes indicated that nitrate was a critical electron acceptor for sulfur- or metal-oxidizing bacteria in these hydrothermal ecosystems. Our results provided information about the compositions and diversity of the 3 important genes involved in nitrogen fixation and nitrate/nitrite reduction processes in hydrothermal ecosystems and is the first comprehensive genetic repertoire of genes related to potential nitrogen fixation and denitrification processes in various hydrothermal environments.

Coupled Ge/Si and Ge isotope ratios as geochemical tracers of seafloor hydrothermal systems: Case studies at Loihi Seamount and East Pacific Rise 9°50′N

Germanium (Ge) and Silicon (Si) exhibit similar geochemical behavior in marine environments but are variably enriched in seafloor hydrothermal fluids relative to seawater. In this study, Ge isotope and Ge/Si ratio systematics were investigated in low temperature hydrothermal vents from Loihi Seamount (Pacific Ocean, 18°54′N, 155°15′W) and results were compared to high-temperature vents from the East Pacific Rise (EPR) at 9°50′N. Loihi offers the opportunity to understand contrasting Ge and Si behavior in low temperature seafloor hydrothermal systems characterized by abundant Fe oxyhydroxide deposition at the seafloor. The results show that both Ge/Si and δ74/70Ge in hydrothermal fluids are fractionated relative to the basaltic host rocks. The enrichment in Ge vs. Si relative to fresh basalts, together with Ge isotope fractionation (Δ74/70Gefluid-basalt up to 1.15‰ at EPR 9°50′N and 1.64‰ at Loihi) are best explained by the precipitation of minerals (e.g. quartz and Fe-sulfides) during higher temperature seawater–rock reactions in the subsurface. The study of Fe-rich hydrothermal deposits at Loihi, largely composed of Fe-oxyhydroxides, shows that Ge isotopes are also fractionated upon mineral precipitation at the seafloor. We obtained an average Ge isotope fractionation factor between Fe-oxyhydroxide (ferrihydrite) and dissolved Ge in the fluid of −2.0±0.6‰ (2sd), and a maximum value of −3.6±0.6‰ (2sd), which is consistent with recent theoretical and experimental studies. The study of a hydrothermal chimney at Bio 9 vent at EPR 9°50′N also demonstrates that Ge isotopes are fractionated by approximately −5.6±0.6‰ (2sd) during precipitation of metal sulfides under hydrothermal conditions. Using combined Ge/Si and estimated Ge isotope signatures of Ge sinks and sources in seawater, we propose a preliminary oceanic budget of Ge which reveals that an important sink, referred as the “missing Ge sink”, may correspond to Ge sequestration into authigenic Fe-oxyhydroxides in marine sediments. This study shows that combining Ge/Si and δ74/70Ge systematics provides a useful tool to trace hydrothermal Ge and Si sources in marine environments and to understand formation processes of seafloor hydrothermal deposits.

Quantitative PCR analysis of functional genes in iron-rich microbial mats at an active hydrothermal vent system (Lō'ihi Seamount, Hawai'i).

The chemolithotrophic Zetaproteobacteria represent a novel class of Proteobacteria which oxidize Fe(II) to Fe(III) and are the dominant bacterial population in iron-rich microbial mats. Zetaproteobacteria were first discovered at Lō'ihi Seamount, located 35 km southeast off the big island of Hawai'i, which is characterized by low-temperature diffuse hydrothermal venting. Novel nondegenerate quantitative PCR (qPCR) assays for genes associated with microbial nitrogen fixation, denitrification, arsenic detoxification, Calvin-Benson-Bassham (CBB), and reductive tricarboxylic acid (rTCA) cycles were developed using selected microbial mat community-derived metagenomes. Nitrogen fixation genes were not detected, but all other functional genes were present. This suggests that arsenic detoxification and denitrification processes are likely cooccurring in addition to two modes of carbon fixation. Two groups of microbial mat community types were identified by terminal restriction fragment length polymorphism (T-RFLP) and were further described based on qPCR data for zetaproteobacterial abundance and carbon fixation mode preference. qPCR variance was associated with mat morphology but not with temperature or sample site. Geochemistry data were significantly associated with sample site and mat morphology. Together, these qPCR assays constitute a functional gene signature for iron microbial mat communities across a broad array of temperatures, mat types, chemistries, and sampling sites at Lō'ihi Seamount.