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Symbioses between invertebrate animals and chemosynthetic bacteria form the basis of hydrothermal vent ecosystems worldwide. In the Lau Basin, deep-sea vent snails of the genus Alviniconchaassociate with either Gammaproteobacteria(A. kojimai, A. strummeri) or Campylobacteria(A. boucheti) that use sulfide and/or hydrogen as energy sources. While the A. bouchetihost–symbiont combination (holobiont) dominates at vents with higher concentrations of sulfide and hydrogen, the A. kojimaiand A. strummeriholobionts are more abundant at sites with lower concentrations of these reductants. We posit that adaptive differences in symbiont physiology and gene regulation might influence the observed niche partitioning between host taxa. To test this hypothesis, we used high-pressure respirometers to measure symbiont metabolic rates and examine changes in gene expression among holobionts exposed to in situ concentrations of hydrogen (H2: ~25 µM) or hydrogen sulfide (H2S: ~120 µM). The campylobacterialsymbiont exhibited the lowest rate of H2S oxidation but the highest rate of H2oxidation, with fewer transcriptional changes and less carbon fixation relative to the gammaproteobacterialsymbionts under each experimental condition. These data reveal potential physiological adaptations among symbiont types, which may account for the observed net differences in metabolic activity and contribute to the observed niche segregation among holobionts.

Symbioses between invertebrate animals and chemosynthetic bacteria form the basis of hydrothermal vent ecosystems worldwide. In the Lau Basin, deep-sea vent snails of the genus Alviniconchaassociate with either Gammaproteobacteria(A. kojimai, A. strummeri) or Campylobacteria(A. boucheti) that use sulfide and/or hydrogen as energy sources. While the A. bouchetihost–symbiont combination (holobiont) dominates at vents with higher concentrations of sulfide and hydrogen, the A. kojimaiand A. strummeriholobionts are more abundant at sites with lower concentrations of these reductants. We posit that adaptive differences in symbiont physiology and gene regulation might influence the observed niche partitioning between host taxa. To test this hypothesis, we used high-pressure respirometers to measure symbiont metabolic rates and examine changes in gene expression among holobionts exposed to in situ concentrations of hydrogen (H2: ~25?µM) or hydrogen sulfide (H2S: ~120?µM). The campylobacterialsymbiont exhibited the lowest rate of H2S oxidation but the highest rate of H2oxidation, with fewer transcriptional changes and less carbon fixation relative to the gammaproteobacterialsymbionts under each experimental condition. These data reveal potential physiological adaptations among symbiont types, which may account for the observed net differences in metabolic activity and contribute to the observed niche segregation among holobionts.

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 situpressure 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.Graphical Abstract

This paper reports the results of quantitative imaging using a stereoscopic, high‐speed camera system at two natural gas seep sites in the northern Gulf of Mexico during the Gulf Integrated Spill Research G07 cruise in July 2014. The cruise was conducted on the E/V Nautilususing the ROV Herculesfor in situ observation of the seeps as surrogates for the behavior of hydrocarbon bubbles in subsea blowouts. The seeps originated between 890 and 1190 m depth in Mississippi Canyon block 118 and Green Canyon block 600. The imaging system provided qualitative assessment of bubble behavior (e.g., breakup and coalescence) and verified the formation of clathrate hydrate skins on all bubbles above 1.3 m altitude. Quantitative image analysis yielded the bubble size distributions, rise velocity, total gas flux, and void fraction, with most measurements conducted from the seafloor to an altitude of 200 m. Bubble size distributions fit well to lognormal distributions, with median bubble sizes between 3 and 4.5 mm. Measurements of rise velocity fluctuated between two ranges: fast‐rising bubbles following helical‐type trajectories and bubbles rising about 40% slower following a zig‐zag pattern. Rise speed was uncorrelated with hydrate formation, and bubbles following both speeds were observed at both sites. Ship‐mounted multibeam sonar provided the flare rise heights, which corresponded closely with the boundary of the hydrate stability zone for the measured gas compositions. The evolution of bubble size with height agreed well with mass transfer rates predicted by equations for dirty bubbles. Seep bubbles survive to the boundary of the hydrate stability zoneRise speed of bubbles are correlated with the trajectoriesSize evolution of the hydrate bubbles agrees with predicted mass transfer rate for dirty bubbles

The exposure of mantle peridotite to water at crustal levels leads to a cascade of interconnected dissolution-precipitation and reduction-oxidation reactions-a process referred to as serpentinization. These reactions have major implications for microbial life through the provision of hydrogen (H2). To simulate incipient serpentinization under well-constrained conditions, we reacted centimeter-sized pieces of uncrushed harzburgite with chemically modified seawater at 300 °C and 35 MPa for ca. 1.5 yr (13 441 h), monitored changes in fluid chemistry over time, and examined the secondary mineralogy at the termination of the experiment. Approximately 4 mol% of the protolith underwent alteration forming serpentine, accessory magnetite, chlorite, and traces of calcite and heazlewoodite. Alteration textures bear remarkable similarities to those found in partially serpentinized abyssal peridotites. Neither brucite nor talc precipitated during the experiment. Given that the starting material contained ~4 times more olivine than orthopyroxene on a molar basis, mass balance requires that dissolution of orthopyroxene was significantly faster than dissolution of olivine. Coupled mass transfer of dissolved Si, Mg, and H+between olivine and orthopyroxene reaction fronts was driven by steep activity gradients and facilitated the precipitation of serpentine. Hydrogen was released in significant amounts throughout the entire experiment; however, the H2release rate decreased with time. Serpentinization consumed water but did not release significant amounts of dissolved species (other than H2) suggesting that incipient hydration reactions involved a volume increase of ~40%. The reduced access of water to fresh olivine surfaces due to filling of fractures and coating of primary minerals with alteration products led to decreased rates of serpentinization and H2release. While this concept might seem at odds with completely serpentinized seafloor peridotites, reaction-driven fracturing offers an intriguing solution to the seemingly self-limiting nature of serpentinization. Indeed, the reacted sample revealed several textural features diagnostic of incipient reaction-driven fracturing. We conclude that fracturing must have far reaching impacts on the rates of serpentinization and H2release in peridotite-hosted hydrothermal systems.

Carboxylic acids are ubiquitous in basinal brines from petroleum-producing sedimentary basins. Although the absolute concentrations of individual short-chain acids vary systematically over many orders of magnitude, relative abundances are characterized by approximately constant ratios. Laboratory experiments have demonstrated that oxidation of aqueous n-alkanes proceeds through a sequence of reactions involving alkene, alcohol, ketone, and carboxylic acid reaction intermediaries. The highly specific nature of these reactions allows the relative distribution of carboxylic acids produced during oxidation of n-alkanes to be predicted quantitatively as a function of petroleum composition. A model based on the results of laboratory experiments was developed to account for the thermogenic production of aqueous carboxylic acids in sedimentary basins.

The concept that aqueous CO2can be reduced to hydrocarbons abiotically during serpentinization of olivine has become widespread in the earth and planetary sciences. This process has been invoked to explain the occurrence of hydrocarbons in crystalline igneous rocks and proposed as a source of prebiotic organic compounds for the origin of life. We reevaluate this scenario through an experimental study of the reaction of dissolved CO2in the presence of olivine under hydrothermal conditions (300°C, 350 bar). Reduction of CO2to formate (HCOO−) was found to proceed rapidly, with H2generated from hydrothermal alteration of olivine serving as the reductant. The reverse reaction, decomposition of formic acid to CO2and H2, was also found to proceed rapidly. Although dissolved hydrocarbon concentrations increased throughout the experiments, isotopic labeling of dissolved CO2with 13C indicated that these compounds were primarily generated from reduced carbon compounds already present in olivine at the beginning of the experiment rather than by reduction of CO2. The only hydrocarbon product from reduction of CO2observed in the experiments was a small amount of methane (<0.04% conversion of dissolved CO2in more than 2500 h of heating). Comparison of the reaction products with thermodynamic data indicates that reactions between dissolved CO2and formate rapidly achieved metastable equilibrium at the experimental conditions, suggesting that similar reactions could control the concentration of formate in geologic fluids. The results indicate that the potential for abiotic formation of hydrocarbons during serpentinization may be much more limited than previously believed, and other mineral catalysts or vapor phase reactions may be required to explain many occurrences of abiotic hydrocarbons in serpentinites and igneous rocks.

The presence of sedimentary organic matter blanketing midocean ridge crests has a potentially strong impact on metal transport in hydrothermal vent fluids. To constrain the role of organic matter in metal mobility during hydrothermal sediment alteration, we reacted organic-rich diatomaceous ooze from Guaymas Basin, Gulf of California, and organic-poor hemipelagic mud from Middle Valley, northern Juan de Fuca Ridge, with seawater and a Na-Ca-K-Cl fluid of seawater chlorinity, at 275° to 400°C, 350 to 500 bars, and initial fluid: sediment mass ratios ranging from 1.6 to 9.8. Reaction of these fluids with both sediment types released CO2and high concentrations of ore-forming metals (Fe, Mn, Zn, Pb) to solution. Relatively low concentrations of Cu were observed in solution and likely reflect the reducing conditions that resulted from the presence of sedimentary organic matter. Both the concentrations of CO2and dissolved metals were lower in fluids reacted with Middle Valley sediment compared with aqueous concentrations in fluids reacted with Guaymas Basin sediment. During alteration of both sediment types, metal concentrations varied strongly as a function of temperature, increasing by up to an order of magnitude over the 75°C range of each experiment. Major element fluid chemistry and observed alteration assemblages suggest that during hydrothermal alteration of organic-lean sediment from Middle Valley a feldspar-quartz-illite mineral assemblage buffered in situ pH. In contrast, data from the experimental alteration of organic-rich Guaymas Basin sediment suggest that a calcite-plagioclase-quartz assemblage regulated in situ pH. Fluid speciation calculations suggest that in situ pH during Guaymas Basin sediment alteration was lower than during alteration of Middle Valley sediment and accounts for the substantially greater metal mobility at a given temperature and pressure during the former experiment. Comparison of our results with the results of basalt alteration experiments indicate that except for Cu, hydrothermal sediment alteration results in equal or greater concentrations of ore-forming metals at a given temperature and pressure. Accordingly, the presence of ore-forming metals in fluids currently venting from sediment-covered hydrothermal systems at concentrations substantially lower than in fluids from bare-rock systems may reflect chemical reequilibration during subsurface cooling within the sediment pile.

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