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Serpentinization, the reaction of water with ultramafic rock, produces reduced, hyperalkaline, and H₂-rich fluids that support a variety of hydrogenotrophic microbial metabolisms. Previous work indicates the occurrence of methanogenesis in fluids from the actively serpentinizing Samail Ophiolite in the Sultanate of Oman. While those fluids contain abundant H₂to fuel hydrogenotrophic methanogenesis (CO₂ + 4H₂➔ CH₄ + 2H₂O), the concentration of CO₂is very low due to the hyperalkalinity (> pH 11) and geochemistry of the fluids. As a result, species such as formate and acetate may be important as alternative methanogenic substrates. In this study we quantified the impact of inorganic carbon, formate and acetate availability for methanogenic metabolisms, across a range of fluid chemistries, in terms of (1) the potential diffusive flux of substrates to the cell, (2) the Affinity (Gibbs energy change) associated with methanogenic metabolism, and (3) the energy “inventory” per kg fluid. In parallel, we assessed the genomic potential for the conduct of those three methanogenic modes across the same set of fluids and consider the results within the quantitative framework of energy availability. We find that formatotrophic methanogenesis affords a higher Affinity (greater energetic yield) than acetoclastic and hydrogenotrophic methanogenesis in pristine serpentinized fluids and, in agreement with previous studies, find genomic evidence for a methanogen of the genusMethanobacteriumto carry out formatotrophic and hydrogenotrophic methanogenesis, with the possibility of even using bicarbonate as a supply of CO₂. Acetoclastic methanogenesis is also shown to be energetically favorable in these fluids, and we report the first detection of a potential acetoclastic methanogen of the familyMethanosarcinaceae, which forms a distinct clade with a genome from the serpentinizing seafloor hydrothermal vent field, Lost City. These results demonstrate the applicability of an energy availability framework for interpreting methanogen ecology in serpentinizing systems. 

Abstract We report the formation of minerals from the tochilinite-valleriite group (TVG) during laboratory serpentinization experiments conducted at 300 and 328 °C. Minerals in the TVG are composed of a mixture of sulfide and hydroxide layers that can contain variable proportions of Fe, Mg, Cu, Ni, and other cations in both layers. Members of this group have been observed as accessory minerals in several serpentinites, and have also been observed in association with serpentine minerals in meteorites. To our knowledge, however, TVG minerals have not previously been identified as reaction products during laboratory simulation of serpentinization. The serpentinization experiments reacted olivine with artificial seawater containing 34S-labeled sulfate, with a small amount of solid FeS also added to the 300 °C experiment. In both experiments, the predominant reaction products were chrysotile serpentine, brucite, and magnetite. At 300 °C, these major products were accompanied by trace amounts of the Ni-bearing TVG member haapalaite, Ni,Fe-sulfide (likely pentlandite), and anhydrite. At 328 °C, valleriite occurs rather than haapalaite and the accompanying Ni,Fe-sulfide is proportionally more enriched in Ni. Reduction of sulfate by H2 produced during serpentinization evidently provided a source of reduced S that contributed to formation of the TVG minerals and Ni,Fe-sulfides. The results provide new constraints on the conditions that allow precipitation of tochilinite-valleriite group minerals in natural serpentinites. 

We assessed the relationship between rates of biological energy utilization and the biomass sustained by that energy utilization, at both the organism and biosphere level. We compiled a dataset comprising >10,000 basal, field, and maximum metabolic rate measurements made on >2,900 individual species, and, in parallel, we quantified rates of energy utilization, on a biomass-normalized basis, by the global biosphere and by its major marine and terrestrial components. The organism-level data, which are dominated by animal species, have a geometric mean among basal metabolic rates of 0.012 W (g C)⁻¹and an overall range of more than six orders of magnitude. The biosphere as a whole uses energy at an average rate of 0.005 W (g C)⁻¹but exhibits a five order of magnitude range among its components, from 0.00002 W (g C)⁻¹for global marine subsurface sediments to 2.3 W (g C)⁻¹for global marine primary producers. While the average is set primarily by plants and microorganisms, and by the impact of humanity upon those populations, the extremes reflect systems populated almost exclusively by microbes. Mass-normalized energy utilization rates correlate strongly with rates of biomass carbon turnover. Based on our estimates of energy utilization rates in the biosphere, this correlation predicts global mean biomass carbon turnover rates of ~2.3 y⁻¹for terrestrial soil biota, ~8.5 y⁻¹for marine water column biota, and ~1.0 y⁻¹and ~0.01 y⁻¹for marine sediment biota in the 0 to 0.1 m and >0.1 m depth intervals, respectively.