Kinetics of H2–O2–H2O redox equilibria and formation of metastable H2O2 under low temperature hydrothermal conditions

Author Posting. © The Author(s), 2010. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 75 (2011): 1594-1607, doi:10.1016/j.gca.2010.12.020.Hydrothermal experiments were conducted to evaluate the kinetics of H2(aq) oxidation in the homogeneous H2-O2-H2O system at conditions reflecting subsurface/near-seafloor hydrothermal environments (55-250 oC and 242-497 bar). The kinetics of the water-forming reaction that controls the fundamental equilibrium between dissolved H2(aq) and O2(aq), are expected to impose significant constraints on the redox gradients that develop when mixing occurs between oxygenated seawater and high- temperature anoxic vent fluid at near-seafloor conditions. Experimental data indicate that, indeed, the kinetics of H2(aq)-O2(aq) equilibrium become slower with decreasing temperature, allowing excess H2(aq) to remain in solution. Sluggish reaction rates of H2(aq) oxidation suggest that active microbial populations in near-seafloor and subsurface environments could potentially utilize both H2(aq) and O2(aq), even at temperatures lower than 40 oC due to H2(aq) persistence in the seawater/vent fluid mixtures. For these H2-O2 disequilibrium conditions, redox gradients along the seawater/hydrothermal fluid mixing interface are not sharp and microbially-mediated H2(aq) oxidation coupled with a lack of other electron acceptors (e.g. nitrate) could provide an important energy source available at low-temperature diffuse flow vent sites. More importantly, when H2(aq)-O2(aq) disequilibrium conditions apply, formation of metastable hydrogen peroxide is observed. The yield of H2O2(aq) synthesis appears to be enhanced under conditions of elevated H2(aq)/O2(aq) molar ratios that correspond to abundant H2(aq) concentrations. Formation of metastable H2O2 is expected to affect the distribution of dissolved organic carbon (DOC) owing to the existence of an additional strong oxidizing agent. Oxidation of magnetite and/or Fe++ by hydrogen peroxide could also induce formation of metastable hydroxyl radicals (•OH) through Fenton-type reactions, further broadening the implications of hydrogen peroxide in hydrothermal environments.This research was conducted with partial support from the NSF OCE-0752221 and the Geophysical Laboratory Postdoctoral Fellowship. We would also like to acknowledge contributions by the W.M. Keck Foundation and Shell towards supporting the hydrothermal lab at the Geophysical Lab. SMS acknowledges support from NSF OCE-0452333 and the Alfried-Krupp Wissenschaftskolleg Greifswald (Germany), while WES acknowledges support from NSF grants OCE-0549457 and OCE- 0813861

Eco-geochemical dynamics of a shallow-water hydrothermal vent system at Milos Island, Aegean Sea (Eastern Mediterranean)

Shallow-water hydrothermal vents are extreme environments that share many characteristics with their deep-sea analogs. However, despite ease of access, much less is known about the geochemical dynamics of these ecosystems. Here, we report on the spatial and temporal geochemical dynamics of a shallow-water vent system at Paleochori Bay, Milos Island, Greece. Our multi-analyte voltammetric profiles of dissolved O2 and hydrothermal tracers (e.g. Fe2 +, FeSaq, Mn2 +) on sediment cores taken along a transection in hydrothermally affected sediments indicate three different areas: the central vent area (highest temperature) with a deeper penetration of oxygen into the sediment, and a lack of dissolved Fe2 + and Mn2 +; a middle area (0.5 m away) rich in dissolved Fe2 + and Mn2 + (exceeding 2 mM) and high free sulfide with potential for microbial sulfide oxidation as suggested by the presence of white mats at the sediment surface; and, finally, an outer rim area (1–1.5 m away) with lower concentrations of Fe2 + and Mn2 + and higher signals of FeSaq, indicating an aged hydrothermal fluid contribution. In addition, high-frequency temperature series and continuous in situ H2S measurements with voltammetric sensors over a 6-day time period at a distance 0.5 m away from the vent center showed substantial variability in temperature (31.6 to 46.4 °C) and total sulfide (488 to 1329 μM) in the upper sediment layer. The analysis of these data suggests that tidal and wind forcing, and abrupt geodynamic events generate intermittent mixing conditions lasting for several hours to days. Despite substantial variability, the concentration of sulfide available for chemoautotrophic microbes remained high. However, the availability of electron acceptors originating from seawater might be more intermittent, which in turn has an effect on the reestablishment of the white mats after wave-induced disturbances. Our results emphasize the importance of transient events in the development of chemoautotrophic communities in the hydrothermally influenced sediments of Paleochori Bay

Ammonia oxidation, denitrification and dissimilatory nitrate reduction to ammonium in two US Great Basin hot springs with abundant ammonia-oxidizing archaea

Summary Many thermophiles catalyse free energy-yielding redox reactions involving nitrogenous compounds; however, little is known about these processes in natural thermal environments. Rates of ammonia oxidation, denitrification and dissimilatory nitrate reduction to ammonium (DNRA) were measured in source water and sediments of two ∼80°C springs in the US Great Basin. Ammonia oxidation and denitrification occurred mainly in sediments. Ammonia oxidation rates measured using 15N-NO3- pool dilution ranged from 5.5 ± 0.8 to 8.6 ± 0.9 nmol N g−1 h−1 and were unaffected or only mildly stimulated by amendment with NH4Cl.Denitrification rates measured using acetylene block ranged from 15.8 ± 0.7 to 51 ± 12 nmol N g−1 h−1 and were stimulated by amendment with NO3- and complex organic compounds. The DNRA rate in one spring sediment measured using an 15N-NO3- tracer was 315 ± 48 nmol N g−1 h−1. Both springs harboured distinct planktonic and sediment microbial communities. Close relatives of the autotrophic, ammonia-oxidizing archaeon \u27Candidatus Nitrosocaldus yellowstonii\u27 represented the most abundant OTU in both spring sediments by 16S rRNA gene pyrotag analysis. Quantitative PCR (qPCR) indicated that \u27Ca. N. yellowstonii\u27 amoA and 16S rRNA genes were present at 3.5-3.9 × 108 and 6.4-9.0 × 108 copies g−1 sediment. Potential denitrifiers included members of the Aquificales and Thermales. Thermus spp. comprised \u3c 1% of 16S rRNA gene pyrotags in both sediments and qPCR for T. thermophilus narG revealed sediment populations of 1.3-1.7 × 106 copies g−1 sediment. These data indicate a highly active nitrogen cycle (N-cycle) in these springs and suggest that ammonia oxidation may be a major source of energy fueling primary production