Subsurface microbial communities undertake many terminal electron-accepting processes, often simultaneously. Using a tritium-based assay, we measured the potential hydrogen oxidation catalyzed by hydrogenase enzymes in several subsurface sedimentary environments (Lake Van, Barents Sea, Equatorial Pacific, and Gulf of Mexico) with different predominant electron-acceptors. Hydrogenases constitute a diverse family of enzymes expressed by microorganisms that utilize molecular hydrogen as a metabolic substrate, product, or intermediate. The assay reveals the potential for utilizing molecular hydrogen and allows qualitative detection of microbial activity irrespective of the predominant electron-accepting process. Because the method only requires samples frozen immediately after recovery, the assay can be used for identifying microbial activity in subsurface ecosystems without the need to preserve live material. We measured potential hydrogen oxidation rates in all samples from multiple depths at several sites that collectively span a wide range of environmental conditions and biogeochemical zones. Potential activity normalized to total cell abundance ranges over five orders of magnitude and varies, dependent upon the predominant terminal electron acceptor. Lowest per-cell potential rates characterize the zone of nitrate reduction and highest per-cell potential rates occur in the methanogenic zone. Possible reasons for this relationship to predominant electron acceptor include (i) increasing importance of fermentation in successively deeper biogeochemical zones and (ii) adaptation of H2ases to successively higher concentrations of H2 in successively deeper zones.
Hydrogen (H2) is produced in geological settings by dissociation of water due to radiation from radioactive decay of naturally occurring uranium (238U, 235U), thorium (232Th) and potassium (40K). To quantify the potential significance of radiolytic H2 as an electron donor for microbes within the South Pacific subseafloor basaltic aquifer, we use radionuclide concentrations of 43 basalt samples from IODP Expedition 329 to calculate radiolytic H2 production rates in basement fractures. The samples are from three sites with very different basement ages and a wide range of alteration types. U, Th, and K concentrations vary by up to an order of magnitude from sample to sample at each site. Comparison of our samples to each other and to the results of previous studies of unaltered East Pacific Rise basalt suggests that significant variations in radionuclide concentrations are due to differences in initial (unaltered basalt) concentrations (which can vary between eruptive events) and post-emplacement alteration. However, there is no clear relationship between alteration type and calculated radiolytic yields. Local maxima in U, Th, and K produce hotspots of H2 production, causing calculated radiolytic rates to differ by up to a factor of 80 from sample to sample. Fracture width also greatly influences H2 production, where microfractures are hotspots for radiolytic H2 production. For example, H2 production rates normalized to water volume are 190 times higher in 1 μm wide fractures than in fractures that are 10 cm wide. To assess the importance of water radiolysis for microbial communities in subseafloor basaltic aquifers, we compare electron transfer rates from radiolysis to rates from iron oxidation in subseafloor basalt. Radiolysis appears likely to be a more important electron donor source than iron oxidation in old (>10 Ma) basement basalt. Radiolytic H2 production in the volume of water adjacent to a square cm of the most radioactive SPG basalt may support as many as 1500 cells.
The nature of the energy yielding mechanisms in the lowenergy organic-poor sedimentary environment underlying the South Pacific Gyre (SPG) is not fully constrained. We used the approach of Wang et al. (2008) to quantify rates of organic-fuelled metabolic activities at most IODP Expedition 329 Sites (U1365 through U1370). At Site U1366 and U1370 net rates of oxygen-reducing organic oxidation averaged 1.77E-2 and 1.64E-3 fmol O2 cell-1 yr-1, respectively, representing a tremendously low cellular metabolism. At Site U1370, we observe net oxygen reduction throughout the entire sediment column. At Site U1366, statistically significant net oxygen reduction is not detected at depths greater than 11 meters below seafloor. Despite these low rates of organic oxidation, most cell counts are above the minimum detection limit throughout the entire sequence at both sites. Hydrogen from natural radioactive splitting of water has been hypothesized to be a significant electron donor in organic-poor sediment of the SPG. Becauses water radiolysis produces H2 and - O2 simultaneously, oxidation of this H2 does not contribute to net O2 reduction in the sediment. Our calculation of radiolytic H2 production, based on radioactive element content and sediment physical properties, indicate that on average 5.63E-1 and 9.79E-2 fmol H2 yr-1 cell-1 is available throughout the sequence at Sites U1366 and U1370, respectively. Despite these relatively high production rates, dissolved H2 abundances are below detection at both sites. These results suggest that H2 from in situ water radiolysis fuels the predominant energy-yielding pathway for microbes in SPG sediment.
Concentrations of uranium (U), thorium (Th), and potassium (K) in geological materials provide insight into many important lithological characteristics and geologic processes. In marine sediment, they can aid in identifying clay compositions, depositional environments, and diagenetic processes. They can also yield information about the alteration and heat production of rocks (Ketcham 1996; Barr et al., 2002; Revillon et al., 2002; Brady et al., 2006; Bartetzko, 2008). Measurements of the concentrations of these elements in geological materials are relatively straightforward in shore-based laboratories. Rapidly determining their abundance within cores of sedimentary and igneous rock sequences onboard a research vessel is a more challenging but potentially very useful method to non-destructively and quickly provide important geochemical information about the concentrations of U, Th, and K within the sequences being cored.