Hydrogen Utilization Potential in Subsurface Sediments

Abstract

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.

Evidence for H2 consumption by uncultured Desulfobacterales in coastal sediments

“…Sulfate respiration is generally considered the most important H 2 ‐scavenging process in marine sediments and is an energy source for SRM in Wadden Sea sediments (Finke and Jørgensen, ) and likely also at site Janssand. The potential H 2 oxidation rates we measured in anoxic slurries were in a similar range as those in other anoxic aquatic sediments and clearly exceeded those commonly measured in (oxic) surface layers (Nedwell and Banat, ; Adhikari et al ., ).…”

“…To further resolve the H 2 -consuming SRM community, it will be of major importance to unravel, how H 2 ase gene expression reflects the actual in situ process. To this end, molecular approaches could be integrated with measurements of in situ 3 H 2 turnover (Adhikari et al, 2016).…”

Microbial Sulfate Reduction Potential in Coal-Bearing Sediments Down to ~2.5 km below the Seafloor off Shimokita Peninsula, Japan

“…Thus, in contrast to microbial activity measurements, it does not require metabolically active cells. However, immediate deep-freezing is important to preserve the enzymes in the sediment and prevent activity loss by oxidation (see Adhikari et al, 2016 for details). These samples were shipped deep-frozen to the home laboratory for shore-based hydrogenase enzyme activity measurements (Supplementary Table 2 ).…”

“…A tritium–based hydrogenase enzyme assay was used to measure potential oxidation rates of hydrogen (Soffientino et al, 2009 ; Adhikari et al, 2016 ) at the University of Bremen. In short, three replicates and a killed control were prepared from frozen sediment for the incubation experiment using anoxic autoclaved synthetic seawater medium.…”

“…100 μL of the supernatant was mixed with 4 mL liquid scintillation fluid (Perkin Elmer Ultima Gold™ LLT) and the radioactivity was measured in a liquid scintillation analyzer (Perkin Elmer TriCarb® TR2810). The increase in radioactivity over time was used as a measure of the potential hydrogenase activity (see (Adhikari et al, 2016 ) for details).…”