Towards constraining H2 concentration in subseafloor sediment: A proposal for combined analysis by two distinct approaches

Abstract

Molecular hydrogen (H2) is a central metabolite that couples organic matter degradation and terminal electron-accepting processes. H2 levels in natural environments are often regulated by microbial syntrophy; therefore, pore-water H2 concentration is a useful parameter for studying biogeochemical processes in sediments. However, little is known about H2 concentrations in marine subsurface sediments. Previous studies applying either a headspace equilibration technique or an extraction method for the analysis of pore-water H2 in deeply buried sediments have generated results that sometimes contradict the principles established based on studies of microbial culture and surface sediments. In this study, we first evaluated and optimized an extraction method, which was then applied in combination with a headspace equilibration method to determine concentrations of pore-water H2 in subseafloor sediments along a transect of five sites of different water depths and geochemical regimes at the continental margin off Namibia, SE Atlantic. The two methods generated depth profiles with some similarities in curve shape, but the extraction method yielded higher H2 values than the headspace equilibration technique. By comparing the two data sets with thermodynamic calculations of potential terminal electron-accepting processes, we were able to provide a first evaluation of syntrophic conditions in subseafloor sediment from the perspective of H2 biogeochemistry. We observed that in the sulfate reduction zone, the H2 concentrations are higher than the H2 threshold allowed for the next most favorable terminal metabolism (methanogenesis), suggesting relaxation of coupling between H2-producing and H2-consuming activities at these depths. In contrast, the H2 concentrations in the upper methanogenic zone are low enough for methanogens to outcompete CO2-reducing acetogens. Our findings suggest the existence of varied extents of syntrophic H2 coupling in subseafloor sediment.