Abstract
Sulfate reduction is the predominant anaerobic microbial process of organic matter mineralization in marine sediments, with recent studies revealing that sulfate reduction not only occurs in sulfate-rich sediments, but even extends to deeper, methanogenic sediments at very low background concentrations of sulfate. Using samples retrieved off the Shimokita Peninsula, Japan, during the Integrated Ocean Drilling Program (IODP) Expedition 337, we measured potential sulfate reduction rates by slurry incubations with 35S-labeled sulfate in deep methanogenic sediments between 1276.75 and 2456.75 meters below the seafloor. Potential sulfate reduction rates were generally extremely low (mostly below 0.1 pmol cm−3 d−1) but showed elevated values (up to 1.8 pmol cm−3 d−1) in a coal-bearing interval (Unit III). A measured increase in hydrogenase activity in the coal-bearing horizons coincided with this local increase in potential sulfate reduction rates. This paired enzymatic response suggests that hydrogen is a potentially important electron donor for sulfate reduction in the deep coalbed biosphere. By contrast, no stimulation of sulfate reduction rates was observed in treatments where methane was added as an electron donor. In the deep coalbeds, small amounts of sulfate might be provided by a cryptic sulfur cycle. The isotopically very heavy pyrites (δ34S = +43‰) found in this horizon is consistent with its formation via microbial sulfate reduction that has been continuously utilizing a small, increasingly 34S-enriched sulfate reservoir over geologic time scales. Although our results do not represent in-situ activity, and the sulfate reducers might only have persisted in a dormant, spore-like state, our findings show that organisms capable of sulfate reduction have survived in deep methanogenic sediments over more than 20 Ma. This highlights the ability of sulfate-reducers to persist over geological timespans even in sulfate-depleted environments. Our study moreover represents the deepest evidence of a potential for sulfate reduction in marine sediments to date.
Highlights
Sulfate reduction is a globally important microbial process in anoxic marine sediments (Canfield, 1991; Jørgensen and Kasten, 2006; Bowles et al, 2014)
Rates measured in slurries with the amendment of methane to the headspace are shown in red, incubations without methane amendment in blue. potential sulfate reduction rates (pSRR) decreased with depth from ∼5 pmol cm−3 d−1 in the upper 1–2 mbsf down to 0.7 pmol cm−3 d−1 in the deepest samples down to 346.28 mbsf
Potential Sulfate Reduction Rates in the Deep Sediments pSRR could be determined in most sample replicates from the cores between 1276.75 and 2456.72 mbsf (Table 1)
Summary
Sulfate reduction is a globally important microbial process in anoxic marine sediments (Canfield, 1991; Jørgensen and Kasten, 2006; Bowles et al, 2014). In recent studies, sulfate reduction was detected in methane zones, operating at low background concentrations of sulfate (Leloup et al, 2006; Holmkvist et al, 2011; Treude et al, 2014; Brunner et al, 2016; Orsi et al, 2016). This shows that there is a general zonation of predominant microbial processes in the sediment column determined by pore water chemistry and thermodynamics, this zonation is not absolute and exceptions are common
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