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Abstract:
Sulfate is the most important terminal electron acceptor on earth and plays a pivotal role in the mineralization of organic matter in marine sediments. Sulfate reduction is expected to cease when sulfate is depleted and methanogenesis starts to occur. However, abundant sulfate reducing microorganisms (SRM) were regularly reported to be present in subsurface sediments. Their relative proportions to bacteria were found to be similar to surface sediments, where sulfate is not limited. A “cryptic” sulfur cycle in the methanic zone has been suggested, which is tightly coupled to reactive iron minerals. The Helgoland mud area is characterized through high sedimentation rates and a shallow sulfate zone. Below, dissolved Fe2+ starts to accumulate through an unknown mechanisms. A possibility is the abiotic oxidation of sulfide by reactive iron minerals, which would require active SRM for the continuous supply of sulfide. This study aimed to assess, if the methanic, Fe2+ rich sediments of the Helgoland mud area possess the potential for active sulfate reduction and if SRM are active. Quantitative analyses of DNA extracts from a 5 m long sediment core, sampled in May 2015, showed high abundances of dissimilatory (bi)sulfite reductase (dsrA) genes with up to 1 x 107 gene copies in the methanic zone. Their relative proportions to bacterial 16S rRNA genes varied between 9 – 17 % and were not related to the availability of sulfate. Slurry incubations with 35SO42- tracer revealed high rates for all selected depths indicating an active sulfate reducing community in sediments from the Helgoland mud area. Rates did not increase by the addition of acetate and sulfate, thus, substrates were not limited. Three week incubations with different potential electron donors showed an increase of Fe2+ not related to sulfate but to acetate, which implies the presence of dissimilatory iron reduction. All in all, the potential and activity of SRM in sediments from the Helgoland mud area was shown and future studies need to reveal the strength of a “cryptic” sulfur cycle.
