Sulfate reduction and methanogenesis in a Thioploca-dominated sediment off the coast of Chile

Abstract

Continental shelf sediments of the central Chile upwelling area are dominated by the presence of dense mats of the filamentous, sulfur-depositing bacterium Thioploca spp. We examined rates and pathways of S and methane cycling in these sediments along a transect from the Bay of Concepción to the continental slope. Sulfate reduction rates (170–4670 nmol cm −3 d −1) were equal to or exceeded rates reported for other subtidal marine sediments. Elemental S and pyrite were the dominant end-products of sulfate reduction in Thioploca mats on the continental shelf, whereas, in the highly-reducing, Beggiatoa-dominated sediments of the nearby Bay of Concepción, acid-volatile S was the principal end-product. Dissolved organic C values were lowest at the stations with the highest sulfate reduction rates and increased offshore. Sediment porewater methane concentrations in all surface sediments were low (<12 nmol cm −3), and methane production rates at the station most dominated by Thioploca were extremely low ( <0.5 nmol cm −3 d −1). Low methane production rates and concentrations were matched by low methane oxidation rates (<0.1 nmol cm −3 d −1). Radio-tracer studies showed that methane production was almost exclusively from methylamines, substrates which are noncompetitive with sulfate reduction, rather than from acetate or CO 2/H 2. Bacterial MPN (most probable number) counts also indicated the presence of a methylotrophic population of methanogens. Surprisingly, high numbers of autotrophic acetogenic bacteria were found, suggesting that the bacterial population involved in anaerobic DOC degradation is more complex than expected. In spite of the high sulfate reduction rates, sulfide concentrations in the shelf and slope were low or undetectable (<0.5 μM), and sulfate concentrations were never depleted below bottom water levels down to depths of 25–30 cm. Calculations suggest that Thioploca were oxidizing a maximum of 35% of sulfide production—not enough to prevent sulfate depletion. Either other sulfide oxidizers were also present or transient hydrodynamic conditions coupled with bioturbation resulted in oxidation of the sediments.