Biogeochemistry of a deep-sea whale fall: sulfate reduction, sulfide efflux and methanogenesis

Deep-sea whale falls create sulfidic habitats supporting chemoautotrophic communities, but microbial processes underlying the formation of such habitats remain poorly evaluated. Microbial degradation processes (sulfate reduction, methanogenesis) and biogeochemical gradients were studied in a whale-fall habitat created by a 30 t whale carcass deployed at 1675 m depth for 6 to 7 yr on the California margin. A variety of measurements were conducted including photomosaicking, microsensor measurements, radiotracer incubations and geochemical analyses. Sediments were studied at different distances (0 to 9 m) from the whale fall. Highest microbial activities and steepest vertical geochemical gradients were found within 0.5 m of the whale fall, revealing ex situ sulfate reduction and in vitro methanogenesis rates of up to 717 and 99 mmol m–2 d–1, respectively. In sediments containing whale biomass, methanogenesis was equivalent to 20 to 30% of sulfate reduction. During in vitro sediment studies, sulfide and methane were produced within days to weeks after addition of whale biomass, indicating that chemosynthesis is promoted at early stages of the whale fall. Total sulfide production from sediments within 0.5 m of the whale fall was 2.1 ± 3 and 1.5 ± 2.1 mol d–1 in Years 6 and 7, respectively, of which ~200 mmol d–1 were available as free sulfide. Sulfate reduction in bones was much lower, accounting for a total availability of ~10 mmol sulfide d–1. Over periods of at least 7 yr, whale falls can create sulfidic conditions similar to other chemosynthetic habitats such as cold seeps and hydrothermal vents.