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

Abstract

Present address: Bundesanstalt für Geowissenschaften und Rohstoffe, Stilleweg 2, 30655 Hannover, GermanyABSTRACT: 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 Skeleton of a whale on the deep-sea floor, covered by chemoautotrophic bacterial mats, anemones, and bone-eating worms Osedax spp., 6 yr after arrival at the seafloor.

Rapid scavenging of jellyfish carcasses reveals the importance of gelatinous material to deep-sea food webs

“…Extensive microbial degradation and low scavenging activity were reported at large, deep-sea jelly-fall deposits ( jelly-detritus cover 100% of the seafloor) in the Gulf of Oman [12] and at large deep-sea pysosome deposits discovered off Ivory Coast [13] and the continental slope off the eastern US coast [21,22]. The difference between our study and these other observations could be related to differences in the sizes of the jelly-fall deposits between studies, with high concentrations of dissolved inorganic carbon (DIC), noxious sulfide and ammonium produced at large jelly-fall deposits deterring scavengers from feeding [14,26,27]. However, the small size of jelly falls in this study does not appear to be the main reason for the rapid scavenging activity observed as comparatively little or a complete lack of scavenging has also been observed at small gelatinous deposits in the deep Gulf of Oman (approx.…”

Microbial Ecology of the Dark Ocean above, at, and below the Seafloor

“…Similar to gas seeps representing conspicuous oases of sedimentary activity, dense organic matter deposits at the seafloor, such as animal carcasses (Fig. 4B) and wood falls, also support microbial and faunal communities that are more active than those in surrounding sediments (29,194,503,504,564). Where such "food falls" occur, a succession of microbial and macrofaunal communities colonize the deposit, and high levels of biological activity result (194).…”

“…Where such "food falls" occur, a succession of microbial and macrofaunal communities colonize the deposit, and high levels of biological activity result (194). The influx of more labile organic carbon to the sediment leads to an increase in sediment microbial activity and the development of shallow sulfate reduction zones in the sediment (564).…”

Microbial methanogenesis in the sulfate-reducing zone of sediments in the Eckernförde Bay, SW Baltic Sea

“…It has been estimated that > 50 % in spring (FebruaryMarch), < 25 % in summer (July-August), and > 75 % in autumn (September-October) of these blooms is reaching the seafloor (Smetacek et al, 1984), resulting in an overall high organic carbon content of the sediment (5 wt %), which leads to high benthic microbial degradation rates including sulfate reduction and methanogenesis (Whiticar, 2002;Treude et al, 2005a;Bertics et al, 2013). Previous studies revealed that high organic matter availability can relieve competition between sulfate reducers and methanogens in sulfatecontaining marine sediments (Oremland et al, 1982;Holmer and Kristensen, 1994;Treude et al, 2009;Maltby et al, 2016). To determine the effect of POC concentration and C / N ratio (the latter as a negative indicator for the freshness of POC) on SRZ methanogenesis, two PCAs were conducted with (a) the focus on the upper 0-5 cm b.s.f., which is directly influenced by freshly sedimented organic material from the water column (Fig.…”