Hydrothermal activity, functional diversity and chemoautotrophy are major drivers of seafloor carbon cycling

Abstract

Hydrothermal vents are highly dynamic ecosystems and are unusually energy rich in the deep-sea. In situ hydrothermal-based productivity combined with sinking photosynthetic organic matter in a soft-sediment setting creates geochemically diverse environments, which remain poorly studied. Here, we use comprehensive set of new and existing field observations to develop a quantitative ecosystem model of a deep-sea chemosynthetic ecosystem from the most southerly hydrothermal vent system known. We find evidence of chemosynthetic production supplementing the metazoan food web both at vent sites and elsewhere in the Bransfield Strait. Endosymbiont-bearing fauna were very important in supporting the transfer of chemosynthetic carbon into the food web, particularly to higher trophic levels. Chemosynthetic production occurred at all sites to varying degrees but was generally only a small component of the total organic matter inputs to the food web, even in the most hydrothermally active areas, owing in part to a low and patchy density of vent-endemic fauna. Differences between relative abundance of faunal functional groups, resulting from environmental variability, were clear drivers of differences in biogeochemical cycling and resulted in substantially different carbon processing patterns between habitats.

Hydrothermal activity lowers trophic diversity in Antarctic hydrothermal sediments

“…This relationship may also suggest that the influence of disturbance gradients created by hydrothermalism can result in an impoverished community (McClain and Schlacher, 2015;Bell et al, 2016b). Productivity-diversity relationships in which higher productivity sustains higher diversity have also been suggested for deep-sea ecosystems (McClain and Schlacher, 2015;Woolley et al, 2016), but this is not supported by the Bransfield Strait sites (Bell et al, 2017a). We suggest that in the Bransfield Strait the environmental toxicity in hydrothermal sediments (from differences in temperature and porewater chemistry) causes a concomitant decline in both trophic and species diversity (Bell et al, 2016b) in spite of the potential for increased localised production (Bell et al, 2017a).…”

“…However, the Bransfield Strait can be subject to substantial export production and it is probable that surface production contributes much more to sea floor OM than is evident from the fatty acid composition. Non-hydrothermal sediments were more enriched in 34 S than hydrothermal sediments, an offset that probably resulted from greater availability of lighter sulfur sources such as sulfide oxidation at Hook Ridge, even if surface-derived OM remained the dominant source of organic matter at the hydrothermal sites (Bell et al, 2017a).…”

“…Fauna with more depleted δ 34 S and/or more enriched δ 13 C values were likely to have derived at least a small amount of their diet from chemosynthetic sources (potentially indirectly through the non-selective consumption of detrital OM) both at hydrothermal and background regions (Bell et al, 2017a). Carbon and sulfur isotopic measurements indicated mixed sources for most consumers between chemosynthetic OM and surface-derived photosynthetic OM.…”

“…Macrofaunal assemblages in Bransfield hydrothermal sediments were strongly influenced by hydrothermal activity (Bell et al, 2016b(Bell et al, , 2017a. Bacterial mats were widespread across Hook Ridge, where variable levels of hydrothermal activity were detected (Aquilina et al, 2013).…”

“…S1, Bell et al, 2017b). All lipid samples were dominated by saturated and monounsaturated fatty acids (SFAs and MUFAs), comprising 91-94 % of FA abundance per site.…”