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.

Figure 1

(A) Percentage contribution of OM inputs at each site (±1 S.D.); (B) Comparison of selected variables of external and internal cycling values (±1 S. D.). POC = Particulate organic carbon. DOC = Dissolved organic carbon. OC = Organic carbon. In situ production, suspension feeding and total OM inputs are given as net values (i.e. corrected for metabolic constraints of the relevant taxa).

Figure 2

Percentage diet composition of deposit feeders and predators/scavengers at each site, along a gradient of hydrothermal activity. Macro. = Macrofauna; Mega. = Megafauna.

Figure 3

Selected mean carbon flows between food web compartments at each site. Arrow thickness = flow rate. White patches = bacterial mat. See supplementary figure for full details of exchanges between compartments.

Figure 4

Differences in the potential contribution of chemosynthetic organic matter to different hydrothermal vent types, with representative taxa included for reference. SPOM = Surface-derived Particulate Organic Matter. White patches = bacterial mats. Figure credit: Alison Manson, University of Leeds.