Benthic carbon fixation and cycling in diffuse hydrothermal and background sediments in the Bransfield Strait, Antarctica

Abstract

Sedimented hydrothermal vents are likely to be widespread compared to hard substrate hot vents. They host chemosynthetic microbial communities which fix inorganic carbon (C) at the seafloor, as well as a wide range of macroinfauna, including vent-obligate and background non-vent taxa. There are no previous direct observations of carbon cycling at a sedimented hydrothermal vent. We conducted 13C isotope tracing experiments at three sedimented sites in the Bransfield Strait, Antarctica, which showed different degrees of hydrothermalism. Two experimental treatments were applied, with 13C added as either algal detritus (photosynthetic C), or as bicarbonate (substrate for benthic C fixation). Algal 13C was taken up by both bacteria and metazoan macrofaunal, but its dominant fate was respiration, as observed at deeper and more food-limited sites elsewhere. Rates of 13C uptake and respiration suggested that the diffuse hydrothermal site was not the hot spot of benthic C cycling that we hypothesised it would be. Fixation of inorganic C into bacterial biomass was observed at all sites, and was measurable at two out of three sites. At all sites, newly fixed C was transferred to metazoan macrofauna. Fixation rates were relatively low compared with similar experiments elsewhere; thus, C fixed at the seafloor was a minor C source for the benthic ecosystem. However, as the greatest amount of benthic C fixation occurred at the “Off Vent” (non-hydrothermal) site (0.077±0.034 mg C m−2 fixed during 60 h), we suggest that benthic fixation of inorganic C is more widespread than previously thought, and warrants further study.