Carbon processing by the benthic ecosystem and benthic C fixation in methane-rich sediments on the South Georgia margin

As bottom water warms, destabilisation of gas hydrates may increase the extent of methane rich sediments. We present an assessment of organic carbon processing by the benthic community in methane rich sediments, including one of the first investigations of inorganic C fixation in a non-hydrothermal vent setting. This topic was previously poorly studied, and there is much need to fill the gaps in our knowledge of such ecosystems. We hypothesised that benthic C fixation would occur, and that a high biomass macrofaunal community would play a substantial role in organic C cycling. Experiments were conducted at a 257 m deep site off South Georgia. Sediment cores were amended with 13C and 15N labelled algal detritus, or 13C labelled bicarbonate solution. In the bicarbonate experiment, labelling of bacteria-specific phospholipid fatty acids provided direct evidence of benthic C fixation, with transfer of fixed-C to macrofauna and DOC. In the algae experiment, macrofauna played an active role in organic carbon cycling. Compared to similar experiments, low temperature supressed the rates of community respiration, and macrofaunal C uptake. While benthic C fixation occurred, the biological processing of organic carbon was dominantly controlled by low temperature and high photic zone productivity