Respiratory quotients of particle-associated microbes track carbon flux attenuation in the mesopelagic Southern Ocean.

Mesopelagic microbes and zooplankton, degrade, and attenuate >90% of the 10 billion tonnes of particulate organic carbon that sinks into the oceans' interior annually. Approaches such as particle interceptors/incubators (called c-respire) can isolate the microbial assemblage attached to particles from that of zooplankton, enabling quantification of microbially mediated particulate organic carbon flux attenuation. This metric yields patterns of particulate organic carbon degradation by microorganisms through the upper mesopelagic (200-500 m depth). Here, we investigate the temporal sequence of particulate organic carbon degradation in two steps. First, we intercept sinking particle assemblages from different depths (180-300 m) and hence with varying degrees of exposure to microbial activity. Second, we incubate these intercepted particles shipboard for 12 h (short-term) and track degradation using apparent respiratory quotients (dDIC/dDO2). We also conducted a 12-h shipboard incubation on a particle assemblage that had already undergone a 36-h in situ c-respire (long-term) incubation. At a subantarctic and two polar sites, apparent respiratory quotients (ARQs) from short-term incubations exhibited a significant decrease with depth, consistent with particles deeper in the upper mesopelagic being exposed to a longer period of degradation and flux attenuation (as they settle). ARQs from all long-term incubations had significantly lower ARQs, and smaller depth-dependent gradients, than the short-term incubations. We interpret these trends as being driven in part by sequential changes in the stoichiometry of the microbially altered particulate organic carbon (POC) substrates. ARQs of <0.5 (less than the theoretical minimum) were observed in long-term incubations suggesting a role for incomplete oxidation of dissolved substrates. This temporal sequence is used to conceptually explore what sets the limits on microbially mediated degradation of POC.