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 (POC) 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 POC flux attenuation. This metric yields patterns of POC degradation by microbes through the upper mesopelagic (200-500 m depth). Here, we investigate the temporal sequence of POC 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 12h (short-term) and track degradation using Apparent Respiratory Quotients (ARQ, dDIC/dDO2). We also incubate shipboard (12h) a particle assemblage previously incubated (36h) in situ using C-RESPIRE (long-term). At a subantarctic and two polar sites, 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 POC substrates. Several ARQs of <0.5 (less than the theoretical minimum) were observed in long-term incubations suggesting a role for incomplete oxidation of substrates. This temporal sequence is used to conceptually explore what sets the limits on microbially-mediated degradation of POC.