Organic matter pulse events and warming drive aerobic and anaerobic bacterial metabolism in hypoxic estuarine waters

Climate change and eutrophication pose serious threats to coastal ecosystems by altering primary productivity and increasing the frequency or magnitude of high organic matter (OM) pulses. Bacterial degradation of this OM contributes to dissolved oxygen depletion and the expansion of hypoxic zones. Yet, temperature-sensitivity (Q10) of bacterial metabolism under hypoxic conditions remains poorly constrained. Using hypoxic deep waters from the Lower St Lawrence Estuary, we conducted experiments to quantify the responses of bacterial production (BP) and respiration (BR) to temperature (Q10 prod and Q10 resp , respectively) under contrasting OM conditions. Our results suggest a co-limitation of OM availability and temperature on Q10 prod with values reaching 338, but not on Q10 resp . Elevated OM availability enabled denitrification and aerobic respiration to coexist under mild hypoxia, likely contributing to high Q10 prod and explaining the weaker response of aerobic respiration to temperature. Moreover, the in-situ OM pool had an amino acids anoxic signature, consistent with anaerobic bacterial degradation despite an oxygen saturation of 15%. Together, these results suggest that OM pulses regulate bacterial metabolism in cold hypoxic ecosystems and may promote the coexistence of aerobic and anaerobic metabolic pathways well before anoxic conditions develop.