Abstract
Although bulk bacterial metabolism in response to temperature has been determined for different oceanic regions, the impact of temperature on the functional diversity of dissolved organic matter (DOM) utilization has been largely unexplored. Here, we hypothesized that besides modifying the rates of carbon utilization, temperature can also alter the diversity of substrates utilized. The patterns of utilization of 31 model DOM compounds (as represented in Biolog EcoPlates (TM)) by bacterioplankton were assessed using inocula from surface waters of the southern Bay of Biscay continental shelf over 1 year. Bacteria utilized more polymers and carbohydrates in late spring and summer than in winter, likely reflecting changes in substrate availability linked to the release and accumulation of DOM in phytoplankton post-bloom conditions. Seawater temperature correlated positively with the number of substrates utilized (i.e. functional richness) and this relationship was maintained in monthly experimental incubations spanning 3 degrees C below and above in situ values. The enhancement of functional richness with experimental warming displayed a unimodal response to ambient temperature, peaking at 16 degrees C. This temperature acted as a threshold separating nutrient-sufficient from nutrient-deficient conditions at the study site, suggesting that trophic conditions will be critical in the response of microbial DOM utilization to future warming.