DOM degradation incubations of prokaryote-derived DOM by Mediterranean Sea microbial communities
Heterotrophic prokaryotes (HP) contribute largely to dissolved organic matter (DOM) processing in the ocean, but they also release diverse organic substances. The bioavailability of DOM released by HP under varying environmental conditions are not fully elucidated.  In this study, we investigated the effect of P-limitation on the bioavailability of DOM released by a single bacterial strain and 2 natural HP communities. These HP were grown under P-replete and P-limited conditions, and their DOM (HP-DOM) was used as a substrate for natural prokaryotic communities from a coastal site in the Northwestern Mediterranean Sea. We followed changes in prokaryotic growth, enzymatic activity, diversity and community composition together with the consumption of HP-DOM fluorescence (FDOM). HP-DOM produced under P-replete and P-limited conditions promoted both significant and similar prokaryotic growth. The humic-like fluorescence, commonly considered recalcitrant, was consumed during the incubations when this peak was initially dominating the FDOM pool, and this consumption coincided with higher alkaline phosphatase activity. HP-DOM supported growth of diverse prokaryotic communities and P-driven differences in HP-DOM quality selected for different indicator taxa in the degrading communities. Taken together, our findings emphasize that HP-DOM lability is dependent on both the DOM quality, shaped by P availability, and the composition of the consumer community.