Changes in DOM Quality Determine Prokaryotic Activities and Extracellular Release in the NW Mediterranean Sea: An Experimental Approach

Marine dissolved organic matter (DOM) has varying degrees of bioavailability from labile to refractory fractions. DOM bioavailability affects its prokaryotic uptake and release, which determines the carbon sequestration mechanism through the microbial carbon pump (MCP). Here, we tested how changes in DOM bioavailability influence prokaryotic communities and DOM transformation during an incubation experiment in the NW Mediterranean Sea. DOM was extracted at 5 m on 31 May and 26 August 2021, added to mesopelagic prokaryotic communities, and incubated for 34 days. Our results suggest that the growth of different prokaryotic communities, linked to the use of alkaline phosphatase and higher viral abundances, results in the release of prokaryotic-derived DOM, which we traced using dissolved hydrolysable amino acids and the accumulation of humic-like fluorescent DOM. The accumulation coincided with the growth of particular prokaryotic members, such as Campylobacterales and Enterobacterales, during the active growth phase, and Flavobacterales of the genera Tenacibaculum, Pedosphaerales and Chitinophagales at the end of the incubation. The parallel analysis of DOM and the prokaryotic community links DOM transformations with a temporal succession of niches in the free-living prokaryotic community, identifying the main actors of the MCP of the Mediterranean Sea.