Water mass age and dissolved organic matter properties drive the diversity of pelagic prokaryotes in the Western Mediterranean Sea

The deep sea is among the largest, yet still poorly known, ecosystems on Earth. This knowledge gap is particularly evident for the bathypelagic layer (between 1000 and 4000 m) of the deep Mediterranean Sea (MS), characterized by peculiar environmental conditions at meso- and bathypelagic depths, such as the thermal signature of the deep waters, showing temperatures approximately 10 °C higher than any oceanic system at comparable depths. This sustains high rates of prokaryotic activities and fast dissolved organic matter (DOM) mineralization rates, and likely selects for unique microbial assemblages. We collected seawater samples in stations representative of different areas of the Western Mediterranean Sea (Algero-Provençal Basin, Alboran Sea and Gulf of Lion), from the surface to the bathypelagic layer (down to 2680 m) and in different water masses, and described the composition of pelagic prokaryotic communities, along with measurements of the main physical-chemical variables, concentration and optical properties (absorption and fluorescence) of DOM. Remarkable differences in DOM optical properties were observed among water masses, with highest dissolved organic carbon (DOC) values in surface waters and lowest in the oldest water masses represented by the Western Mediterranean Deep Water (WMDW) and the Levantine Intermediate Water (LIW), which were characterized by the highest concentration of recalcitrant DOM. The water mass had a significant partitioning effect on microbial community composition, which showed the highest richness in LIW. DOC, the marine humic-like component of DOM (C1mh) and oxygen were the main drivers of prokaryotic community structure. Changes in quality of DOM were reflected in shifts in community composition, supporting the existence of strong relationships between DOM quality and microbial community composition in the deep MS. Our data shed light on the community composition and diversity patterns of prokaryotic plankton in the deep western MS, helping to elucidate the major microbial players in the DOM cycling, and to progress towards a better comprehension of its future trends in light of changing conditions that are modifying the oceanography of the entire MS basin.