The dark portion of the Mediterranean Sea is a bioreactor of organic matter cycling

Total prokaryotic abundance, prokaryotic heterotrophic production and enzymatic activities were investigated in epi-, meso- and bathypelagic waters along a longitudinal transect covering the entire Mediterranean Sea. The prokaryotic production and enzymatic activities in deep waters were among the highest reported worldwide at similar depths, indicating that the peculiar physico-chemical characteristics of the Mediterranean Sea, characterized by warm temperatures (typically 13°C also at abyssal depths), support high rates of organic carbon degradation and incorporation by prokaryotic assemblages. The higher trophic conditions in the epipelagic waters of the Western basin resulted in significantly higher prokaryotic production and enzymatic activities rates than in the Central-Eastern basin. While all of the variables decreased significantly from epi- to meso- and bathypelagic waters, cell-specific hydrolytic activity and cell-specific carbon production significantly increased. In addition, the deep-water layers were characterised by low half-saturation constants (Km) of all enzymatic activities. These findings suggest that prokaryotic assemblages inhabiting the dark portion of the Mediterranean Sea are able to channel degraded carbon into biomass in a very efficient way, and that prokaryotic assemblages of the deep Mediterranean waters work as a “bioreactor” of organic matter cycling. Since prokaryotic production and enzymatic activities in deep water masses were inversely related with oxygen concentration, we hypothesise a tight link between prokaryotic metabolism and oxygen consumption. As climate change is increasing deep-water temperatures, the predicted positive response of prokaryotic metabolism to temperature increases may accelerate oxygen depletion of deep Mediterranean waters, with cascade consequences on carbon cycling and biogeochemical processes on the entire deep basin.