Linking Microbial Functioning and Trophic Pathways to Ecological Status in a Coastal Mediterranean Ecosystem

Coastal marine ecosystems host complex microbial communities, and both continental inputs and water masses properties influence microbial composition and its metabolism. The identifying traits of the phytoplankton and bacteria such as biomass, size, shape and their metabolism related to organic matter production and degradation, recognized as indicators of the functioning of an ecosystem, were observed in the Gulf of Manfredonia in late spring. This Gulf area, located in the South Adriatic Sea, is characterized by terrestrial inputs and mesoscale circulation influence such as coastal waters flowing southward from the North Adriatic and off-shore waters interested by the Ionian Sea. Water samples were grouped in clusters (Coastal, Intermediate, Offshore and Deep Systems) according to the water column properties. Phytoplankton community biomass and composition, autotrophic and total prokaryotic abundances and microbial metabolism such as enzyme activity rates and prokaryotic heterotrophic production were analysed to elucidate the trophic pathways with the objective to infer on the ecosystem status. As expected, size-fractionated phytoplankton biomass and production, showed greater concentration in Coastal waters with prevalence of the largest fractions (micro- and nano-) supported by the diatoms. Conversely, lower biomass and production were measured in all off-shore waters, mainly sustained by smallest fractions (nano-sized phytoflagellates and picophytoplankton). Total and autotrophic prokaryotic abundance decreased from coastal to off-shore stations, inversely with respect to cell volume. Prokaryotic heterotrophic production was just below 50% compared to that of phytoplankton in all waters, evidencing an active biomass synthesis. High alkaline phosphatase and leucine aminopeptidase in Coastal and Offshore waters suggested the quick regeneration of Phosphorous and protein decomposition, respectively. Different levels of phytoplankton-bacteria association might provide a tool to define the ecological status of the studied system in the observed period, and a method of classification exportable to other coastal systems is proposed.