Prokaryotic activities and abundance in pelagic areas of the Ionian Sea

The Ionian Sea represents a suitable basin for studying the biogeochemical processes mediated by microbial activities. Due to its characteristics of a crossing region between the Western and Eastern Mediterranean Sea, it is one of the most affected sites by changes in water mass composition and dynamics, caused by the Eastern Mediterranean Transient (EMT). To date, relatively few data exist on microbial activities in pelagic areas of the Ionian Sea. From 1998 to 2004 during different research cruises, prokaryotic parameters (abundance, extracellular enzyme activities -leucine aminopeptidase, ß-glucosidase, alkaline phosphatase-, bacterial production and respiration) were measured together with the culturable bacteria and the main physical, chemical and trophic parameters (temperature, salinity, particulated organic matter). The aim of the study was to describe the spatial and temporal variability of microbial activities involved in the carbon and phosphorus cycles, in different layers. Results showed that organic matter transformation mediated by the microbial community displayed a significant increase in Autumn, pointing out the occurrence of significant changes also at meso- and bathy- pelagic depths. Unlike the dark ocean, the bacterial growth efficiency in the Ionian Sea, which increased with depth, seemed to vary its role from a source to a sink of carbon in the epi- and in meso-and bathypelagic layers, respectively. The mechanism of phosphatase regulation showed a weak inverse correlation between specific phosphatase and inorganic P in all seasons except Autumn. It is worth mentioning that the reported results constitute, to the best of our knowledge, one of the available datasets, giving information about microbial activities in the Ionian Sea.