Phytoplankton diversity and size structure in the Central-Southern Tyrrhenian Sea: implications for microbial functioning

Microbial community dynamics in relation to mesoscale hydrographical features are almost unknown particularly in the pelagic Central-Southern Tyrrhenian Sea. To get a more comprehensive view of phytoplankton community structure and microbial community functioning, datasets of phytoplankton abundance, composition and some microbial enzyme activities (leucine aminopeptidase, LAP, beta-glucosidase, GLU and alkaline phosphatase, AP) from six cruises carried out twenty years ago were analyzed. Hydrographic characteristics identified the presence of both Atlantic Waters (AW) and Tyrrhenian Intermediate Waters (TIW). Size structure of phytoplankton biomass showed an unexpected high contribution of the pico-phytoplankton to the total primary production (> 60%) determining a predominant microbial food web. Phytoplankton distribution patterns varied more significantly on a seasonal rather than spatial scale. Autumn assemblages were characterized by the highest abundance and carbon content, with species mainly belonging to dinoflagellates whose growth was supported by intense microbial activities. In contrast, in the summer diatoms developed in unstable TIW where microbial activity was declining. Enzymatic activities varied in the different water masses and seasons, with high LAP activity in summer AW (s-AW) as well as in deep TIW (d-TIW), while AP and GLU reached their maximum in autumn AW (a-AW), suggesting quick organic matter recycling. Coupled primary production and hydrolysis in mixed AW (m-AW) and in a-AW indicated synchronized autotrophic and heterotrophic processes, while in TIW organic matter was only partially recycled. Overall, microbial metabolism was closely shaped by hydrographic and seasonal dynamics, confirming its key role in biogeochemical cycles. Our data could provide a baseline study for future research dealing with the microbial functioning in this Mediterranean region.