All-in-one: microbial response to natural and anthropogenic forcings in a coastal Mediterranean ecosystem, the Syracuse bay (Ionian Sea, Italy)

Bacterial and phytoplankton communities are in close relationships, but how natural and anthropogenic stressors can affect their dynamics is not fully known. To study the response of microbial communities to environmental and human-induced perturbations, phytoplankton and bacterial communities were seasonally monitored in a Mediterranean coastal ecosystem, the Syracuse Bay, where multiple conflicts co-exist. Quali-quantitative, seasonal surveys of the phytoplankton communities (diatoms, dinoflagellates and other taxa), the potential microbial enzymatic activity rates (leucine aminopeptidase, beta-glucosidase and alkaline phosphatase) and heterotrophic culturable bacterial abundance, together with the thermoaline structure and trophic status in terms of nutrient concentrations, phytoplankton biomass (as Chlorophyll-a), and total suspended and particulate organic matter, were carried out to integrate microbial community dynamics in the context of environmental characterization and disentangle microbial patterns related to natural changes from those driven by anthropic impact over this ecosystem. In spite of the complex relationships between the habitat characteristics, microbial community abundance and metabolic potential, in the Syracuse Bay the availability of organic substrates differently originated by the local conditions appeared to drive the distribution and activity of microbial assemblage. A seasonal pattern of microbial abundances was observed, with the highest concentrations of phytoplankton in spring and low values in winter, whereas heterotrophic bacteria were more abundant during autumn period. The autumn peaks of the rates of enzymatic activities suggested that not only phytoplankton-derived but also allocthonous organic polymers strongly stimulated the microbial metabolism. An increased microbial response in terms of abundance and metabolic activities was detected especially at the sites directly affected by organic matter inputs related to agriculture or aquaculture activities. Nitrogen salts such as nitrate, rather than orthophosphate, were primary drivers of phytoplankton growth. This study also provides insights on the different seasonal scenarios of water quality in the Syracuse Bay, that could be helpful for management studies of this Mediterranean coastal environment.