Picocyanobacteria in Adriatic transitional environments

In the marine realm picoplanktonic cyanobacteria (PC) show a broad distribution from the surface ocean to the offshore and coastal waters of semi-enclosed seas and, because of their photoheterotrophy, they are responsible for a substantial fraction of both primary production and organic matter degradation/utilization. In the present study, we investigated PC distribution related to some physical–chemical parameters (temperature, salinity and dissolved oxygen), in the most productive periods of the year, spring and fall, at 37 stations located in seven transitional systems all along the Italian coast of the Adriatic Sea: Grado and Marano, Venezia, Pialassa Baiona, Margherita di Savoia, Torre Guaceto, Cesine and Alimini. The aim of the study was to determine if PC are common elements of these transitional systems (lagoons, marshes and solar salterns) or whether they are only occasionally important during stochastic bloom events. PC mean abundances were all over the seven habitats more than one order of magnitude higher (>5 × 107 cells L−1) than those found in coastal and offshore waters of the Adriatic Sea. Their range of variability was also broader (105–109 cells L−1 vs. 106–108 cells L−1). PC populations appeared well adapted to ample variations in salinity (5–87). The highest abundances (close to 109 cells L−1) were found at extremely high/low salinity values. Even oxygen concentrations close to hypoxia did not seem to negatively affect PC abundances. Because of their ability in adapting to extreme conditions, PC may become the prevailing fraction of the phototrophic plankton in these sites and their role in such ecosystems should not be underestimated.