Production and utilization of organic matter in different P-availability conditions: a mesocosm experiment in the Northern Adriatic Sea.

The growth, activity and structure of a marine plankton assemblage from the Gulf of Trieste (NE Adriatic Sea) were monitored in a short-term mesocosm experiment (25 L) in order to investigate the influence of phosphorous availability on the production and utilization of organic matter. The coastal community, dimensionally selected (b200 μm), was grown under different P-concentrations. The results obtained from the nutrient enriched enclosures highlighted the microbial food web potential in high productivity systems: the phytoplankton community grew to a bloom-like state and resulted particularly productive enhancing the development of the prokaryotic population (especially of its active fraction). Glucidic exoenzymes operated at high hydrolysis rates in order to recycle extracellular carbohydrate/exudates produced by the abundant phytoplankton community. Intense prokaryotic production, directed prevalently to replication rather than to biomass production, increased the density of prokaryotic cells on levels that particularly favoured virus proliferation, acting thus as feedback control on the bacterial population. Different P concentrations also strongly influenced the structure of the bacterial community (either directly or indirectly) analyzed through denaturing gradient gel electrophoresis (DGGE), by selecting two diverse assemblages at the end of the experiment. Conversely to high N:P ratios observed throughout the year in the Northern Adriatic Sea, our study simulated a phosphorous-balanced condition and pointed out how nutrient limitation affects the entire microbial plankton community, reducing the trophic status of the environment through the control of phytoplankton production.