Phosphatase activities of the microbial community during the bloom of Ostreopsis cf. ovata in the northern Adriatic Sea: the role of temperature and organic phosphorus sources.

During the last few decades, recurrent and intense blooms of the toxic benthic dinoflagellate Ostreopsis cf. ovata have been frequently reported during summer along several sections of the Mediterranean coast. Field studies about Ostreopsis blooms highlighted the important role of hydrodynamics, water temperature and inorganic nutrients, among the other environmental parameters. However, all factors (and the interactions among them) that drive the bloom development throughout its phases are not totally understood yet. For example, the potential utilization of organic phosphorus (DOP) and its role on the Ostreopsis bloom are still unclear. The potential utilization of dissolved organic phosphorus (DOP) by the toxic dinoflagellate Ostreopsis cf. ovata within its microbial-mat community was investigated during a bloom along the Conero Riviera (N Adriatic Sea). Measurements of phosphomonoesterase (PMEase) and phosphodiesterase (PDEase) activities of the epiphytic mats (including cells and exopolymeric substances) and a range of chemico- physical parameters were performed from late summer to early autumn at Passetto station. Analyses of ambient nutrient fractions revealed very high N:P values, very low filterable reactive P (FRP) concentrations and DOP concentrations that were on average 85% of the total dissolved P. A rapid increase in PMEase and PDEase activities in the microbial community was recorded coinciding with the onset of the Ostreopsis bloom. Chromogenic staining of samples showed that activity was closely associated with the Ostreopsis cells, located both extracellularly (cell surface and within the EPS) and intracellularly (ventral cytoplasm). The increase in both phosphatases indicates that Ostreopsis-mat community can utilize a wide range of DOP types confirmed through tests in laboratory. Ostreopsis cf. ovata can utilize both phosphomonoester (D- Fructose 1,6-disphosphate, β-Glycerophosphate, α-D-Glucose 1-phosphate, Guanosine 5’-monophosphate and Phytic acid) and phosphodiester (DNA and RNA) sources to grow. The experiments also demonstrated that PMEase and PDEase were strongly influenced by water temperature, with maximum values recorded at 30-35 °C. Based on the present findings, the Ostreopsis bloom seems to be maintained thanks to some adaptations that allow it to thrive in P-limited environments where organic P is the main source of P, until water temperature is enough high to allow PMEase and PDEase activity.