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.

Increased organic nutrient loads deriving from anthropogenic activities and natural processes frequently cause eutrophication of coastal waters. An increasing number of phototrophs have been shown to make use of organic nutrients. The potential utilization of dissolved organic phosphorus (DOP) by microbial-mats associated with the toxic dinoflagellate Ostreopsis cf. ovata, was investigated throughout a full cycle of a bloom occurred in 2015 in the Conero Riviera (N Adriatic Sea). Measurements of phosphomonoesterase (PMEase) and phosphodiesterase (PDEase) activities of the epiphytic mats (including cells and exopolymeric substances) and chemical-physical parameters were made from late summer to early autumn with a weekly frequency. Analyses of ambient inorganic nutrient fractions revealed 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 a proliferation of the Ostreopsis population. 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 phosphatase activities indicates that Ostreopsis-mat community can utilize a wide range of DOP types. Tests in laboratory confirmed that 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, Ostreopsis seems to have 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.