How harmful algae compete for limited nutrient resources. Lessons from the northern Adriatic Sea

Imbalanced nutrient supply is a general and common challenge to marine microalgae. The competition for the limiting resource quickly becomes a shaping pressure for microalgae community structures. Nitrogen and phosphorous are often key nutrients and limiting supply of phosphorous is hypothesized to be a global trend in marine systems. The northern Adriatic is a model system with respect to this trend and marine microalgae can be observed under a variety of steep ecological gradients. We observed the in situ “behavior” of microalgae with respect to competition for phosphorous, generally the limiting nutrient. Our results suggest a number of key strategies for the successful competition for this element: Increased phosphate uptake rates, specialization on organic phosphate sources at high or low concentrations by the use of alkaline phosphatases, delayed bloom initialization, phosphate pool formation, morphological adaptations and alternations of cellular nutrient requirements. This results in the characterization of a multitude of spatiotemporal niches across ecological gradients. We performed in vitro experiments on monoclonal algal cultures to quantify the performances of microalgal species in phosphate limited conditions. Here we present the relative performances of harmful algae with respect to the abovementioned mechanisms in the context of other common northern Adriatic microalgae and explain the spatio-temporal distribution of harmful algae from the genera (Skeletonema, Chaetoceros, Pseudo-nitzschia, Prorocentrum and Ostreopsis).