Mixotrophic phytoplankton dynamics in a shallow Mediterranean water body: how to make a virtue out of necessity

Mixotrophy is a combination of photosynthesis and direct access to organic carbon sources, mainly through osmotrophy or phagotrophy. This strategy is adopted by several, phylogenetically distinct, phytoplankton groups and is commonly occurring in marine, brackish and freshwater ecosystems. Traditionally, it has been put in relation to both scarcity of inorganic nutrients and poor light conditions. However, we observed blooms of the mixotrophic, toxic haptophyte Prymnesium parvum in different periods of the year and under variable resources availability. The analysis of a 6.5-year data set of phytoplankton weekly records from a Sicilian shallow lake (Biviere di Gela, south-eastern Sicily) allowed us to hypothesise that a depleted condition as regards inorganic nutrients is not the main fuel to the growth of P. parvum, neither this is due to light limitation. The results achieved show that an increased availability in suitable preys can stimulate the growth of this phagotrophic photoautotroph. Contemporarily, it was not found any clear environmental patterns to explain species dominance and growth patterns as related to inorganic nutrient availability. Moreover, it is shown that these organisms tend to monopolise resources when these become available irrespective of seasons, and under variable conditions as regards inorganic nutrient availability.