The reaction of Prymnesium parvum to a sudden salinity decrease

• Brackish inland water is sufficient for Prymnesium parvum population to grow. • Changes in osmotic pressure cause the release of P.parvum toxins into water. • Massive fish kills are caused by changes in the osmotic pressure of P.parvum cells. • The critical value to maintain cell water homeostasis is ca. 0.88 psu. Prymnesium parvum N. Carter, 1937 (the golden algae) inhabits salty inland environments and significantly impacts the fitness of other water species. These algae produce many cytotoxic metabolites, thus, during their blooms, massive fish kill can be observed. It is so far unknown what triggers the sudden release of toxins from the algal cells. We hypothesise that a sudden decrease of water salinity with an increase of P. parvum to be the major factor contributing to the substantial increase of biologically active substances (i.e. prymnesin, secondary metabolites, enzymes, etc.) due to the massive mortality of the algae cells. At the practical salinity value of 0.68, the increase of osmotic pressure to 360 hPa caused cell bursts after 10 min of exposure. The cell size of P. parvum , especially in the lateral axis, increased significantly, changing the volume of the cells from 234±79 µm 3 (1.55) to 534±149 µm 3 (0.68). The cell burst followed by the release of the cellular components was noted during particle analysis of cell suspensions in different water salinities. The appearance of enormous amounts of small particles in water derived from suddenly broken cells of the golden algae is probably responsible for the massive death of gill breathing organisms. Thus, P. parvum, as well as other organisms' survival in brackish water, depends on water salinity homeostasis that is recognised to be met above the value of practical salinity equal to 0.88.