Ecophysiology of a successful phytoplankton competitor in the African flamingo lakes: the green alga Picocystis salinarum (Picocystophyceae)

Picocystis salinarum is a globally widespread picoplanktonic green alga of saline lakes. This tiny alga has been recorded in four continents, and in some cases, it has become the dominant phytoplankton species. We examined the ecophysiology of a Kenyan strain of P. salinarum, collected from Lake Nakuru. The photosynthetic activity of the species was measured in seventy-two combinations of light intensity and temperature. The photosynthetic activity was low along the temperature gradient with an optimum at 31.9 degrees C; it varied between 0.097 and 1.233 mu g C mu g(-1) Chl a h(-1). The ability of P. salinarum to utilize low light intensity (alpha varied between 0.0061 and 0.1 (mu g C mu g(-1) Chl a h(-1)) (mu mol photons m(-2) s(-1))(-1)) and its susceptibility to photoinhibition at different temperatures confirm the species' preference for low light intensity, which is already shown by its I-k values (1.0-89.3 mu mol photons m(-2) s(-1)). The salinity tolerance of P. salinarum, more specifically the effect of changes in the concentration of chloride (NaCl) and carbonate forms (Na2CO3 and NaHCO3), was investigated in continuous cultures. The dominant ion of the medium greatly affected the growth of P. salinarum: significantly higher growth rates were recorded in carbonate form-dominated media as compared to chloride-dominated ones; the highest growth rate was observed at the highest concentration. The observed physiological properties (slow growth, low photosynthetic activity) are not typical of a dominant species; however, the high conductivity tolerance could be a key factor explaining the success of P. salinarum.