Salt tolerance of Planktothrix agardhii (Gomont) Anagnostidis & Komárek (cyanoprokaryota) between freshwater and brackish strains in batch cultures

Planktothrix agardhii (P. agardhii), a filamentous cyanobacterium commonly found in freshwater environments, has also been reported in brackish areas. In order to test the hypothesis of an acclimation of P. agardhii to its environmental salinity, we monitored growth and morphometry of P. agardhii brackish and freshwater strains under various salinities (0 to 15 psu). Despite differences in their response under increasing salinities (0–15 psu), the brackish strains show: (i) growth less gradually inhibited by increasing salinities than the freshwater strains; (ii) growth nearly optimal in the salinity of their natural environment (iii) a salt tolerance range wider (0 to [12.5–15 psu]) than that of freshwater P. agardhii (0 to [7.5–10 psu]). Our findings suggest that this tolerance may be due to an acclimation of the strains to their environment by effective salt extrusion and compatible solutes accumulation. This salt acclimation, just as the population response to various salinities, is well reflected by the filaments morphometry. This study highlights the ability of potentially harmful cyanobacteria to acclimate to salinity in a salinization context.