Salt tolerance of the harmful cyanobacterium Microcystis aeruginosa

Increasing salinities in freshwater ecosystems caused by agricultural practices, droughts, or rise in sea level are likely to affect the species composition of phototrophic microorganisms. Cosmopolitan freshwater cyanobacteria of the Microcystis genus can produce the toxin microcystin, and present a potential health risk in many eutrophic lakes. In this study, M. aeruginosa Strain PCC7806 was grown in semi-continuous turbidostats to investigate the effect of increasing salinity on growth rate, microcystin cell quota, microcystin production and extracellular microcystin concentration. Specific growth rate, microcystin cell quota and microcystin production remained more or less unaffected by salinity levels up to 10 g l(-1). Specific growth rate collapsed when salinity was increased beyond 10 g l(-1) for several weeks. Cell size and microcystin cell quota decreased while extracellular microcystin concentrations increased at salinities above 10 g l(-1), indicating leakage and/or cell lysis. Salt-shock experiments revealed that M aeruginosa can temporarily endure salinities as high as 17.5 g l(-1). These results indicate that, for a freshwater species, M aeruginosa has a high salt tolerance. Rising salinities in freshwater ecosystems are therefore unlikely to suppress M aeruginosa blooms, and may in fact enhance the exposure of aquatic organisms to elevated concentrations of extracellular microcystins.