Salinity stress response of the invasive dinoflagellate Prorocentrum minimum

Estuarine and coastal environments are vulnerable to alien species invasions. Many nonindigenous organisms demonstrate high ecological plasticity, which is particularly well expressed in unicellular eukaryotes. In this study, responses of the potentially toxic invasive dinoflagellate Prorocentrum minimum, initially acclimated to salinity 17 (control), to short-term stress by salinities 4, 8 and 35 were investigated experimentally by determination of mortality, alterations in cell cycle and chromosome fine structure, RNA synthesis and DNA replication. The percentage of dead cells within the P. minimum population after stress by critical salinity 8 was lower compared to the other tested conditions. Salinity stress caused only moderate impact on the cell cycle pattern; changes of chromosome fine structure at salinities 4 and 8 were evident though reversible. We disclosed the elevated RNA synthesis and DNA replication which can play a 'compensational' role by speeding up the metabolic (synthetic) activity of these protists at the critical salinity conditions. The increased RNA synthesis in phase G1 and additional DNA replication in phase S of the cell cycle likely enhance salinity tolerance and cause relatively low mortality of these micro-eukaryotes at salinity 8. We infer that the enhanced synthesis of DNA and RNA can be one of the advanced ecological strategies of P. minimum which along with the other cellular and physiological characteristics (small body size, planktonic mode of life, fast reproduction, high evolution rates, mixotrophy, ability to form cysts, etc.) helps these dinoflagellates to invade successfully and populate permanently coastal ecosystems, remaining highly competitive in the unstable brackishwater environments.