Influence of salinity on growth and cell volume in three strains of Prorocentrum cordatum (Dinophyceae)

The aim of this work was to test whether salinity is the driving factor for size and physiological differences in the dinoflagellate Prorocentrum cordatum. Three strains, isolated from 3 sites with different salinities (the central Baltic Sea, Sweden, salinity 8; Chesapeake Bay, Maryland, USA, salinity 16; and the Gulf of Trieste, northern Adriatic Sea, Italy, salinity 32), were included in this study. Following their molecular and morphological characterisation, specific growth rates and dimensions were examined for each strain cultivated at all 3 salinities (i.e. 8, 16 and 32). The Adriatic strain always showed the largest cells and the Baltic strain the smallest. Adriatic and American strains showed lower specific growth rates at salinity 8 and highest at salinity 16. The Baltic strain growth rates were similar at all salinities. As salinity 16 proved to be optimal for the strains, additional parameters were further compared at this salinity: cell volume, particulate organic carbon, particulate nitrogen, delayed fluorescence and in vivo chl a fluorescence. The Adriatic strain had the highest particulate organic carbon, particulate nitrogen, and delayed and in vivo fluorescence. Conversely, the Baltic strain had the lowest values. Although the strains had different specific growth rates with respect to salinity, this was not the main driving factor for the different sizes and photosynthetic activity among the strains. Thus, although there was low genetic variability among strains from the 3 geographical areas, intraspecific variability was suggested.