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oxic Pseudo‐nitzschia australis strains isolated from French coastal waters were studied to investigate their capacity to adapt to different salinities. Their acclimation to different salinity conditions (10, 20, 30, 35 and 40) was studied on growth, photosynthetic capacity, cell biovolume and domoic acid (DA) content. The strains showed ability to acclimate to a salinity range from 20 to 40, with optimal growth rates between salinities 30 and 40. The highest cell biovolume was observed at the lowest salinity 20 and was associated with the lowest growth rate. Salinity did not affect the photosynthetic activity; Fv/Fm values and the pigment contents remained high with no significant difference among salinities. An enhanced production of zeaxanthin was, however, observed in the late stationary and decline phases in all cultures except for those acclimated to salinity 20. In terms of cellular toxin content, DA concentrations were 2 to 3‐fold higher at the lowest salinity (20) than at the other salinities and were combined with a low amount of dissolved DA. The fact that P. australis accumulate more DA per cell in less saline waters, illustrates that climate‐related changes in salinity may affect Pseudo‐nitzschia physiology through direct effects on growth, physiology and toxin content.

Freshwater cyanobacteria are known for their ability to produce bioactive compounds, some of which have been described as allelochemicals. Using a combined approach of co‐cultures and analyses of metabolic profiles, we investigated chemically mediated interactions between two cyanobacterial strains, Microcystis aeruginosa PCC7806 and Planktothrix agardhii PCC7805. More precisely, we evaluated changes in growth, morphology and metabolite production and release by both interacting species. Co‐culture of Microcystis with Planktothrix resulted in a reduction of the growth of Planktothrix together with a decrease of its trichome size and alterations in the morphology of its cells. The production of intracellular compounds by Planktothrix showed a slight decrease between mono and co‐culture conditions. Concerning Microcystis, the number of intracellular compounds was higher under co‐culture condition than under monoculture. Overall, Microcystis produced a lower number of intracellular compounds under monoculture than Planktothrix, and a higher number of intracellular compounds than Planktothrix under co‐culture condition. Our investigation did not allow us to identify specifically the compounds causing the observed physiological and morphological changes of Planktothrix cells. However, altogether, these results suggest that co‐culture induces specific compounds as a response by Microcystis to the presence of Planktothrix. Further studies should be undertaken for identification of such potential allelochemicals.

Several coastal countries including France have experienced serious and increasing problems related to Pseudo‐nitzschia toxic blooms. These toxic blooms occur in estuarine and coastal waters potentially subject to fluctuations in salinity. In this study, we document for the first time the viability, growth, photosynthetic efficiency and toxin production of two strains of Pseudo‐nitzschia australis grown under conditions with sudden salinity changes. Following salinity variation, the two strains survived over a restricted salinity range of 30 to 35, with favorable physiological responses, as the growth, effective quantum yield and toxin content were high compared to the other conditions. In addition, high cellular quotas of domoic acid (DA) were observed at a salinity of 40 for the strain IFR‐PAU‐16.1 in comparison with the other strain IFR‐PAU‐16.2 where the cell DA content was directly released into the medium. On the other hand, the osmotic stress imposed at lower salinities, 20 and 10, resulted in cell lysis and a sudden DA leakage in the medium. Intra‐specific variability was observed in growth and toxin production, with the strain IFR‐PAU‐16.1 apparently able to withstand higher salinities than the strain IFR‐PAU‐16.2. On the whole, DA does not appear to act as an osmolyte in response to sudden salinity changes. Since most of the shellfish harvesting areas of bivalve molluscs in France are located in areas where the salinity generally varies between 30 and 35, Pseudo‐nitzschia australis blooms might potentially impact public health and commercial shellfish resources in these places.