Growth response of the ichthyotoxic haptophyte, Prymnesium parvum Carter, to changes in sulfate and fluoride concentrations
Golden alga Prymnesium parvum Carter is a euryhaline, ichthyotoxic haptophyte (Chromista). Because of its presumed coastal/marine origin where SO42- levels are high, the relatively high SO 4 2- concentration of its brackish inland habitats, and the sensitivity of marine chromists to sulfur deficiency, this study examined whether golden alga growth is sensitive to SO 4 2- concentration. Fluoride is a ubiquitous ion that has been reported at higher levels in golden alga habitat; thus, the influence of F- on growth also was examined. In low-salinity (5 psu) artificial seawater medium, overall growth was SO 4 2— dependent up to 1000 mg l-1 using MgSO 4 or Na 2 SO 4 as source; the influence on growth rate, however, was more evident with MgSO 4 . Transfer from 5 to 30 psu inhibited growth when salinity was raised with NaCl but in the presence of seawater levels of SO 4 2- , these effects were fully reversed with MgSO 4 as source and only partially reversed with Na 2 SO 4 . Growth inhibition was not observed after acute transfer to 30 psu in a commercial sea salt mixture. In 5-psu medium, F- inhibited growth at all concentrations tested. These observations support the hypothesis that spatial differences in SO 4 2- –but not F-–concentration help drive the inland distribution and growth of golden alga and also provide physiological relevance to reports of relatively high Mg 2+ concentrations in golden alga habitat. At high salinity, however, the ability of sulfate to maintain growth under osmotic stress was weak and overshadowed by the importance of Mg 2+ . A mechanistic understanding of growth responses of golden alga to SO 4 2- , Mg 2+ and other ions at environmentally relevant levels and under different salinity scenarios will be necessary to clarify their ecophysiological and evolutionary relevance.