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 SO(4)(2-) 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.