Variability in Toxicity of the Dinoflagellate Alexandrium Tamarense Isolated from Hiroshima Bay, Western Japan, as a Reflection of Changing Environmental Conditions

Abstract

The variability of cellular toxin content in the dinoflagellate Alexandrium tamarense isolated from Hiroshima Bay was analyzed under a variety of culture conditions. Growth and toxicity were represented as a function of light (80, 90, 110, 160 and 350 μmol m^–2 s ^–1), temperature (12, 17 and 22°C), salinity (13, 16.5, 19.5, 25, 29, 33, 36.5 and 38 PSU) and ammonium concentration (0.11, 0.22 and 0.44 mM). Toxicity was measured by the tissue culture bioassay using mouse neuroblastoma cells, and expressed as saxitoxin concentration equivalents. Cellular toxicity increased with decreasing salinity. At temperatures of 17 and 22°C, maximum toxin content was observed at the lowest light intensity and growth rate. At the lowest temperature of 12°C, maximum toxin content was observed at intermediate light intensities and growth rates. A drastic increase in toxin content with an increase in ammonium concentration from 0.11 to 0.22 mM supported the idea that ammonium utilization for toxin production directly brings about a high toxin content inA. tamarense. Our results ecologically imply that the cells become highly toxic in environments with low salinity and high ammonium concentration, and successive cloudy days. Such environmental conditions may lead to increasing risk of shellfish toxification.