Effects of salinity on growth and toxin production of a noxious phytoflagellate, Heterosigma akashiwo (Raphidophyceae)

Growth and toxin production of a noxious phytoflagellate Heterosigma akashiwo (Kagoshima strain) were investigated at a salinity range from 20-40parts per thousand. The maximum growth (0.68 +/- 0.03 divisions day(-1)) was obtained at 25parts per thousand, while the lowest growth (0.48 +/- 0.02 divisions day(-1)) was attained at 40parts per thousand. The lower salinity was found suitable for maintaining a better morphological condition and motility of the cells. The Kagoshima strain became more toxic at 20parts per thousand, and the toxicity decreased with increased salinity. A negative correlation between salinity increase and toxin production was found on HPLC analysis of H. akashiwo toxins (HaTxI, II, III and IV).Toxin HaTxIV (corresponding to oxidized PbTx-2) was most dominant and peaked at 20parts per thousand with a yield of 0.3 pg cell-(1). Considerable amounts of HaTxI and III (corresponding to PbTx-2 and 3) were also detected. However, HaTxII (corresponding to PbTx-9) was detected only as a trace at 20 and 25 parts per thousand. A sharp decrease in all toxins was found at salinities above 30 parts per thousand.The increase of specific growth rate raised the toxin content per cell of the strain.