Effects of Salinity on Growth and Toxin Production of a Noxious Phytoflagellate, Heterosigma akashiwo (Raphidophyceae)

Abstract

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