Effects of temperature and light intensity on the growth and toxicity of Heterosigma akashiwo (Raphidophyceae)

The effects of temperature and light intensity on the growth and toxicity o f Heterosigma akashiwo (Hada) were studied in the laboratory. The growth an d toxicity of this organism were found to be greatly influenced by temperat ure and light intensity. The best growth was found at 25 degrees C where th e toxicity was very low. On the other hand, cells were found to be most tox ic at 20 degrees C (0.087 +/- 0.005 FU), although the growth was much less than at 25 degrees OC. The lowest toxicity was observed at 30 degrees C wit h a toxicity of 0.011 +/- 0.005 FU. The toxicity of this species also diffe red markedly at different light intensities. It gave the maximum toxicity a t 200 mu Em(-2) s(-1), but poor growth. The best growth occurred at 100 mu Em(-2) s(-1), but with a marked toxicity drop. In toxin composition studies , the amounts of most of the toxic fractions were lower at 25 degrees C and 100 mu Em(-2) s(-1) where the growth was most conspicuous.