Ecological implications of the production of toxic substances by fish killing phytoplankton species grown under variable N:P-ratios

Sammanfattning: The influence of different nutrient conditions on the production of toxins was studied in certain fishkilling haptophytes (e.g. Prymnesium parvum and Chrysochromulina polylepis). The toxicity was strongly influenced by the nutrient status of the medium, resulting in a significantly higher toxicity when grown under nutrient limited conditions compared to nutrient replete conditions. This occurred independently of the growth-limiting nutrient (N or P), suggesting a relationship between high toxicity and limitation of nutrients. The increase in toxicity was correlated to a decrease in intracellular N or P levels, indicating that the enhancement of toxicity is a response to cellular physiological stress (i.e. limitation of growth), due to low levels of N or P in the water. I also found that ingestion of bacteria provided an immediate source of phosphorus to the algae, which enhanced the growth of P-limited cells. In addition, the ingestion of bacteria indirectly reduced the toxicity of previously P-limited cells, by increasing the cellular P-content and subsequently decreasing the cellular physiological stress. This is further evidence that toxin production in these haptophyte species is related to physiological stress. I also investigated how these toxins affect other parts of the marine ecosystem. Cell-free filtrates of P. parvum cultures had a strong negative effect on the growth of several different species of microalgae, implying that P. parvum excretes toxic substances with an allelopathic function. In addition, P. parvum had a negative effect on the survival of the ciliate Euplotes affinis. The toxic effect of P. parvum on other organisms was dependent on the physiological status of the P. parvum cells, with the highest toxicity in nutrient stressed cultures. This was shown by a significant reduction of cell numbers in algal cultures exposed to cell-free filtrates of nutrient limited P. parvum cultures, whereas addition of filtrates of nutrient replete cultures did not inhibit the growth. It was also shown by a higher mortality when E. affinis were incubated with P. parvum cells grown under nutrient limited (N or P) conditions, compared to when incubated with nutrient sufficient cells. The ability to destroy or inhibit the growth of other co-existing algal species or grazers will substantially increase the competitive ability of fishkilling haptophytes under nutrient limiting conditions, and may thus be of importance for the development of toxic blooms of these species.