The role of cyanobacterial toxins as grazing inhibitors in the fresh water cladoceran Daphnia magna straus

Life history parameters of the cladoceran Duphnia magna were compared under several combinations of defined media and green alga! diets in order to select a standardized and therefore repeatable culture system for D. magna. It was found that none of the defined media tested were nutritionally sufficient to replace natural media. However, Elendt M4 medium when used in conjunction with Chlorella minutissima was deemed the best combination representing a defined medium for Daphnia culture. It was noted that bacterial contaminants in algal diets can compensate for some of the nutritional deficiencies present in defined culture media. The effects of short term (24 h) and long term (4 - 5 d) exposure to toxic Microcystis aeruginosa on the feeding rate of D. magna were investigated by comparing 24 h feeding rates on Chlorella vulgaris, non-toxic M. aeruginosa (strain CYA 43) and toxic M. aeruginosa (strain PCC 7820). Toxic M. aeruginosa suppressed the feeding rate of D. magna not only on M. aeruginosa itself but afso on other algae present. However the cyanobacterial toxin, microcystin-LR, was not the main causative factor for the feeding inhibition. It was hypothesised that factors present on the surface of M. aeruginosa cells caused a rapid suppression of feeding rate. Inhibition of feeding was further suppressed by the poor quality of M. aeruginosa as a food source for D. magna. Microcystin-LR was the primary cause of the death of daphnids which were exposed to toxic M. aeruginosa cells, confirming that microcystin-LR is toxic to D. magna. However, at concentrations which occur in natural waters, dissolved purified microcystin-LR had no measurable effect on the feeding rate or life history of D. magna. The toxicity of intact cells of M. aeruginosa was three orders of magnitude greater than that of purified microcystin-LR. A difference in bioavailability of microcystin-LR or a cumulative effect of an undescribed toxicant were thought to be the reason for toxicity differences