Effects of cell-bound microcystins on survival and feeding of Daphnia spp.

The influence of cell-bound microcystins on the survival time and feeding r ates of six Daphnia clones belonging to five common species was studied. To do this, the effects of the microcystin-producing Microcystis strain PCC78 06 and its mutant, which has been genetically engineered to knock out micro cystin synthesis, were compared. Additionally, the relationship between mic rocystin ingestion rate by the Daphnia clones and Daphnia survival time was analyzed. Microcystins ingested with Microcystis cells were poisonous to a ll Daphnia clones tested. The median survival time of the animals was close ly correlated to their microcystin ingestion rate. It was therefore suggest ed that differences in survival among Daphnia clones were due to variations in microcystin intake rather than due to differences in susceptibility to the toxins. The correlation between median survival time and microcystin in gestion rate could be described by a reciprocal power function. Feeding exp eriments showed that, independent of the occurrence of microcystins, cells of wild-type PCC7806 and its mutant are able to inhibit the feeding activit y of Daphnia. Both variants of PCC7806 were thus ingested at low rates. In summary, our findings strongly suggest that (i) sensitivity to the toxic ef fect of cell-bound microcystins is typical for Daphnia spp., (ii) Daphnia s pp. and clones may have a comparable sensitivity to microcystins ingested w ith food particles, (iii) Daphnia spp. may be unable to distinguish between microcystin-producing and -lacking cells, and (iv) the strength of the tox ic effect can be predicted from the microcystin ingestion rate of the anima ls.