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

Abstract

The influence of cell-bound microcystins on the survival time and feeding rates of six Daphnia clones belonging to five common species was studied. To do this, the effects of the microcystin-producing Microcystis strain PCC7806 and its mutant, which has been genetically engineered to knock out microcystin synthesis, were compared. Additionally, the relationship between microcystin ingestion rate by the Daphnia clones and Daphnia survival time was analyzed. Microcystins ingested with Microcystis cells were poisonous to all Daphnia clones tested. The median survival time of the animals was closely correlated to their microcystin ingestion rate. It was therefore suggested 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 ingestion rate could be described by a reciprocal power function. Feeding experiments showed that, independent of the occurrence of microcystins, cells of wild-type PCC7806 and its mutant are able to inhibit the feeding activity of Daphnia. Both variants of PCC7806 were thus ingested at low rates. In summary, our findings strongly suggest that (i) sensitivity to the toxic effect of cell-bound microcystins is typical for Daphnia spp., (ii) Daphnia spp. and clones may have a comparable sensitivity to microcystins ingested with food particles, (iii) Daphnia spp. may be unable to distinguish between microcystin-producing and -lacking cells, and (iv) the strength of the toxic effect can be predicted from the microcystin ingestion rate of the animals.

FIG. 1

Feeding rates of Daphnia clones on Scenedesmus, wild-type PCC7806, and mutant PCC7806. The data represent mean values of five replicates and the respective standard errors (SEs).

FIG. 2

Survivorship of Daphnia clones fed with either the wild-type PCC7806 (black squares) or the mutant (open squares). The data represent mean values of three (D. galeata clone A and B, D. hyalina, D. magna) or four (D. pulex, D. pulicaria) replicates and the respective SEs.

FIG. 3

Regression analyses of the relationship between microcystin ingestion rate and LT₅₀. Black squares represent data obtained with different Daphnia clones, with the wild-type PCC7806 as food (r = 0.84, F = 9.3, P < 0.04). The open squares correspond to data obtained with D. galeata and four microcystin-producing Microcystis strains (data from reference 38). The equation for the whole data set is LT₅₀ = 3.28 · microcystin ingestion rate^−0.58 (r = 0.92, F = 42.1, P < 0.001).