The effects of microcystins on Daphnia galeata, a typical filter-feeding grazer in eutrophic lakes, were investigated. To do this, the microcystin-producing wild-type strain Microcystis aeruginosa PCC7806 was compared with a mcy− PCC7806 mutant, which could not synthesize any variant of microcystin due to mutation of a microcystin synthetase gene. The wild-type strain was found to be poisonous toD. galeata, whereas the mcy− mutant did not have any lethal effect on the animals. Both variants of PCC7806 were able to reduce the Daphnia ingestion rate. Our results suggest that microcystins are the most likely cause of the daphnid poisoning observed when wild-type strain PCC7806 is fed to the animals, but these toxins are not responsible for inhibition of the ingestion process.
Various sites of Phragmites australis in Germany (Uckermark and Mark Brandenburg) and sites in Hungary and Denmark were investigated by PCR fingerprinting in order to determine their genetic diversity. Genetic similarities were calculated and. based on this, a cluster analysis was performed. The genetic similarity of reed clones around one lake ranged from between 70 to loo%, that of clones from different lakes was clearly lower. The further apart the clones were situated from each other, the less similar they were. At a wetland site near the shore a sudden colonisation by several new Phragmites clones was observed. This site was located in front of a well established genetically almost homogenous stand within the eulittoral. All the results are discussed in connection with a proposed model of establishment and development of water reed stands: colonization, selection of best adapted clones by competition and the stability of stands.
Ingestion rates where estimated for daphnids, Cyclops spp. and Bosmina (Eubosmina) coregoni thersites fed hepatotoxic and non-toxic M. aeruginosa either separate or mixed with the readily available food alga Ankistrodesmus falcatus. The ingestion rates of hepatotoxic strains of M. aeruginosa are very low compared with those of A. falcatus or non-toxic M. aeruginosa HUB 5-3 fed to Daphnia magna or D. longispinu. However, a close relationship between ingestion rate of different M. aeruginosa strains and their toxicity could not be observed. Addition of the toxic strain M. aeruginosa HUB 5-24 reduces the ingestion rates of A. falcatus progressively due to increased food rejection by D . magna. Additionally, the assimilation efficiency of M. ueruginosa HUB 5-2-4 is two times lower compared with A.falcatus and M. aeruginosa HUB 5-3 leading to strong starvation.
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