Mechanisms of the inhibitory effect of the cyanobacterium Microcystis aeruginosa on Daphnia galeata's ingestion rate

The possible effects of five Microcystis aeruginosa strains (two unicellular and three colony forming) on the filtering and feeding behavior of Daphnia galeata were investigated to clarify the mechanisms of the inhibitory effect on the ingestion rate of the animals. Of the tested cyanobacterial strains, three inhibited Daphnia's food ingestion. This was primarily caused by affecting the frequency and food transport efficiency of the maxillary contractions. As a result, Daphnia's swallowing rate, and in one case the amount of food contained in the swallowed boluses, were lower than in suspensions of well-ingestible food sources. In contrast, there is no indication that Microcystis can influence thoracic appendage beat or rejection rate. It is likely that colony-forming and unicellular strains can affect the ingestion process due to different factors. The mucilage of the colony-forming Microcystis strains HUB-W75 and HUB-W334 possibly produced a mechanical hindrance of the maxillules, which resulted in an abnormal movement and consequently in the inhibitory effects described above. This was related to the sugar composition of the mucilage polysaccharides, but not to the cellular microcystin content or occurrence of accompanying bacteria. The mucilage-lacking strain HUB 5-2-4, however, must contain another factor which interfered with the maxillules' movement in an unknown way.