Sensitivity and adaptation of aquatic organisms to oscillapeptin J and microcystin-RR

Abstract

Planktothrix rubescens is a frequently found cyanobacterium in deep stratified mesotrophic lakes, and metalimnetic mass developments as well as surface blooms are frequently observed in these waterbodies. P. rubescens contains toxins, ([D-Asp 3 ,(E)-Dhb 7 ]microcystin-RR and oscillapeptin J), numerous other cyclic peptides and depsipeptides which are potent inhibitors of digestive proteases, and so far unknown infochemicals (deterrents) that are bound to the cell surface and allow grazers to discriminate between cyanobacteria and more suitable food particles. Purified [D-Asp 3 ,(E)-Dhb 7 ]microcystin-RR was toxic to all tested aquatic grazers, but the sensitivity varied. The selective feeder Eudiaptomus showed mortality (LC 50 ) at 1.2 μM, while Daphnia sp., Chironomus riparius and Brachionus calyciflorus were less sensitive (21.1 μM, 52μM and 157μM). Toxicity of oscillapeptin J was only found for crustacean grazers (Eudiaptomus sp. LC 50 = 58 μM, Daphnia sp. LC 50 = 203 μM). Toxicity of this compound to mice, when administered intraperitoneally, was not observed. The data characterize oscillapeptin J as a specific crustacean toxin directed against this most dangerous group of grazers for planktonic cyanobacteria. Oscillapeptin J is a very potent inhibitor of trypsin (IC 50 = 20nM) and a moderate inhibitor of chymotrypsin (IC 50 = 2.8 μM). Adaptation to this toxin was found when the sensitivity of Daphnia sp., collected from a lake with annual mass developments of P. rubescens (Lake Hallwilersee) and a lake virtually free of this cyanobacterium (Lake Walensee), were compared with each other. Daphnia sp. from Lake Walensee was significantly more sensitive than Daphnia sp. which has experienced P. rubescens.