Acute, chronic and reproductive toxicity of complex cyanobacterial blooms in Daphnia magna and the role of microcystins

Abstract

Toxic cyanobacterial blooms are a global threat to human health and aquatic biota. While the ecotoxicity of cyanobacterial toxins such as microcystins has been studied extensively, little is known about the risks they pose in the wild, i.e. within complex biomasses. In this work, crustaceans (Daphnia magna) were exposed to varying concentrations (0-405 mg d.w L(-1)) of eight complex cyanobacterial water bloom samples in a series of acute (48 h) and chronic (21 day) toxicity experiments. Further acute and chronic exposure assays were performed using aqueous extracts of the crude biomass samples and two fractions prepared by solid phase extraction (SPE) of the aqueous extracts. The cyanobacterial biomasses differed with respect to their dominant cyanobacterial species and microcystin contents. High acute toxicity was observed for 6 of the 8 crude biomass samples. Chronic exposure assays were performed using one complex biomass sample and its various subsamples/fractions. The complex biomass, the crude aqueous extract, and the microcystin-free SPE permeate all elicited similar and significant lethal effects, with LC50 values of around 35.6 mg biomass d.w L(-1) after 21 days. The cyanobacterial biomass samples also affected reproductive health, significantly increasing the time to the first brood (LOEC = 45 mg d.w L(-1) exposure) and inhibiting fecundity by 50% at 15 mg d.w L(-1). Conversely, the microcystin-containing C18-SPE eluate fraction had only weak effects in the chronic assay. These results indicate that cyanobacterial water blooms are highly toxic to zooplankton (both acutely and chronically) at environmentally relevant concentrations. However, the effects observed in the acute and chronic assays were independent of the samples' microcystin contents. Our results thus point out the importance of other cyanobacterial components such as lipopolysaccharides, various peptides and depsipeptides, polar alkaloid metabolites or other unidentified metabolites in the overall ecotoxicity of complex cyanobacterial blooms.

Keywords: Cyanobacterial bloom; Cyanotoxin; Daphnia magna; Microcystin; Reproduction.