Interactions between cyanobacteria and gastropods I. Ingestion of toxic Planktothrix agardhii by Lymnaea stagnalis and the kinetics of microcystin bioaccumulation and detoxification.

The last two decades have been marked by an increasing occurrence of toxic cyanobacterial blooms in aquatic ecosystems. These pose an expanding threat to the environment and to human health. Among the intracellular toxins produced by cyanobacteria, microcystins (hepatotoxins) are the most frequent and widely studied. As an ubiquitous herbivore living in eutrophic freshwaters, the freshwater snail Lymnaea stagnalis (Gastropoda: Pulmonata) is particularly exposed to cyanobacteria. The toxic filamentous Planktothrix agardhii is common in temperate lakes and is, therefore, a potential food resource for gastropods. We have studied the consumption of P. agardhii by L. stagnalis juveniles and adults in the presence or absence of non-toxic food (lettuce) over a 5-weeks period. Intoxication was followed by a 3-week detoxification period when snails were fed only on lettuce. The kinetics of microcystin accumulation and detoxification in the gastropods were established using the ELISA analytical method. The results showed an ingestion of toxic P. agardhii by L. stagnalis, even in the presence of lettuce, and the absence of food selection regardless of the age of the snails. Juveniles and adults consumed the same number of cells per millilitre and consumption was proportional to food availability. On average, 63% of cyanobacteria available were taken up during the first 24h. After 5 weeks of intoxication, 61% of the toxins present in the ingested cyanobacterial cells had accumulated in snail tissues (95% in the digestive-genital gland complex) with a concentration up to 80.4+/-4.9microggDW(-1). Toxin accumulation was greater in the gastropods fed on P. agardhii alone than those fed on the mixed diet, and was also greater in juveniles than in adults. After the removal of toxic cyanobacteria, detoxification was rapid: 64% of the toxins disappeared from snail tissues during the first week, but microcystins were still detected after 3 weeks (on average, 3.5+/-0.9microggDW(-1)). These results are discussed in terms of potential contamination to the food web.