Dynamics of microcystins in the mussel Mytilus galloprovincialis

The accumulation and depuration of hepatotoxins produced by the freshwater cyanobacterium Microcystis aeruginosa in the mussel Mytilus galloprovincialis was studied. Mussels were fed daily 10(5) cells/ml of the toxic cyanobacterium that produces microcystin-LR (MCYST-LR), for four days. After that period animals were placed in toxin free water and were fed the diatom Nitzschia sp. During two weeks the concentration of the toxin in the mussels, as also in their feces and in the water where animals were placed individually during 24 h, were monitored using an ELISA assay. No mussel mortality was registered during the whole experiment. Mussels showed a maximum detectable level of MCYST of 10.7 microg/g mussels dry weight (DW) during the accumulation period, rising to 16.0 microg MCYST/g mussel DW by day two of the depuration period. Then there was a decrease trend with peaks of toxin at days 6, 8, 11 and 14. The rise of the toxin level on day two of the depuration period seems to have been due to the reingestion of contaminated feces. In fact, feces showed high amounts of MCYST during the first days of depuration with a maximum of 140 microg/d DW on day 3. This coincided with a 50% decrease on the detectable toxin in the mussels reflecting the emptiness of their digestive tract. In the water the highest level of the toxin was 2.5 microg MCYST/liter and some toxin peaks were also observed during the depuration period. This fluctuation of the toxin levels in the mussels, feces and water may be related to the renewal of protein phosphatases and subsequent release of unbound toxins. Results show that depuration of MCYST by mussels is not a very rapid process and contamination by feces containing MCYST is likely to occur and increase the persistence of these toxins in the mussels after the bloom disappearance. Monitoring programs for harmful algal blooms usually include only toxic dinoflagellates and diatoms and their toxins in bivalves. Taken into account the present work they should also include hepatotoxins from cyanobacteria, namely in brackish waters such as estuaries of eutrophic rivers in order to avoid human health hazard.