Elimination of the Cyanobacterial Hepatotoxin Microcystin from the freshwater Pulmonate Snail Lymnaea stagnalis jugularis (SAY)

Abstract

It has been suggested that little to no microcystin (MC), a cyanobacterial hepatotoxin, accumulates within freshwater pulmonate snails because the toxin is associated primarily with undigested gut contents that are eliminated from the animal via egestion. To test this, Lymnaea stagnalis exposed to MC-containing cyanobacteria were placed into toxin-free environments and sampled over short (24 h at 21° C) and long (30 d at 22 and 10° C) time periods. Within 8 h after being removed from exposure to microcystin-containing phytoplankton, the gizzard and cecal string fractions of the feces were eliminated, accounting for 57% of the initial MC concentration. However, detectable concentrations remained beyond 24 h, likely in association with the digestive-gland contents, which can be retained up to 100 h. Long-term MC loss was biphasic at two ambient temperatures. The greatest change (fast phase) occurred over the first 3 d after exposure. By 6 d, the cumulative MC loss from L. stagnalis was 80 and 95% at 10 and 22° C, respectively. Toxin loss over this period was attributed to egestion of indigestible cells/colonies from gizzard and cecum, as well as elimination of unassimilated MC-laden fragments and vacuolate excretion of residues from the digestive gland. The fast-phase depuration rate constant was significantly higher at 22 than at 10° C, indicating an influence of ambient temperature on the rate of toxin loss from pulmonate snails. Depuration continued at slower rates until 30 d, when most (97.5 and 99.5% at 10 and 22° C, respectively) of the initial MC was eliminated.

Acknowledgements

Our special thanks to S. Keehn for field and laboratory assistance during the course of the 30-d depuration study, M. Keohane for field assistance during the 24-h gut clearance study, and J. Silveira for the preparation of samples for MC analysis. We also thank Meanook Biological Research Station for the use of field equipment, Kawtikh Resort (Hastings Lake) for boat launch access, B. Rolseth of the Limnology Laboratory (University of Alberta) for assistance in the laboratory, C. Paszkowski for use of the aquatics laboratory space, Jeff Goldberg for gastropod aquaria design and testing methodology, and K. Baier and M. Craig for guidance and assistance during PP1c assay analyses of phytoplankton and gastropod tissues. Special thanks to J. Burke for her critical reviews of and contributions to earlier manuscripts. This study was funded by a Natural Sciences and Engineering Research Council (NSERC) of Canada Research (Discovery) Grant to E. E. Prepas.

Notes

^aValues represent % decrease in toxin concentration compared to initial mean concentration at d 0 (1239 ng MCLR eq/g dry weight).