High toxin content in the dinoflagellate Gonyaulax excavata in nature

Measurements of toxin content in motile cells of the marine dinoflagellate Gonyaulax excavata were made during its annual blooms in the Bay of Fundy in 1980 through 1983. Plankton organisms were collected with a 20 micron mesh net, filtered to remove zooplankton, centrifuged and extracted by boiling in 0.1 N HCl. Toxin content was measured by mouse bioassay. The plankton material was dominated by G. excavata, with Scrippsiella trochoidea sometimes co-dominating. Before extraction, G. excavata cells were counted microscopically. Toxin values ranged from 2.7 X 10(-6) to 1.1 X 10(-3) micrograms saxitoxin (STX) equivalents per G. excavata cell. The mean of 32 determinations was 2.1 X 10(-4) micrograms STX equivalents per cell (+/- 0.5 X 10(-4) S.E.), which is four times greater than the maximum toxin content recorded for seven strains of G. excavata from the Bay of Fundy when grown in culture. Results show that G. excavata motile cells can contain high levels of toxins in nature and strongly suggest their toxin content in nature can be much greater than under normal laboratory culture conditions. This implies that toxin level data derived from Gonyaulax cultures should be interpreted with caution. Calculations suggest the toxins are major constituents of wild G. excavata cells, accounting on average for about 4% of the total dry weight.