Experimental study on the impact of dinoflagellate Alexandrium species on populations of the rotifer Brachionus plicatilis

In contrast, the remaining seven algal strains (A. tamarense ATHK, AT5-1, AT5-3, ATCI02, ATCI03; also Alexandrium sp1 and Alexandrium sp2) all have adverse effects on the rotifers. Dosing rotifers with respective algal cell densities of 2000 cells ml−1 each, for Alexandrium sp1, Alexandrium sp2, and A. tamarense strains ATHK and ATCI03 showed mean lethal time (LT50) on rotifer populations of 21, 28, 29, and 36h, respectively. The remaining three species (A. tamarense strains AT5-1, AT5-3, ATCI02) caused respective mean rotifer LT50s of 56, 56, and 71 h, compared to 160 h for the unexposed “starved control” rotifers. Experiments to determine ingestion rates for the rotifers, based on changes in their Chlorophyll a content, showed that the rotifers could feed on A. lusitanicum, A. minutum and A. tamarense strain AT-6, but could graze to little or no extent upon algal cells of the other seven strains. The effects on rotifers exposed to different cell densities, fractions, and growth phases of A. tamarense algal culture were respectively compared. It was found that only the whole algal cells had lethal effects, with strongest impact being shown by the early exponential growth phase of A. tamarense. The results indicate that some toxic mechanism(s), other than PSP and present in whole algal cells, might be responsible for the adverse effects on the exposed rotifers.