The toxigenic red-tide-forming dinoflagellates Alexandrium leei and Alexandrium catenella differ in terms of the sensitivity to strong light and low temperature of their photosynthetic machinery

Alexandrium leei and Alexandrium catenella are noxious dinoflagellates that form red tides but the two species differ substantially in optimal growth conditions. For formation of blooms in natural environments, A. leei favors moderate temperatures (around 20 °C) while A. catenella favors much lower temperatures (around 10 °C). We examined the effects of temperature and light on cell proliferation and photosynthesis in cell cultures of the two species. Under light at 100 μmol photons m−2 s−1, the growth rate of both species was highest at 20 °C. At 25 °C, A. leei continued to proliferate but A. catenella did not survive. Conversely, at lower temperatures 12 °C and 15 °C, the growth rate of A. leei was very low while that of A. catenella remained high. The net photosynthetic activity, as determined in terms of oxygen evolution, was highest at 20 °C in A. leei. By contrast, it was highest at 15 °C in A. catenella. Illumination of cells with strong light at 1000 μmol photons m−2 s−1 decreased the activity of photosystem II (PSII), as determined in terms of Fv/Fm, in both species. In A. leei, the activity of PSII declined more rapidly at 12 °C and 15 °C than at 20 °C and 25 °C under strong light. In A. catenella, the activity of PSII declined more rapidly at 20 °C than at 12 °C and 15 °C. Thus, A. catenella appeared to be more resistant to photoinhibition of PSII at low temperatures than A. leei. In addition, the ability to repair photodamaged PSII at 12 °C and 15 °C was greater in A. catenella than that in A. leei. Thus, the tolerance of PSII to strong light and low temperatures might determine the range of temperatures that supports the formation of blooms of the two Alexandrium species.