Opposite Growth Responses of Alexandrium minutum and Alexandrium catenella to Photoperiods and Temperatures

Shift of phytoplankton niches from low to high latitudes has altered their experienced light exposure durations and temperatures. To explore this interactive effect, the growth, physiology, and cell compositions of smaller Alexandrium minutum and larger A. catenella, globally distributed toxic red tide dinoflagellates, were studied under a matrix of photoperiods (light:dark cycles of 8:16, 16:8, and 24:0) and temperatures (18 °C, 22 °C, 25 °C, and 28 °C). Under continuous growth light condition (L:D 24:0), the growth rate (µ) of small A. minutum increased from low to medium temperature, then decreased to high temperature, while the µ of large A. catenella continuously decreased with increasing temperatures. Shortened photoperiods reduced the µ of A. minutum, but enhanced that of A. catenella. As temperature increased, cellular Chl a content increased in both A. minutum and A. catenella, while the temperature-induced effect on RubisCO content was limited. Shortened photoperiods enhanced the Chl a but reduced RubisCO contents across temperatures. Moreover, shortened photoperiods enhanced photosynthetic capacities of both A. minutum and A. catenella, i.e., promoting the PSII photochemical quantum yield (F(V)/F(M), Φ(PSII)), saturation irradiance (E(K)), and maximum relative electron transfer rate (rETRmax). Shortened photoperiods also enhanced dark respiration of A. minutum across temperatures, but reduced that of A. catenella, as well as the antioxidant activities of both species. Overall, A. minutum and A. catenella showed differential growth responses to photoperiods across temperatures, probably with cell size.