Abstract
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 (FV/FM, ΦPSII), saturation irradiance (EK), 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.
Highlights
Introduction published maps and institutional affilGlobal warming has increased the temperature in surface oceans [1,2], extending the low thermal limit of phytoplankton and shifting their niches from low to high latitudes [3,4,5], which has led to the new appearance of temperate species at higher latitudes and even subpolar regions [3]
Temperature is well known to regulate phytoplankton growth through affecting their cellular biochemical reactions that are catalyzed by the involved enzymes, such as ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO), and altering their physiological activities [6]
The response of μ to increasing temperatures was strongly influenced by the photoperiod for large A. catenella (p < 0.01), but not for small A. minutum
Summary
Global warming has increased the temperature in surface oceans [1,2], extending the low thermal limit of phytoplankton and shifting their niches from low to high latitudes [3,4,5], which has led to the new appearance of temperate species at higher latitudes and even subpolar regions [3]. Such a niches shift varies the temperatures and light exposure durations (i.e., diel light:dark cycle) experienced by phytoplankton. The photoperiod that usually alters the cell-received maximum irradiance or iations.
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