We studied the photosynthetic acclimatization of two populations of Asparagopsis taxiformis from two biogeographic regions (Santa Catalina Island, California, and Kaneohe Bay, Hawaii) in two seasons. We compared thermal variability between locations and estimated the environmental optima and tolerance limits of photosynthesis to understand the relationship between phenotypic plasticity and thermal variability at each location. Photosynthetic performance was assessed using oxygen evolution and pulsed amplitude modulated fluorometry to quantify responses to thermal variation. A. taxiformis from both locations had similar high temperature tolerances for maximum net photosynthesis (Pnet(max)) and maximum relative electron transport rate rETR(max) (similar to 30 degrees C), but different cold temperature tolerances. Respiration rates from both locations (summer and winter) always increased above 25 degrees C. Chlorophyll fluorescence yield from dark-adapted samples from both locations decreased at warmer temperatures in the winter and at colder temperatures during the summer. Populations exposed to greater thermal variability (across both short and long time scales) displayed greater capacity for photosynthetic acclimatization. A. taxiformis from California showed seasonal changes in photosynthetic performance within the environmental temperature range (14-21 degrees C), whereas individuals from Hawaii did not. The narrower temperature tolerance (short-term response) of subtropical Asparagopsis may make it more susceptible than temperate populations to the predicted increases in global sea surface temperatures. Acclimatization of photosynthesis may play an important role in enabling A. taxiformis to respond to a variable environment and to persist in different biogeographic regions.