Acclimation of thermotolerant algae to light and temperature interaction¹

Here, we explore the responses of photosynthesis and related cellular processes in the thermotolerant microalga Micractinium sp. acclimated to limiting and saturating irradiances combined with elevated temperatures, using a novel computer-controlled multi-sensor system. This system allows for the monitoring of online values of oxygen exchange during photosynthesis and respiration with high accuracy. Micractinium sp. cells showed maximum growth and net oxygen production rates under the optimal temperature of 25°C regardless of the light acclimation conditions. Our results show that the upper thermal threshold for Micractinium sp. photosynthesis and growth ranges between 35°C and 40°C. This microalga exhibited stable photosynthetic efficiency and effective non-photochemical quenching (NPQ) under saturating light, and was more susceptible to temperature change when acclimated to limiting light levels. These results demonstrate that the acclimation of thermotolerant microalgae to saturating light helps to enhance the thermal tolerance of PSII. This feature results from enhanced heat stability of PSII photochemistry and oxygen evolution.