Temperature responses of pigment contents, chlorophyll fluorescence characteristics, and antioxidant defenses in Gracilariopsis lemaneiformis (Gracilariales, Rhodophyta) under different CO2 levels

Abstract

Gracilariopsis lemaneiformis was cultured at different temperatures (15, 23, and 31 °C) under CO2 levels of 390 and 800 μL L−1 for 2 weeks, to elucidate the physiological and biochemical responses to variable temperature at elevated CO2 in this marine-cultivated species in China. The relative growth rates (RGR) was highest at 23 °C among the three temperature levels. Photosynthetic pigments (Chl a, Car, PB) were CO2 independent, but the 15 °C-grown algae were always larger. In contrast, the Chl a/PB ratios increased with increasing growth temperature. The maximum relative electron transport rates (rETRm) presented a strong potential to acclimate lower growth temperature. Moreover, the photosynthetic acclimation was enhanced by elevated CO2. Adjustments of Chl a fluorescence and antioxidant enzymes were responsible for this photosynthetic acclimation strategy. A rise of Chl a fluorescence (α, Ek, qP, and Y(II)) and superoxide dismutase (SOD) and peroxidase (POD) activities was observed when growth temperature decreased. These changes indicate a strong defense capability for reactive oxygen species (ROS) and a crucial role of elevated CO2 during long-term photosynthetic acclimation. Additionally, a decrease of the maximal quantum yield (Fv/Fm) was found when measured at 39 °C. Up-regulation of thermal dissipation was observed, as NPQ significantly increased at 31 °C at both CO2 levels.