PSII photochemical efficiency and chlororespiration of Acropora millepora zooxanthellae in carbonated seawater

Abstract

Abstract Global trends of increasing atmospheric CO2, warming and eutrophication enrich seawater with CO2—either by absorption from the atmosphere or by microbial respiration—increasing seawater [H+] and [HCO3–] but decreasing [CO32–]. We asked whether, in the absence of warming, these changes in inorganic carbon chemistry affect the PSII photochemical efficiency of zooxanthellae in the reef building coral Acropora millepora. We assessed this efficiency experimentally using pulse-amplitude-modulation fluorometry at a temperature of 25°C, a daily maximum photosynthetically active radiation of ~ 65 µmol quanta m–2 s–1, and a seawater pCO2 that we gradually increased post-acclimation over nine days from ~ 496 to ~ 1290 µatm by injection of CO2 enriched air. Statistical analyses of fluorescence variables derived from saturation pulses delivered every 30 minutes and during induction–recovery experiments did not reveal evidence for an effect of CO2 enrichment on the PSII photochemical efficiency. Two of the three experiments, however, indicate the possibility of a positive effect of CO2 counteracting the gradual decrease in the photochemical efficiency that was observed under conditions of ambient seawater pCO2. Furthermore, we present evidence for chlororespiration—a nocturnal decrease in the PSII photochemical efficiency followed by a sharp increase at the onset of low irradiance. We conclude that seawater CO2 enrichment may not decrease the capacity for photosynthesis in A. millepora zooxanthellae to supply carbon of use in meeting the coral’s energy expenditures.