The effect of ocean acidification on symbiont photorespiration and productivity in Acropora formosa

Ocean acidification is expected to lower the net accretion of coral reefs yet little is known about its effect on coral photophysiology. This study investigated the effect of increasing CO2 on photosynthetic capacity and photoprotection in Acropora formosa. The photoprotective role of photorespiration within dinoflagellates (genus Symbiodinium) has largely been overlooked due to focus on the presence of a carbon-concentrating mechanism despite the evolutionary persistence of a Form II Rubisco. The photorespiratory fixation of oxygen produces phosphoglycolate that would otherwise inhibit carbon fixation though the Calvin cycle if it were not converted to glycolate by phosphoglycolate phosphatase (PGPase). Glycolate is then either excreted or dealt with by enzymes in the photorespiratory glycolate and/or glycerate pathways adding to the pool of carbon fixed in photosynthesis. We found that CO2 enrichment led to enhanced photoacclimation (increased chlorophyll a per cell) to the subsaturating light levels. Light-enhanced dark respiration per cell and xanthophyll de-epoxidation increased, with resultant decreases in photosynthetic capacity (P-nmax) per chlorophyll. The conservative CO2 emission scenario (A1B; 600-790 ppm) led to a 38% increase in the P-nmax per cell whereas the 'business-as-usual' scenario (A1F1; 1160-1500 ppm) led to a 45% reduction in PGPase expression and no change in P-nmax per cell. These findings support an important functional role for PGPase in dinoflagellates that is potentially compromised under CO2 enrichment.