Increased light intensity enhances photosynthesis and biochemical components of red macroalga of commercial importance, Kappaphycus alvarezii, in response to ocean acidification.

The concentration of atmospheric carbon dioxide (CO) has increased drastically over the past several decades, resulting in the pH of the ocean decreasing by 0.44 ± 0.005 units, known as ocean acidification (OA). The Kappaphycus alvarezii (Rhodophyta, Solieriaceae), is a commercially and ecologically important red macroalga with significant CO absorption potential from seawater. The K. alvarezii also experienced light variations from self-shading and varied cultivation depths. Thus, the aim of present study was to investigate the effects of two pCO levels (450 and 1200 ppmv) and three light intensities (50, 100, and 150 μmol photons·m·s) on photosynthesis and the biochemical components in K. alvarezii. The results of the present study showed that a light intensity of 50 μmol photons·m·s was optimal for K. alvarezii photosynthesis with 0.663 ± 0.030 of F/F and 0.672 ± 0.025 of F'/F'. Phycoerythrin contents at two pCO levels decreased significantly with an increase in light intensity by 57.14-87.76%, while phycocyanin contents only decreased from 0.0069 ± 0.001 mg g FW to 0.0047 ± 0.001 mg g FW with an increase in light intensity at 1200 ppmv of pCO. Moreover, moderate increases in light intensity and pCO had certain positive effects on the physiological performance of K. alvarezii, specifically in terms of increasing soluble carbohydrate production. Although OA and high light levels promoted total organic carbon accumulation (21.730 ± 0.205% DW) in K. alvarezii, they had a negative impact on total nitrogen accumulation (0.600 ± 0.017% DW).