Increased irradiance availability mitigates the physiological performance of species of the calcifying green macroalga Halimeda in response to ocean acidification

Although negative responses of tropical calcifying organisms to ocean acidification have been widely reported, the modulating potential of irradiance combined with elevated pCO2 has not been well studied. In this study, the interactive effects of elevated pCO2 and irradiance availability on the physiology of calcifying macroalgae Halimeda cylindracea and Halimeda lacunalis were investigated using a fully factorial, 28-day aquaria coupling experiment. The results of the present study demonstrate that elevated pCO2 negatively influences growth, photosynthesis, calcification and other physiological processes of both Halimeda species. However, these negative effects could be mitigated to some extent by increased irradiance availability. Specific growth rate (SGR), net calcification rates (Gnet) and maximum quantum yield (Fv/Fm) decreased significantly by 6.84%–86.70%, 51.78%–62.29% and 2.37%–28.91% in elevated pCO2 treatments. However, SGR, Gnet and Fv/Fm increased by 3.39%–84.78%, 29.61%–40.68% and 1.68%–6.92% in high irradiance conditions, respectively. Chl-a in elevated pCO2 treatments was 7.75%–61.25% lower than ambient pCO2 conditions, while the carotenoid content increased by 12.12%–57.45% in low irradiance conditions from day 20–28. Malondialdehyde (MDA) content was higher in elevated pCO2 treatments. However, there was also a two- to four-fold increase in proline content in elevated pCO2 treatments. Tissue total organic carbon (TCorg) and nitrogen (TN) were positively correlated to CO2 enrichment. The results of the current study suggested that elevated pCO2 negatively influenced the physiological responses of Halimeda, while increased irradiance availability may enhance the metabolic performance in response to ocean acidification.