Calcification and photophysiology responses to elevated pCO2 in six Halimeda species from contrasting irradiance environments on Little Cayman Island reefs

• All six Halimeda species formed new calcified segments under elevated pCO2.
• Aragonite crystals precipitated under ambient and elevated pCO2 were similar.
• Heavily cemented Halimeda sediments may be resistant to dissolution under elevated pCO2.

While elevated pCO2 associated with ocean acidification has been shown to lower calcification rates in some marine calcifiers, the modulating potential of irradiance is not well studied. Therefore, we examined the interactive effects of irradiance (high and low) and elevated pCO2 on six species of the macroalgal genus, Halimeda, an ecologically important tropical calcifier. Calcification, growth, and aragonite crystal formation were studied in a 42 d aquaria experiment. Species-specific photophysiology and inorganic C saturation levels were ascertained by establishing photosynthesis to irradiance and inorganic C (Ci) relationships potentially linking photophysiology to growth and calcification responses to elevated pCO2. Because of Halimeda's role as a sediment producer on tropical reefs, the effect of elevated pCO2 on dissolution of non-living segments was also examined. Net calcification rates varied among species, with no significant pCO2 or irradiance effects. However, low irradiance enhanced new apical segment growth in all six species. Crystals formed in new apical segments under elevated pCO2. Non-living segment mineral:organic content and internal calcium carbonate (CaCO3) crystal microstructure remained stable over 21 d at elevated pCO2. Species-specific photosynthetic responses corresponded to irradiance conditions at collection sites. For example, high irradiance-adapted species photosynthetic rates were enhanced under increased availability of Ci. In contrast, low irradiance-adapted species photosynthetic rates were saturated under current Ci levels. While species-specific photosynthetic responses to elevated pCO2 occurred, these photophysiology shifts did not result in reduced calcification or microstructural alteration of carbonate crystals at pCO2 levels predicted for the year 2100 in any of the six Halimeda species examined.