Calcification and Crystallography Responses of a Tropical Macroalgal Reef Sediment Producer, Halimeda spp., to Year 2100 pCO2 Levels Under High and Low Irradiance

Even though calcifying reef macroalgae of the Halimeda genus occur across a wide range of irradiances, the interactive effects of elevated pCO₂ and irradiance remain largely uncertain. To investigate how elevated pCO₂ may affect future shallow and deep Halimeda populations, aquaria experiments were conducted to examine calcification responses of seven species to elevated pCO₂ levels predicted for year 2100 (~1000 μatm) under low, sub-saturating (~75 μmol photons m^-2 s^-1) and high, saturating (~300 μmol photons m^-2 s^-1) irradiance. Crystallography of aragonite produced within new segments on adult thalli was characterized using scanning electron microscopy (SEM). We also examined non-living segment crystallography and changes in inorganic content to assess elevated pCO₂ effects on carbonate sediments generated from four Halimeda species. Net calcification rates varied highly among the species, but were unaffected by exposure to elevated pCO₂ for up to 42 d, regardless of irradiance intensity. Aragonite crystals produced within new segments exposed to elevated pCO₂ and contrasting irradiance treatments were indistinguishable based on crystal measurements and SEM imagery. There were also no observable differences in crystallography from non-living, heavily cemented segments after 21 d exposure to elevated pCO₂ compared to controls. By contrast, less-cemented segments contained more narrow crystals after 27 d exposure to elevated pCO₂ relative to controls. However, these differences were small (0.03 µm) and did not contribute significantly to changes in inorganic content. Based on these results, Halimeda is likely to produce new calcified segments with intact aragonite crystals under year 2100 pCO₂ levels at high and low irradiance. Halimeda carbonate sediment production under elevated pCO₂ may show a modest decline in species retaining fine aragonite crystals post-mortality, in contrast to those producing highly cemented segments more resistant to dissolution.