Effects of elevated pCO₂ and irradiance on growth, photosynthesis and calcification in Halimeda discoidea

Ocean acidification (OA) effects on photophysiology and calcification were examined in Halimeda discoidea, a calcifying macroalga that produces tropical reef sediments. Photosynthetic parameters, including maximum photosynthetic rate (P max), photosynthetic efficiency (α) and compensating irradiance (I c) were determined in short-term assays on live thalli after a 10 d exposure to 4 levels of CO₂ partial pressures (pCO₂; 491, 653, 982 and 1201 μatm) under satura ting (300 μmol photons m−2 s−1) and sub-saturating (90 μmol photons m−2 s−1) irradiance in an aquaria study. Morphology of aragonite crystals produced in segments formed on adult thalli was characterized using scanning electron microscopy (SEM). Further, we examined crystal morphology and changes in inorganic content of non-living segments exposed to elevated (1201 μatm) and ambient pCO₂ for 27 d to assess OA effects on carbonate sediments generated from H. discoidea. Even though P max was higher under elevated pCO₂, this photophysiological response did not result in higher calcification rates. Based on crystal measurements and SEM imagery, aragonite crystals within new segments were indistinguishable across pCO₂ and irradiance treatments. Under high irradiance, new segments showed a greater investment in organic versus inorganic production. Non-living segments contained narrower crystals after 27 d exposure to elevated pCO₂ relative to controls, but differences were small (0.03 μm) and did not contribute significantly to changes in normalized biomass or inorganic content. Based on these results, H. discoidea will likely produce new calcified segments with intact aragonite crystals under year 2100 pCO₂ levels at high and low irradiance, while aragonite crystals of the sediment may produce thinner needle carbonate muds.