Journal Article

Effects of elevated p CO sub(2) and irradiance on growth, photosynthesis and calcification in Halimeda discoidea

Blackwelder, P L, Koch, M S, Peach, K E

2016

Overview

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 sub(max)), photosynthetic efficiency ([alpha]) and compensating irradiance (I sub(c)) were determined in short-term assays on live thalli after a 10 d exposure to 4 levels of CO sub(2) partial pressures (p CO sub(2); 491, 653, 982 and 1201 [mu]atm) under saturating (300 [mu]mol photons m super(-2) s super(-1)) and sub-saturating (90 [mu]mol photons m super(-2) s super(-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 [mu]atm) and ambient p CO sub(2) for 27 d to assess OA effects on carbonate sediments generated from H. discoidea. Even though P sub(max) was higher under elevated p CO sub(2), 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 p CO sub(2) 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 p CO sub(2) relative to controls, but differences were small (0.03 [mu]m) 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 p CO sub(2) levels at high and low irradiance, while aragonite crystals of the sediment may produce thinner needle carbonate muds.

• Subject: Brackish / Halimeda discoidea / Marine