Effects of Increased pCO2 on Aragonite Crystal Morphology in Halimeda Spp

Atmospheric CO₂ concentrations are expected to be double pre-industrial levels before the end of this century. The ocean CO₂ reservoir is simultaneously absorbing increased quantities of CO₂, with concomitant increases in carbonic acid and decreases in pH. Open-ocean pH values have decreased by 0.1 since 1980 and are predicted to decrease 0.3 to 0.5 in the next 80 years. This process of ocean acidification will likely modify the biogeochemical processes of calcification and carbonate sediment production. Members of the genus Halimeda spp., a calcareous green segmented macroalgae, are important producers of carbonate sediments in tropical, shallow-water, carbonate settings. Aragonite crystals begin to precipitate approximately 36 hours after the growth of a new segment and continue until available space within the segment is completely filled. Halimeda spp. were grown in sealed aquaria containing an aragonite substrate, air bubbler, temperature and pH probes, and a CO₂-injection apparatus. The water in the aquaria was maintained at pH values of 8.2 (control), 8.0, 7.9, 7.8, and 7.5 by controlling the levels of pCO₂ in the water. Scanning electron microscopy was used to study ultrastructural details of aragonite crystal morphology. As levels of pCO₂ increased, changes in crystal growth density and individual crystal size were noted. This work should provide the foundation for an index linking atmospheric CO₂ concentrations to unaltered Halimeda spp. crystal morphology.