Coral calcification : insights from inorganic experiments and coral responses to environmental variables

Abstract

Coral calcification is examined using a laboratory model and living corals. In the laboratory model, abiogenic aragonite formed at high saturation state (Ω>~20) had a granular appearance and was enriched in trace elements, similar to centers of calcification and dark bands in corals. Abiogenic aragonite formed fibrous crystals at lower saturation states, similar to crystals which radiate out from centers of calcification. These similarities suggest the calcifying environment of the coral experiences a range of saturation states. To estimate when high or low saturation states occur within the coral, living corals were stained, staining patterns suggest dark bands form between dusk and dawn. A model is proposed in which daytime saturation state is limited by the availability of CO2. To test the potential for photosynthesis to limit CO2 availability to calcification, corals were grown under altered CO2 and nutrient levels. Elevated CO2 levels decreased calcification in zooxanthellate corals, however addition of nutrients reduced the negative impact of CO2. This suggests nutrient availability may limit photosynthesis under elevated pCO2 conditions. The effects of nutrients, CO2, and temperature were further tested on both zooxanthellate and azooxanthellate coral colonies. Unexpectedly, a gender difference was found in the effect of CO2 on calcification.