Journal Article

External carbonic anhydrase in three Caribbean corals: quantification of activity and role in CO sub(2) uptake

Hopkinson, Brian M, Fitt, William K, Tansik, Anna L

2015

Overview

Scleractinian corals have complicated inorganic carbon (C sub(i)) transport pathways to support both photosynthesis, by their symbiotic dinoflagellates, and calcification. The first step in C sub(i) acquisition, uptake into the coral, is critical as the diffusive boundary layer limits the supply of CO sub(2) to the surface and HCO sub(3) super(-) uptake is energy intensive. An external carbonic anhydrase (eCA) on the oral surface of corals is thought to facilitate CO sub(2) uptake by converting HCO sub(3) super(-) into CO sub(2), helping to overcome the limitation imposed by the boundary layer. However, this enzyme has not yet been identified or detected in corals, nor has its activity been quantified. We have developed a method to quantify eCA activity using a reaction-diffusion model to analyze data on super(18)O removal from labeled C sub(i). Applying this technique to three species of Caribbean corals (Orbicella faveolata, Porites astreoides, and Siderastrea radians) showed that all species have eCA and that the potential rates of CO sub(2) generation by eCA greatly exceed photosynthetic rates. This demonstrates that eCA activity is sufficient to support its hypothesized role in CO sub(2) supply. Inhibition of eCA severely reduces net photosynthesis in all species (on average by 46 plus or minus 27 %), implying that CO sub(2) generated by eCA is a major carbon source for photosynthesis. Because of the high permeability of membranes to CO sub(2), CO sub(2) uptake is likely driven by a concentration gradient across the cytoplasmic membrane. The ubiquity of eCA in corals from diverse genera and environments suggests that it is fundamental for photosynthetic CO sub(2) supply.

• Subject: Porites astreoides / Scleractinia / Siderastrea radians