Coral hosts actively accumulate inorganic carbon for zooxanthellae to maintain symbiotic relationships

Abstract

AbstractHigh primary productivity of coral reefs is widely attributed to the mutualistic symbiosis between corals and zooxanthellae. The mechanisms that maintain this symbiosis remain poorly explored. Given the high carbon demands by zooxanthellae, we hypothesized that corals may have evolved strategies to actively accumulate dissolved inorganic carbon (DIC) to maintain the host-algae symbiosis. Carbon supplies and consumption were evaluated in the scleractinian coral (Goniopora lobata) and zooxanthellae under light and dark conditions. Results suggest that zooxanthellae are high DIC consumers, requiring about 2-3 fold more DIC than free-living species. The corals were high DIC producers, but were unable to effectively eliminate all the DIC they produced and actively accumulated DIC in the light, with concentrations up to 4.2 fold higher than in the surrounding seawater. Transcriptomic analysis found several DIC enrichment pathways, including CO2concentrating mechanisms, respiration, calcification and phosphoenolpyruvate carboxylase. It appears that corals actively enrich DIC for zooxanthellae.