Quantification and modeling of inorganic carbon processing in scleractinian coral
Tansik, Anna Louisa
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Scleractinian corals build extensive reef frameworks that house extraordinary biodiversity and provide numerous ecosystem services to local human populations. Corals depend upon endosymbiotic dinoflagellates to provide them energy via photosynthesis, which supports both tissue and skeletal growth. Unfortunately, corals, and consequently reefs, are under threat from local and global stressors, including climate change. Increased anthropogenic CO2 emissions causing CO2 to invade the surface ocean, lowering pH levels and CO32- concentrations; this is ocean acidification (OA). Changes to the dissolved inorganic carbon (DIC) system have the potential to alter both photosynthesis (CO2 dependent) and calcification (CO32- dependent). Scleractinian corals are faced with the possibility of impacts to both of these vital processes, yet studies have shown no clear picture of what is likely to occur as pH continues to decline over the course of this century. Much of how corals process DIC is still unquantified, making it difficult to understand the scale of potential changes and what the drivers might be. In order to address these knowledge gaps, membrane inlet mass spectrometry methods, which have been used to describe carbon concentrating mechanisms and photosynthetic DIC kinetics in phytoplankton, were adapted for use with corals. Three different species of Caribbean coral were found to have high levels of carbonic anhydrase (CA) activity on the coral surface, within the coral tissues and in their symbionts. This enzyme is a key component of carbon concentrating mechanisms, and the surface CA provided CO2 to support half of net photosynthesis. Taxanomic variation was found at the coral level with regard to DIC saturation for photosynthesis, but all symbionts had a high affinity for carbon, suggestive of host controlled DIC delivery. This information more completely quantified photosynthesis with regard to DIC, and allowed a model of DIC flow through a coral to be developed. Running the model under projected OA conditions showed no change to photosynthesis, but slight reductions in both calcification and calcifying fluid pH. Investigating the drivers of these changes indicated that increased fluxes of CO2 into the calcifying fluid led to the declines, but biological modification of the calcifying fluid mitigated the effect.