Effects of pCO(2) on photosynthesis and respiration of tropical scleractinian corals and calcified algae

The effects of ocean acidification (OA) on coral reefs have been studied thoroughly with a focus on the response of calcification of corals and calcified algae. However, there are still large gaps in our knowledge of the effects of OA on photosynthesis and respiration of these organisms. Comparisons among species and determination of the functional relationships between pCO(2) and either photosynthesis or respiration are difficult using previously published data, because experimental conditions typically vary widely between studies. Here, we tested the response of net photosynthesis, gross photosynthesis, dark respiration, and light-enhanced dark respiration (LEDR) of eight coral taxa and seven calcified alga taxa to six different pCO(2) levels (from 280 to 2000 mu atm). Organisms were maintained during 7-10 days incubations in identical conditions of light, temperature, and pCO(2) to facilitate comparisons among species. Net photosynthesis was not affected by pCO(2) in seven of eight corals or any of the algae; gross photosynthesis did not respond to pCO(2) in six coral taxa and six algal taxa; dark respiration also was unaffected by pCO(2) in six coral and six algae; and LEDR did not respond to pCO(2) in any of the tested species. Overall, our results show that pCO(2) levels up to 2000 mu atm likely will not fertilize photosynthesis or modify respiration rates of most of the main calcifiers on the back reef of Moorea, French Polynesia.