Does elevated pCO2 affect reef octocorals?

Increasing anthropogenic pCO₂ alters seawater chemistry, with potentially severe consequences for coral reef growth and health. Octocorals are the second most important faunistic component in many reefs, often occupying 50% or more of the available substrate. Three species of octocorals from two families were studied in Eilat (Gulf of Aqaba), comprising the zooxanthellate Ovabunda macrospiculata and Heteroxenia fuscescens (family Xeniidae), and Sarcophyton sp. (family Alcyoniidae). They were maintained under normal (8.2) and reduced (7.6 and 7.3) pH conditions for up to 5 months. Their biolological features, including protein concentration, polyp weight, density of zooxanthellae, and their chlorophyll concentration per cell, as well as polyp pulsation rate, were examined under conditions more acidic than normal, in order to test the hypothesis that rising pCO₂ would affect octocorals. The results indicate no statistically significant difference between the octocorals exposed to reduced pH values compared to the control. It is therefore suggested that the octocorals’ tissue may act as a protective barrier against adverse pH conditions, thus maintaining them unharmed at high levels of pCO₂.

Ocean acidification does not affect the physiology of the tropical coral Acropora digitifera during a 5-week experiment

The increase in atmospheric CO₂ concentration, which has resulted from the burning of fossil fuels, is being absorbed by the oceans and is causing ocean acidification. Ocean acidification involves the decrease of both the pH and the calcium carbonate saturation state. Ocean acidification is predicted to impact the physiology of marine organisms and reduce the calcification rates of corals. In the present study, we measured the rates of calcification, respiration, photosynthesis, and zooxanthellae density of the tropical coral Acropora digitifera under near-natural summertime temperature and sunlight for a 5-week period. We found that these key physiological parameters were not affected by both mid-CO₂ (pCO₂ = 744 ± 38, pH = 7.97 ± 0.02, Ω_arag = 2.6 ± 0.1) and high-CO₂ conditions (pCO₂ = 2,142 ± 205, pH = 7.56 ± 0.04, Ω_arag = 1.1 ± 0.2) throughout the 35 days experimental period. Additionally, there was no significant correlation between calcification rate and seawater aragonite saturation (Ω_arag). These results suggest that the impacts of ocean acidification on corals physiology may be more complex than have been previously proposed.

Major cellular and physiological impacts of ocean acidification on a reef building coral

As atmospheric levels of CO₂ increase, reef-building corals are under greater stress from both increased sea surface temperatures and declining sea water pH. To date, most studies have focused on either coral bleaching due to warming oceans or declining calcification due to decreasing oceanic carbonate ion concentrations. Here, through the use of physiology measurements and cDNA microarrays, we show that changes in pH and ocean chemistry consistent with two scenarios put forward by the Intergovernmental Panel on Climate Change (IPCC) drive major changes in gene expression, respiration, photosynthesis and symbiosis of the coral, Acropora millepora, before affects on biomineralisation are apparent at the phenotype level. Under high CO₂ conditions corals at the phenotype level lost over half their Symbiodinium populations, and had a decrease in both photosynthesis and respiration. Changes in gene expression were consistent with metabolic suppression, an increase in oxidative stress, apoptosis and symbiont loss. Other expression patterns demonstrate upregulation of membrane transporters, as well as the regulation of genes involved in membrane cytoskeletal interactions and cytoskeletal remodeling. These widespread changes in gene expression emphasize the need to expand future studies of ocean acidification to include a wider spectrum of cellular processes, many of which may occur before impacts on calcification.