Effects of light and darkness on pH regulation in three coral species exposed to seawater acidification

Coral reefs are forecast to undergo radical changes in a high CO₂ world.  Research at tropical locations where the seawater is naturally acidified by volcanic CO₂ seeps suggests that future coral reefs will be characterised by and “winner” species more capable of tolerating ocean acidification than “loser” species.
A recent study conducted by the physiology and biochemistry team at CSM, directed by Dr Sylvie Tambutté, compared the capacity of three widespread coral species to regulate pH under varying degrees of acidification. As corals contain symbiotic algae that can increase coral internal pH when they photosynthesize, the study also carried out the interspecies comparison in light and dark conditions.
Results show that the coral Stylophora pistillata, previously dubbed an ocean acidification resistant coral by other studies, does indeed live up to its reputation – as this species was able to maintain intracellular pH and at elevated pH at the site of calcification even under acidification in light and darkness. By contrast the other species tested, Pocillopora damicornis and Acropora hyacinthus displayed a weaker capacity to maintain pH against seawater acidification.  In particular, Acropora hyacinthus was most vulnerable in darkness. Measurements of calcification of this species revealed the skeleton of this species actually started to dissolve under acidification in darkness, perhaps due to its weaker capacity to regulate pH.
Overall, the study shows that the corals species do indeed vary in their ability to regulate their pH against acidification, suggesting there are physiological winners and losers under acidification. Importantly, exposure to light can help mitigate the effects of acidification in certain coral species. The study was laboratory based and carried out on cultured corals, so the results cannot be used with confidence to predict what happens in the natural environment.  However, lab studies such as these are important as they can rule-out confounding environmental factors that are impossible to control in the field, giving us clear insight into the comparative physiology of reef corals.
 

• Venn, A. A., Tambutté, E., Caminiti-Segonds, N., Techer, N., Allemand, D., & Tambutté, S. (2019). Effects of light and darkness on pH regulation in three coral species exposed to seawater acidification. Scientific reports, 9(1), 2201.

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