Decline in symbiont densities of tropical and subtropical scleractinian corals under ocean acidification

Ocean acidification changes the carbonate chemistry of seawater in a manner that reduces the biomineralisation rate of reef-building corals. Other effects of acidification on coral physiology are less well explored, and recent debate has focused on whether ocean acidification causes a change in Symbiodinium densities within tropical and subtropical reef-building corals. Within the framework of null-hypothesis significance testing, some aquaria experiments have provided evidence for a decrease in symbiont densities within coral tissue under ocean acidification (whilst others have suggested an increase). However, null effects have prevailed in the majority of such experiments, and so the question has remained unresolved. This study attempted to resolve this question using a meta-analytic framework, by establishing the effect sizes for symbiont density change under ocean acidification from a structured search of the literature. A regression of effect size (Hedge’s d) versus level of ocean acidification revealed a statistically significant negative relationship, with symbiont density per cm2 decreasing as the level of ocean acidification increased. The decline amounted to an additional 0.07 standard deviations of difference in symbiont density between corals in control (near present day) and acidified seawater with every 100 μatm of increase in partial pressure of CO2 in seawater (a relationship with an r2 of 0.24). A further unresolved question is whether ocean acidification will synergistically exacerbate (or diminish) symbiont density reductions caused by elevated temperature. An analysis of covariance did not reveal a greater decline in symbiont densities with increasing acidification at elevated temperature compared to non-stressful temperature, though this latter analysis should be viewed as exploratory due to a lower sample size. The well-supported evidence for a decline in symbiont densities in tropical and subtropical corals under ocean acidification now provides an impetus for sustained investigation of the consequences of such a change for holobiont functioning and the broader function of the coral reef ecosystem.