by Tim Bateman
Recently, researchers have invested a considerable amount of effort investigating the effects of ocean acidification (OA) on coral reefs and specific coral species. Ocean acidification, the lowering of the ocean’s pH due to anthropogenic CO2 input, can have profound impacts on the calcification rate of corals and will only continue to get worse unless we find a way to curb carbon emissions. Corals secrete skeletons of aragonite, a form of calcium carbonate (CaCO3), to grow, in a process called calcification, but this process is energy intensive and the stability of CaCO3 is pH dependent. As pH decreases and acidity increases, oceans become less saturated with aragonite, making it harder for organisms to calcify. Scientists have recorded conflicting responses to acidification for different coral species, including small and large decreases in calcification or no response in calcification at all (Langdon et al. 2005, Ries et al. 2009, Nakamura et al. 2017, Cole et al. 2018, Zheng et al. 2018). All of the aforementioned results confound the understanding of corals to OA. To further complicate OA, there are interactions with other abiotic factors on reefs that can affect coral calcification including temperature, light, and nutrient availability. Together, this convolution makes it difficult, even for researchers in the field, to reconcile the effects of OA on a reef-wide scale and thus tease apart the mechanisms affecting ecological processes. OA is a considerable threat to coral reefs worldwide, but to facilitate management and mitigation we must first understand exactly how it affects these systems.
One way OA threatens reefs is by contributing to the dissolution of CaCO3, the process by which CaCO3 dissolves. This process is essentially the opposite of calcification and thus works to dissolve coral skeletons, preventing growth. Dissolution is not the same as a reduction in calcification even though the outcome of both is largely the same. The key ecological difference for reefs is in the large uniform increase in dissolution seen across reef systems as pH decreases. If OA continues at the current rate, before the end of the century corals will not be able to grow their skeletons fast enough to keep up with dissolution (Dove et al. 2013, Eyre et al. 2018). Figure 1 shows how changes in aragonite saturation state impact three different processes critical to reef growth. Aragonite saturation state dictates how easy it is for aragonite that is dissolved in the water to be used for calcification. Coral calcification and sediment dissolution both contribute significantly to the third process, net ecosystem calcification which is the balance between calcification and dissolution across the whole reef. The response of coral calcification to OA is variable across studies but overall decreases slightly with aragonite saturation while dissolution increases significantly.
In conclusion, OA presents considerable challenges to the survival of coral reef communities by impacting CaCO3 chemistry. Even though corals may not display a uniform decrease in calcification with decreasing pH, net ecosystem calcification is declining due to dissolution. By the end of the 21st century, coral reefs will be in a state of net dissolution and CaCO3 will be dissolving faster than organisms can calcify.
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