Light availability determines susceptibility of reef building corals to ocean acidification

Elevated seawater pCO₂, and in turn ocean acidification (OA), is now widely acknowledged to reduce calcification and growth of reef building corals. As with other environmental factors (e.g., temperature and nutrients), light availability fundamentally regulates calcification and is predicted to change for future reef environments alongside elevated pCO₂ via altered physical processes (e.g., sea level rise and turbidity); however, any potential role of light in regulating the OA-induced reduction of calcification is still unknown. We employed a multifactorial growth experiment to determine how light intensity and pCO₂ together modify calcification for model coral species from two key genera, Acropora horrida and Porites cylindrica, occupying similar ecological niches but with different physiologies. We show that elevated pCO₂ (OA)-induced losses of calcification in the light (G _L) but not darkness (G _D) were greatest under low-light growth conditions, in particular for A. horrida. High-light growth conditions therefore dampened the impact of OA upon G _L but not G _D. Gross photosynthesis (P _G) responded in a reciprocal manner to G _L suggesting OA-relieved pCO₂ limitation of P _G under high-light growth conditions to effectively enhance G _L. A multivariate analysis of past OA experiments was used to evaluate whether our test species responses were more widely applicable across their respective genera. Indeed, the light intensity for growth was identified as a significant factor influencing the OA-induced decline of calcification for species of Acropora but not Porites. Whereas low-light conditions can provide a refuge for hard corals from thermal and light stress, our study suggests that lower light availability will potentially increase the susceptibility of key coral species to OA.

Ocean acidification

The oceans play a central role in the maintenance of life on Earth. Oceans provide extensive ecosystems for marine animals and plants covering two-thirds of the Earth’s surface, are essential sources of food, economic activity, and biodiversity, and are central to the global biogeochemical cycles. The oceans are the largest reservoir of carbon in the Planet, and absorb approximately one-third of the carbon emissions that are released to the Earth’s atmosphere as a result of human activities. Since the beginning of industrialization, humans have been responsible for the increase in one greenhouse gas, carbon dioxide (CO₂), from approximately 280 parts per million (ppm) at the end of the nineteenth century to the current levels of 390ppm. As well as affecting the surface ocean pH, and the organisms living at the ocean surface, these increases in CO₂ are causing global mean surface temperatures to rise.

Ocean acidification (video)