REPORT Light availability determines susceptibility of reef building corals to ocean acidification D. J. Suggett • L. F. Dong • T. Lawson • E. Lawrenz • L. Torres • D. J. Smith Received: 20 June 2012 / Accepted: 5 December 2012 / Published online: 22 December 2012 Ó Springer-Verlag Berlin Heidelberg 2012 Abstract Elevated seawater pCO 2 , 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 nutri- ents), light availability fundamentally regulates calcifica- tion and is predicted to change for future reef environments alongside elevated pCO 2 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 multifacto- rial growth experiment to determine how light intensity and pCO 2 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 pCO 2 (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 pCO 2 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. Keywords Coral  Ocean acidification  Light  Acropora  Porites Introduction Almost one-third of all CO 2 emitted to the atmosphere over the last 200 years has been absorbed by the oceans (Sabine et al. 2004). Importantly, elevated seawater pCO 2 drives a complex change in carbonate chemistry lowering pH (Caldeira and Wickett 2005) and the availability of car- bonate (aragonite saturation, X) required for coral calcifi- cation and growth. Long-term records suggest that coral growth across entire reef systems has already declined in recent decades (De’ath et al. 2009) and hence reductions in ocean pH, or ‘‘ocean acidification’’ (OA), predicted for this century as CO 2 emissions continue to rise (Caldeira and Wickett 2005; IPCC 2007), could potentially be cata- strophic for the future form and function of coral reefs (Hoegh-Guldberg et al. 2007; Pandolfi et al. 2011). A growing wealth of experiments and observations has attempted to quantity the extent with which elevated pCO 2 impacts coral growth (Pandolfi et al. 2011; Chan and Connolly 2012; McCulloch et al. 2012); however, while these efforts provide a strong mechanistic understanding as to how pCO 2 (pH) controls biogeochemical and ecological Communicated by Biology Editor Dr. Anastazia Banaszak Electronic supplementary material The online version of this article (doi:10.1007/s00338-012-0996-7) contains supplementary material, which is available to authorized users. D. J. Suggett (&)  L. F. Dong  T. Lawson  E. Lawrenz  L. Torres  D. J. Smith Coral Reef Research Unit, School of Biological Sciences, University of Essex, Colchester CO4 3SQ, UK e-mail: dsuggett@essex.ac.uk 123 Coral Reefs (2013) 32:327–337 DOI 10.1007/s00338-012-0996-7