REPORT Antioxidant responses to heat and light stress differ with habitat in a common reef coral Thomas D. Hawkins 1,2 • Thomas Krueger 1,3 • Shaun P. Wilkinson 1 • Paul L. Fisher 1,4 • Simon K. Davy 1 Received: 25 February 2015 / Accepted: 2 September 2015 Ó Springer-Verlag Berlin Heidelberg 2015 Abstract Coral bleaching—the stress-induced collapse of the coral–Symbiodinium symbiosis—is a significant driver of worldwide coral reef degradation. Yet, not all corals are equally susceptible to bleaching, and we lack a clear understanding of the mechanisms underpinning their differential susceptibilities. Here, we focus on cellular redox regulation as a potential determinant of bleaching susceptibility in the reef coral Stylophora pistillata. Using slow heating (1 °Cd -1 ) and altered irradiance, we induced bleaching in S. pistillata colonies sampled from two depths [5–8 m (shallow) and 15–18 m (deep)]. There was signif- icant depth-dependent variability in the timing and extent of bleaching (loss of symbiont cells), as well as in host enzymatic antioxidant activity [specifically, superoxide dismutase and catalase (CAT)]. However, among the coral fragments that bleached, most did so without displaying any evidence of a host enzymatic antioxidant response. For example, both deep and shallow corals suffered significant symbiont loss at elevated temperature, but only deep colonies exposed to high temperature and high light dis- played any up-regulation of host antioxidant enzyme activity (CAT). Surprisingly, this preceded the equivalent antioxidant responses of the symbiont, which raises ques- tions about the source(s) of hydrogen peroxide in the symbiosis. Overall, changes in enzymatic antioxidant activity in the symbionts were driven primarily by irradi- ance rather than temperature, and responses were similar across depth groups. Taken together, our results suggest that in the absence of light stress, heating of 1 °Cd -1 to 4 °C above ambient is not sufficient to induce a substantial oxidative challenge in S. pistillata. We provide some of the first evidence that regulation of coral enzymatic antioxi- dants can vary significantly depending on habitat, and, in terms of determining bleaching susceptibility, our results suggest a significant role for the host’s differential regu- lation of cellular redox status. Keywords Coral bleaching  Cnidarian–dinoflagellate symbiosis  Symbiodinium  Stylophora pistillata  Oxidative stress  Climate change Introduction The productivity of coral reefs depends on a symbiosis between reef corals and photosynthetic dinoflagellates (Symbiodinium) (Muscatine and Hand 1958; Davy et al. 2012). Despite their ecological success, reef corals are sensitive to high temperature and high light, which induce a pathology known as coral bleaching. Bleaching describes the whitening of coral tissues due to the loss of symbiont cells and pigments (Goreau 1990; Lesser 2011). The Communicated by Biology Editor Dr. Line K. Bay Electronic supplementary material The online version of this article (doi:10.1007/s00338-015-1345-4) contains supplementary material, which is available to authorized users. & Simon K. Davy simon.davy@vuw.ac.nz 1 School of Biological Sciences, Victoria University of Wellington, Wellington 6140, New Zealand 2 College of Earth, Ocean and Environment, University of Delaware, Lewes, DE 19958, USA 3 Laboratory for Biological Geochemistry, School of Architecture, Civil and Environmental Engineering (ENAC), E ´ cole Polytechnique Fe ´de ´rale de Lausanne (EPFL), 1015 Lausanne, Switzerland 4 School of Civil Engineering, University of Queensland, Brisbane, QLD 4072, Australia 123 Coral Reefs DOI 10.1007/s00338-015-1345-4