Variation in antioxidant gene expression in the scleractinian coral Acropora millepora under laboratory thermal stress

Abstract

The effects of climate change over the last few decades have pushed the majority of reef-building corals close to their upper thermal limit. Upon exposure to thermal stress, the intra-cellular formation of harmful oxygen intermediates can lead to the disruption of the obligate symbiosis between the coral host and their dinoflagellate endosymbionts (zooxanthellae), a process known as coral bleaching. Applications of molecular techniques to cnidarian research have recently enhanced our understanding of the magnitude of corals’ transcriptional response to various stressors. In the present study, we developed a quantitative real-time polymerase chain reaction assay to assess expression levels of 4 genes involved in the corals’ oxidative stress response (HSP70, MnSOD, ferritin, Zn2+-metalloprotease) after exposure to laboratory-controlled thermal stress. Using the Indo-Pacific reef coral Acropora millepora, our study provides the first population-scale analysis of antioxidant gene expression in coral. Despite the significant up-regulation of those 4 genes in the thermally stressed samples relative to non-stressed samples, our results show that there is an enormous intra- as well as inter-colony variation in transcript abundance at a particular point in time. We discuss the potential roles of ferritin and Zn2+-metalloprotease in the break down of the intra-cellular Fe2+-homeostasis and in coral host cell detachment, respectively, during bleaching conditions. Our results emphasise the importance of measuring inter-individual variation to gain an insight into the population response to a common and increasingly encountered environmental stressor.