Transcriptional profiles inferring thermal stress responses of scleractinian corals from Mediterranean Sea

Shallow-water corals of the Mediterranean Sea are facing a dramatic increase in water temperature due to climate change. In many cases, a shift toward environmental conditions outside the normal range of variability may cause suboptimal physiological performance, such as enhanced respiration, higher susceptibility to pathogens, bleaching, reduced calcification and tissue necrosis. However, not all corals are thought to be affected equally, because physiological performances and responses are the principal determinant of a species tolerance to environmental variability and may change the ability of corals to cope with changing conditions. In this study, transcriptional profiles of four genes involved in apoptosis (bax and bcl-2), redox regulation (sod) and energy metabolism (gapdh) have been analyzed in three common shallow-water Mediterranean scleractinian, namely Astroides calycularis, Balanophyllia europaea and Leptopsammia pruvoti. The main aim was to assess the contribution of these cytoprotective mechanisms to the physiological plasticity of the species, that possess different growth modes (solitary vs colonial) and trophic strategies (zooxanthellate vs azooxanthellate). Using quantitative real-time PCR, expression profiles after an acute heat-shock exposure (2h at 32°C + 0-24 h post-stress recovery) were assessed, and a potential timeline for the events occurring in the investigated species was disclosed. The overall transcriptional response to heat stress aligns with relative differences in stress susceptibility of the species and suggests a contribution of trophic strategy in the degree of coral resilience to stress. Understanding the overall molecular processes is demanding for corals inhabiting the Mediterranean (a biodiversity “hot spot”), in light of projected scenarios of anthropogenic global change.