Coral physiological adaptations to air exposure: heat shock and oxidative stress responses in Veretillum cynomorium

Abstract

Intertidal environments are boundaries between marine and terrestrial systems and are subject to rapid fluctuations in physical characteristics across tidal cycles. Although extreme low-tide, air exposure (hypoxia), high-irradiance and thermal stress are structuring forces of intertidal coral communities, some octocorals have developed the ability to withstand emersion. Here, we investigated, for the first time, the physiological mechanisms that enable an octocoral, Veretillum cynomorium, to tolerate the rapid cyclical fluctuations of the intertidal environment. A significant increase in expression of two molecular chaperones, heat shock cognate 70 (HSC70) and heat shock protein 70 (HSP70), was observed throughout the emersion period, which was then followed by a drop during immersion. An opposite trend was observed for the peroxidase activity, as there was no malondialdehyde (MDA) build-up in coral tissues during air exposure. Thus, coral tissues were not subjected to any peroxidative damage during air exposure. Additionally, during the emersion phase, there was a significant increase in CAT and GST activities, whereas SOD activity remained stable. Altogether, our results suggest that HSC70/HSP70, CAT and GST constitute an integrated response during oxygen-restricted periods. In fact, this strategy may constitute an anticipatory protective response to the oxidative stress caused by the reoxygenation event. The present study constitutes the first evidence of cellular preparation to post-hypoxic free radical damage in octocorals, and we argue that such physiological strategy can allow these organisms to thrive in intertidal habitats and withstand air exposure during low tides.