Oxidative stress responses of the scleractinian coral Acropora digitifera from the tropical Bidong Island, Malaysia - focus on protein thiol groups' detection

Over the past 40 years, recurrent episodes of coral bleaching have been observed around the world, which have led to coral reef degradation. The investigation of several biomarkers' responses is essential to understand the phenomenon. The present field experiment study was conducted on nine healthy colonies of the scleractinian coral Acropora digitifera collected in Bidong Island (north-eastern Peninsular Malaysia) between 2019 and 2020 to evaluate their oxidative stress status through the assessment of Symbiodiniaceae (SD) density, antioxidant enzyme activities, lipid oxidation, and protein thiol, with special emphasis on temperature. This study revealed that the presence of the dominant SD ITS2 type C3 from 2019 to 2020 suggests that the relationship between A. digitifera and the genus Cladocopium remains unaffected by the temperature. Concomitantly, there was no significant increase in antioxidant enzyme activities (glutathione S-transferase and catalase) with temperature, indicating that A. digitifera was capable of managing oxidative stress even in a warm environment. The monthly mean antioxidant enzyme activities and malondialdehyde (MDA) levels were lowest in July 2020 and then increased significantly in the subsequent sampling month, indicating a biochemical regulation to maintain cellular homeostasis. Meanwhile, the level of sensitive protein thiol was highest at 30.8 °C and significantly reduced up to 60% as temperature fluctuated, showing that proteins undergo only a small range of oxidative modifications. In addition, we found a weak negative correlation between SD density and thiol content, which may indicate that (1) the presence of SD constantly causes subtle oxidative pressure on coral hosts and (2) the presence of high SD density increases the potential for thiol oxidation. We conclude that there is no direct evidence of A. digitifera experiencing considerable oxidative stress and cellular damage, provided the rate of disturbance remains low.