Increased seawater temperature triggers thermal, oxidative and metabolic response of Ostrea edulis, leading to anaerobiosis

Bivalves are among the marine organisms most influenced by climate change. Despite the flat oyster's Ostrea edulis high economic value, its culture is developed on a very small scale, since this species possesses a strong susceptibility to abiotic stressors. Due to climate change, temperature is one of the most critical environmental parameters for the welfare of the Mediterranean basin's marine inhabitants. The present study's purpose was to investigate the physiological performance of the Mediterranean's native O. edulis as it faces exposure to different temperatures. Since juveniles are more susceptible to abiotic stressors, this experimental procedure was focused on young individuals. The seawater temperatures studied included a standard control temperature of 21 °C (often observed in several marine areas throughout the Mediterranean), as well as increased seawater temperatures of 25 °C and 28 °C, occasionally occurring in shallow Mediterranean waters inhabited by bivalve spat. These were selected since the tissues of O. edulis becomes partly anaerobic in temperatures exceeding 26 °C, while cardiac dysfunction (arrhythmia) emerges at 28 °C. The results demonstrate that temperatures above 25 °C trigger both the transcriptional upregulation of hsp70 and hsp90, and the antioxidant genes Cu/Zn sod and catalase. Enhancement of thermal tolerance and increased defense against increased ROS production during thermal stress, were observed. As the intensity and duration of thermal stress increases, apoptotic damage may also occur. The increased oxidative and thermal stress incurred at the highest temperature of 28 °C, seemed to trigger the switch from aerobic to anaerobic metabolism, reflected by higher pepck mRNA expressions and lower ETS activity.