Establishment of oxidative stress biomarkers in oocytes from healthy and bleached scleractinian corals

Corals have a symbiotic relationship with photosynthetic dinoflagellates associated with the host's tissue, those responsible for most of their daily energy gain. Warming ocean water breaks down symbiosis, leading to a phenomenon known as bleaching. Without their primary nutritional source, corals depend on heterotrophy to survive, which can stagnate the reproductive cycle. There are corals, however, that manage to maintain gametogenesis even when bleached. However, the physiology of these gametes in such a situation is unknown. Our study is the first to evaluate the effects of bleaching on the oocytes of a scleractinian coral, as well as the strategies of these cells to maintain the balance of the antioxidant system and cellular homeostasis. We evaluated and quantified markers of oxidative balance in oocytes released from colonies of Mussismilia harttii with different physiological conditions: bleached and healthy. Healthy and bleached coral oocytes were collected considering a post-spawning time curve (0, 5 and 10 h) and frozen to evaluate the relationship between oxidative balance markers and the physiological conditions of coral colonies. Total protein levels, the activity of the antioxidants superoxide dismutase (SOD) and catalase (CAT), and the levels of lipoperoxidation (TBARS), a marker of lipid damage, were measured. The oocytes presented a significant difference between 0 h and 5 h after spawning for all the parameters regardless of the colony's health. Health status modulated SOD activity and TBARS levels, with oocytes from the bleached colony suffering the most lipid damage. These organisms seem to preserve the quality of female gametic cells even 10 h after spawning in both colonies, suggesting a robust antioxidant system capable of prolonging their lifespan and, possibly, their fertilization capability. This response may be related to an intensification of heterotrophy, ensuring nutritional support and thus reproductive effort and quality of gametes even in bleached corals.