The antioxidant capacity of Aiptasia is influenced by its symbiotic state

The cnidarian-Symbiodiniaceae symbiosis is sustained by the exchange of nutrients and protection between partners: the host provides a stable environment and key metabolites, while the algae supply photosynthate and oxygen. However, daytime hyperoxia can promote the formation of reactive oxygen species (ROS). Under normal conditions, ROS are neutralized by enzymatic and non-enzymatic antioxidants, but stress can increase ROS production to levels that exceed antioxidant defences, triggering oxidative damage and potentially leading to bleaching. Although redox balance is central to symbiotic stability, the role of host antioxidant regulation remains poorly understood.
Here, we examined how host antioxidant dynamics vary with symbiotic state in the model sea anemone Aiptasia. We used two strains of Exaiptasia diaphana, F003 (female) and CC7 (male), which differ in both host genotype and associated Symbiodiniaceae species. Symbiotic and aposymbiotic individuals of each strain were exposed to low (20 µmol photons m-2 s-1) and high (160 µmol photons m-2 s-1) light intensities. Antioxidant capacity was quantified using the ORAC (Oxygen Radical Absorbance Capacity) assay and by the measurement of antioxidants. Samples were collected after one and three days of exposure, at both the beginning and end of the photoperiod. Our results reveal that antioxidant dynamics vary markedly with symbiotic state, light environment, time of day and host genotype. By providing comprehensive antioxidant profiles across multiple strains and symbiotic conditions, this study offers new insights into redox homeostasis and its role in regulating the cnidarian-algal symbiosis.