The antioxidant capacity of Aiptasia is affected by its symbiotic state

In the cnidarian-Symbiodiniaceae photosymbiosis, the host provides protection and nutrients to its algal partner, which in turn supplies photosynthetic products such as sugars and oxygen. During the day, cellular conditions are hyperoxic and under stress, this hyperoxia can drive the production of reactive oxygen species (ROS). Under some conditions, ROS production can overwhelm the antioxidant defences leading to the loss of algal symbionts by the host (i.e., bleaching). Despite its importance, the role of host redox homeostasis in regulating symbiosis remains poorly understood. Here, we investigated how antioxidant capacity varies between individuals with and without symbionts in the model sea anemone Aiptasia. Symbiotic and aposymbiotic populations of two Exaiptasia diaphana strains (CC7 and F003) were exposed to 30 and 200 µmol photons m-2 s-1. Host antioxidant capacity was assessed using the ORAC (Oxygen Radical Absorbance Capacity) assay after one and three days of light exposure, sampled at the start and end of the photoperiod. For F003, aposymbiotic anemones displayed higher antioxidant capacity than symbiotic ones under low light only. By contrast, for CC7, the ORAC values at high light were lower in the aposymbiotic state. Overall, these results show that the host antioxidant capacity is affected by the symbiotic state, but also the host genotype and possibly the species of Symbiodiniaceae hosted.