THE ROLE OF LIGHT AND PHOTO‐OXIDATIVE STRESS IN CORAL BLEACHING

Abstract

Octocorals remain one of the understudied groups of cnidarians. Nevertheless, energy‐converting symbionts and organelles may be central to the cnidarian stress response. Stress may slow photochemistry in Symbiodinium spp., leaving photosystems I and II relatively reduced and increasing reactive oxygen species (ROS). Alternatively, ROS may emanate from mitochondria. These sources of ROS can be distinguished by using microscopy to examine the effects of light on stressed cnidarians incubated in the dark with a fluorescent, ROS‐detecting probe (H2DCFDA). Experiments were carried out with three species of alcyonacean octocoral, Phenganax parrini, Sarcothelia sp., and Sympodium sp. After incubating colonies for 1 h at elevated temperature, imaging and illumination (ex. 450 – 490, em. 515 – 565 nm) were begun simultaneously. ROS formation largely corresponded to the onset of illumination. On the other hand, chlorophyll fluorescence (ex. 530 – 580, em. 620 – 690 nm) did not conform to this pattern. This difference is consistent with the expected rates of reaction. Remarkably, treatment with the inhibitor 3‐(3,4‐dichorophenyl)‐1,1‐dimethylurea (DCMU) resulted in dramatically higher levels of light‐induced ROS. Chlorophyll fluorescence exhibited higher levels in DCMU treatment but not significantly so. By controlling for variation between individual symbionts, however, DCMU produced significantly greater levels of chlorophyll fluorescence, indicating the expected greater reduction of photosystem II. A brief exposure to light and thermal stress produced a similar effect in all three species. In addition to ROS being initiated by light, these results indicate: (i) a brief period of stress shifts photosystem redox state toward reduction, (ii) photosystem II can donate electrons to oxygen when blocked with DCMU, (iii) chlorophyll fluorescence is highly variable between individual Symbiodinium. Imaging of individual symbionts in hospite thus provides a powerful method for understanding the initial steps of the cnidarian stress response.