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Figure 2


Fig. 2. Schematic representation of oxygen handling pathways in Symbiodinium resident in host cells under ambient (A), and elevated temperature and light (B) conditions. Under ambient conditions, the photosynthetic apparatus, consisting of photosystem II (PSII) and photosystem I (PSI) on the thylakoid, operates normally and produces large quantities of oxygen that diffuse into the host. ROS that are produced are converted back to oxygen with superoxide dismutase (SOD) and ascorbate peroxidase (APX). Under stressed conditions, damage to the photosynthetic apparatus occurs in at least three places (depicted as `flashes' in the figure): the D1 protein in PSII; in the Calvin cycle; and on the thylakoid membranes. This damage acts to generate large amounts of ROS in the form of singlet oxygen (1O2) and superoxide (O2–) that overwhelm the oxygen-handling pathways. O2– is converted to both the most highly reactive hydroxyl radical (·OH) and the more stable and highly diffusible hydrogen peroxide (H2O2), which can move into host tissues. Figure adapted from Venn and colleagues (Venn et al., 2008).





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