Increased rate of D1 repair in coral symbionts during bleaching is insufficient to counter accelerated photo‐inactivation

We dissect the primary photo‐inactivation and the counteracting metabolic repair rates in fragments of the scleractinian coral, Pocillopora damicornis, subjected to a combined stress of a shift to elevated temperature (from 26°C to 32°C) and increased light (from 200 µmol photons m−2 s−1 to 400 µmol photons m−2 s−1) to induce bleaching. During the bleaching treatment the dinoflagellate symbionts showed a 5.5‐fold acceleration in their photosystem II (PSII) repair rate constant, demonstrating that they maintain strong metabolic capacity to clear and replace photo‐damaged D1 protein at the elevated temperature and light conditions. Nevertheless, the symbionts concurrently suffered a seven‐fold increase in the rate constant for PSII photo‐inactivation. This rapid photo‐inactivation exceeded the PSII repair capacity, therefore tipping the symbionts, and by implication the symbiosis, into net photo‐inhibition. Increased photo‐inactivation in hospite, rather than an inhibition of PSII repair, is the principle trigger for net photo‐inhibition under bleaching conditions.