Temporal patterns in effective quantum yield of individual zooxanthellae expelled during bleaching

Bleaching is a worldwide phenomenon affecting coral reefs. During elevated temperature and light conditions (bleaching), expelled zooxanthellae show distinct patterns in photosynthetic health. An innovative new device was used to collect individual expelled zooxanthellae, when a coral was exposed to bleaching conditions. This has provided new insight into the photosynthetic condition and abundance of expelled zooxanthellae. It has been assumed that expelled zooxanthellae were dead or moribund; however, we have found individual cells can have healthy effective quantum yields (ϕPSII) >0.65 after 8 h of bleaching conditions (500 μmol photons m−2 s−1, 33 °C). The population of expelled zooxanthellae from Cyphastrea serailia and Pocillopora damicornis showed distinct patterns in the frequency distribution of ϕPSII over time and between locations (sun versus shade) within a colony. During the first 4 h of exposure to bleaching conditions, only 5% of expelled individual cells from P. damicornis were photosynthetically inactive (ϕPSII<0.05), whereas for C. serailia, this was 30%. The overall photosynthetic health of expelled zooxanthellae from C. serailia was better than P. damicornis (0.53±0.13 and 0.38±0.13 after 8 h, respectively). This was generally reflected by the in hospite measurement of the coral, yet, the in hospite cells always had a higher ϕPSII than expelled cells, suggesting that host tissue provided added photoprotection for the zooxanthellae.