Even a slight temperature increase triggers the inhibition of Symbiodiniaceae cell division and promotes cell expulsion in the coral Acropora selago

SUMMARY Coral bleaching, characterized by the loss of Symbiodiniaceae symbionts from corals, is promoted both by acute high‐temperature events and by prolonged moderate thermal stress. However, the mechanisms responsible for decreases in Symbiodiniaceae cell densities within corals remain unclear. Symbiodiniaceae cells within corals proliferate through cell division; therefore, their population density is generally understood to be regulated by the host coral through symbiont cell expulsion. Coral bleaching may reflect a disruption of this balance, specifically between symbiont proliferation and host‐mediated expulsion. In the coral Acropora selago , extreme temperatures (32°C) induce extensive symbiont expulsion, whereas physiological damage is already evident at more moderate temperatures (30°C). To detect early signals preceding symbiosis breakdown in this species, we conducted high‐resolution monitoring of symbiont expulsion and cell division at 3‐h intervals under thermal stress (30°C), using 27°C as control. At 30°C, symbiont expulsion increased 1.7–2.3‐fold, while dividing symbiont cells decreased by nearly 40% relative to 27°C. To assess direct thermal effects on Symbiodiniaceae cells, eight culture strains from six genera (two free‐living species and six host‐derived species) were used to observe changes in growth and photosynthetic performance at 27°C and 30°C. Except for two strains of free‐living species, photosynthetic performance showed no significant change, whereas four of the remaining host‐derived species exhibited significantly reduced growth at 30°C. Based on observations of Symbiodiniaceae both within A . selago and in culture strains, our results suggest that even slight increases in temperature can substantially suppress their proliferation. Although the increase in expulsion at 30°C was less pronounced than the massive loss at 32°C, acceleration of symbiont expulsion is already evident at this moderate temperature. Visible bleaching was not observed; nonetheless, these results indicate that even moderate thermal stress can trigger an imbalance between symbiont proliferation and host‐mediated expulsion, processes essential for maintaining appropriate symbiont density within the coral.