Ciliary flows in corals ventilate target areas of high photosynthetic oxygen production.

Most tropical corals live in symbiosis with Symbiodiniaceae algae whose photosynthetic production of oxygen (O) may lead to excess O in the diffusive boundary layer (DBL) above the coral surface. When flow is low, cilia-induced mixing of the coral DBL is vital to remove excess O and prevent oxidative stress that may lead to coral bleaching and mortality. Here, we combined particle image velocimetry using O-sensitive nanoparticles (sensPIV) with chlorophyll (Chla)-sensitive hyperspectral imaging to visualize the microscale distribution and dynamics of ciliary flows and O in the coral DBL in relation to the distribution of Symbiodiniaceae Chla in the tissue of the reef building coral, Porites lutea. Curiously, we found an inverse relation between O in the DBL and Chla in the underlying tissue, with patches of high O in the DBL above low Chla in the underlying tissue surrounding the polyp mouth areas and pockets of low O concentrations in the DBL above high Chla in the coenosarc tissue connecting neighboring polyps. The spatial segregation of Chla and O is related to ciliary-induced flows, causing a lateral redistribution of O in the DBL. In a 2D transport-reaction model of the coral DBL, we show that the enhanced O transport allocates parts of the O surplus to areas containing less chla, which minimizes oxidative stress. Cilary flows thus confer a spatially complex mass transfer in the coral DBL, which may play an important role in mitigating oxidative stress and bleaching in corals.