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Abstract

The exchange of metabolites between environment and coral tissue depends
on the flux across the diffusive boundary layer (DBL) surrounding the
tissue. Cilia covering the coral tissue have been shown to create
vortices that enhance mixing in the DBL in stagnant water. To study the
role of cilia under simulated ambient currents, we designed a new
light-sheet microscopy based flow chamber setup. Microparticle
velocimetry was combined with high-resolution oxygen profiling in the
coral Porites lutea under varying current and light conditions with
natural and arrested cilia beating. Cilia-generated vortices in the
lower DBL mitigated extreme oxygen concentrations close to the tissue
surface. Under light and arrested cilia, oxygen surplus at the tissue
surface increased to 350 mu M above ambient, in contrast to 25 mu M
under ciliary beating. Oxygen shortage in darkness decreased from 120 mu
M (cilia arrested) to 86 mu M (cilia active) below ambient. Ciliary
redistribution of oxygen had no effect on the photosynthetic efficiency
of the photosymbionts and overall oxygen flux across the DBL indicating
that oxygen production and consumption was not affected. We found that
corals actively change their environment and suggest that ciliary flows
serve predominantly as a homeostatic control mechanism which may play a
crucial role in coral stress response and resilience.