Shaping the Coral Diffusive Boundary Layer by Cilia Beating

In the diffuse boundary layer (DBL) surrounding all sessile marine organisms, molecular diffusion is the dominant transport mechanism which often limits the mass exchange with the environment. In the case of coral individuals, the thickness of the DBL can vary depending i.a. on the water flow and on the morphology of the coral. Nevertheless, recent reports show that corals are able to modify the surrounding DBL by cilia beatings which generate mixing vortices at the tissue surface in stagnant water. We combined particle image velocimetry (PIV), particle tracking velocimetry (PTV) with high resolution oxygen profiling to observe flow fields and oxygen concentrations in the boundary layer of the reef building tropical coral Porites lutea under natural and arrested ciliary motion (sodium orthovanadate) and under varying current and light conditions. Cilia generated a series of vortices in the lower DBL which directed water towards and away from the coral tissue altering oxygen concentrations close to the tissue surface. Interestingly, the oxygen flux across the DBL was not enhanced by the cilia activity. Instead, the vortex mixing caused the erosion of oxygen gradients only in the lower DBL. This, however, led to a substantial mitigation of extreme oxygen concentrations under both light and dark conditions. Under high light, oxygen levels fell from 350µM (arrested cilia) to only 25µM (ciliary beating) above ambient concentrations. In darkness, oxygen levels increased from 120µM (arrested cilia) to only 86µM (ciliary beating) below ambient. Further, by applying a numerical transport-reaction model we found that vortex formation by cilia increased the surface area of the surrounding DBL, thus facilitating the solute transfer across the coral tissue. Our findings showcase the plasticity of a previously believed fixed barrier and how corals can alter it to their advantage.