Anomalous swimming of a ciliary microorganism adjacent to a wall

Numerous swimming microorganisms exist in the Earth’s environment and impact on maintaining aquatic ecosystems. Some ciliates, which live in ponds, lakes, etc., are known to accumulate on the solid-fluid interfaces in nature, where they can obtain enough foods and stable environment. However, the mechanism of this accumulation had been unclear. The swimming of ciliates in bulk water is described by “neutral swimmer” in squirmer model, which is standard hydrodynamic model of active microswimmer. The neutral swimmer is repelled from a wall by hydrodynamic interaction. Where does this contradiction come from? Here, we tried to elucidate the mechanism of the accumulation through comparing the difference between actual ciliate and neutral swimmer. Experimental observation on the ciliate, Tetrahymena pyriformis, swimming near a wall without cell adhesion confirmed that the cell slid adjacent to the wall, in which the novel mechano-sensing response of cilia was observed. Hydrodynamic simulation proved that not only the ciliary response above mentioned but also the anisotropic shape of the cell was crucial for the adjacent swimming.