Simple dynamics underlying the survival behaviors of ciliates

Abstract

Edited by Tamiki Komatsuzaki Ciliates are swimming microorganisms in aquatic environments. Habitats where ciliates accumulate include nutrient-rich solid-liquid interfaces such as pond bottom walls and waterweed surfaces. The ciliates stay near the walls to survive. We investigated the dynamics of the near-wall behavior of ciliates. In experiments, the ciliates were made to slide on a flat wall of glass substrate. When encountering the wall, the wall-side cilia of the cells stop their motion and lose their propelling activity, which indicates that the ciliates have a mechano-sensing system for cilia beating. Based on the experimental results, we hypothesized that the ciliary thrust force that propels the cell body becomes asymmetric, and the asymmetry of the thrust force generates a head-down torque to keep the cell sliding on the wall. To prove this hypothesis, we performed numerical simulations by using a developed hydrodynamic model for swimming ciliates. The model revealed that the loss of cilia activity on the wall side physically induces a sliding motion, and the aspect ratio of the cell body and effective cilium area are critical functions for the sliding behavior on a wall. In addition, we investigated the stability of the sliding motion against an external flow. We found that ciliates slide upstream on a wall. Interestingly, the dynamics of this upstream sliding, called rheotaxis, were also explained by the identical physical conditions for no-flow sliding. Only two simple physical conditions are required to explain the dynamics of ciliate survival behavior. This review article is an extended version of the Japanese article, Fluid Dynamic Model Reveals a Mechano-sensing System Underlying the Behavior of Ciliates, published in

Swimming ability and flagellar motility of sperm packets of the volvocine green algaPleodorina starrii

“…Therefore, the timeaveraged force distribution generated by flagella is supposedly uniform over the asexual colony (i.e., neutral swimming), and this results in straight swimming. In other neutral swimmers [40] such as the ciliates Paramecium [41] and Tetrahymena [42], helical trajectories are frequently observed possibly because the metachronal waves are oblique against the body axis [41,43] or the cilia around the mouth (oral groove) move differently from the cilia in the other parts [41]; however, these were not the case in the asexual colonies of P. starrii in the present study. In contrast, sperm packets were pullers with tens of flagella in the anterior part (Supplementary Movie S1).…”

Swimming ability and flagellar motility of sperm packets of the volvocine green alga Pleodorina starrii

“…Therefore, the time-averaged force distribution generated by flagella is supposedly uniform over the asexual colony (i.e., neutral swimming), and this results in straight swimming. In other neutral swimmers [ 40 ] such as the ciliates Paramecium [ 41 ] and Tetrahymena [ 42 ], helical trajectories are frequently observed possibly because the metachronal waves are oblique against the body axis [ 41 , 43 ] or the cilia around the mouth (oral groove) move differently from the cilia in the other parts [ 41 ]; however, these were not the case in the asexual colonies of P . starrii in the present study.…”