Small-scale Fluid Physics and Ecology of Planktonic Ciliates
Jumping and overcoming diffusion limitation of nutrient uptake in the photosynthetic ciliate Mesodinium rubrum
Fast, frequent, and spontaneous jumping by the photosynthetic ciliate Mesodinium rubrum is highly adaptive for overcoming diffusion limitation of nutrient uptake. Using high-speed, high-magnification, digital imaging, jumping behaviors of an Antarctic and temperate North American strain of M. rubrum were investigated. Both strains displayed multiple-beat long jumps, wherein maximum jump speeds and total jump durations varied significantly with strain, temperature, and illumination. However, jump distances were surprisingly similar with means approximating six body lengths, which were just above the thickness of nutrient diffusive boundary layer surrounding the cell. Total jump durations scaled by diffusion time scale were < 1 for almost all observed jumps. Moreover, jump distances and square roots of pre-jump residence times were linearly correlated. Thereby, jumping by M. rubrum is physically constrained by the small-scale advection-diffusion physics of the cell’s immediately surrounding water. Additionally, to achieve similar jump distances as the temperate strain at warm temperature, the Antarctic strain lengthened each beat cycle of cilia to kinematically compensate low jump speed at cold temperature, but the ratio of power to recovery stroke duration (6 : 1) remained unchanged with temperature. As further shown by computational fluid dynamics simulations driven by empirical data, multiple-beat long jumping allows both strains to completely detach the boundary layer and achieve similar Sherwood numbers significantly > 1, despite substantially different jumping kinematics. All these results support the notion that jumping is an essential behavior for M. rubrum to enhance nutrient uptake, and help to explain their high photosynthetic rates and ecological success.
Jiang, H. and Johnson, M. D. (2017) Jumping and overcoming diffusion limitation of nutrient uptake in the photosynthetic ciliate Mesodinium rubrum. Limnology and Oceanography, 62, 421-436.
Why does the jumping ciliate Mesodinium rubrum possess an equatorially located propulsive ciliary belt?
It has long been thought that jumping by the ciliate Mesodinium rubrum can enhance its nutrient uptake. However, jumping can be energetically costly and also dangerous by inducing hydrodynamic disturbances detectable by rheotactic predators. Here, a computational fluid dynamics (CFD) model, driven by published empirical data, is developed to simulate the jump-induced unsteady flow as well as chemical field around a self-propelled jumping ciliate. The associated phosphorus uptake, hydrodynamic signal strength, mechanical energy cost and Froude propulsion efficiency are also calculated. An equatorial ciliary belt (ECB), i.e. the morphology used by M. rubrum for propulsion, is considered. For the purpose of comparison, three other strategies (pulled or pushed by cilia, or towed) are also considered. Comparison of the CFD results among the four strategies considered suggests: (i) jumping enhances phosphorus uptake with simulated values consistent with available field data; (ii) the M. rubrum-like propulsion generates the weakest and spatially most limited hydrodynamic disturbance and therefore may effectively minimize the jump-induced predation risk; and (iii) the M. rubrum-like propulsion achieves a high Froude propulsion efficiency (0.78) and is least costly in mechanical energy expenditure among the three self-propelled strategies considered. Thus, using the ECB for propulsion can be essential in ensuring that M. rubrum is a successful ‘fast-jumping’ primary producer.
Jiang, H. (2011) Why does the jumping ciliate Mesodinium rubrum possess equatorially located propulsive ciliary belt? Journal of Plankton Research, 33, 998-1011 (Featured article).