State Key Laboratory of Simulation and Regulation of Water Cycle in River Basin, China Institute of Water Resources and Hydropower Research, Beijing 100038, China.
Department of Hydraulic Engineering, Tsinghua University, Beijing 100084, China.
Proc Natl Acad Sci U S A. 2022 Oct 18;119(42):e2206738119. doi: 10.1073/pnas.2206738119. Epub 2022 Oct 11.
The accumulation of swimming microorganisms at surfaces is an essential feature of various physical, chemical, and biological processes in confined spaces. To date, this accumulation is mainly assumed to depend on the change of swimming speed and angular velocity caused by cell-wall contact and hydrodynamic interaction. Here, we measured the swimming trajectories of (a biflagellate marine alga) near vertical and horizontal rigid boundaries. We observed that the probability of sharp turns is greatly increased near a vertical wall, resulting in significant changes in the distributions of average swimming speed, angular velocity, and rotational diffusivity near the wall (a quantity that has not previously been investigated) as functions of both distance from the wall and swimming orientation. These cannot be satisfactorily explained by standard hydrodynamic models. Detailed examination of an individual cell trajectory shows that wall contact by the leading flagellum triggers complex changes in the behavior of both flagella that cannot be incorporated in a mechanistic model. Our individual-based model for predicting cell concentration using the measured distributions of swimming speed, angular velocity, and rotational diffusivity agrees well with the experiment. The experiments and model are repeated for a cell suspension in a vertical plane, bounded above by a horizontal wall. The cell accumulation beneath the wall, expected from gyrotaxis, is considerably amplified by cell-wall interaction. These findings may shed light on the prediction and control of cell distribution mediated by gyrotaxis and cell-wall contact.
游泳微生物在表面的积累是各种受限空间物理、化学和生物过程的一个基本特征。迄今为止，这种积累主要被认为取决于细胞壁接触和水动力相互作用引起的游泳速度和角速度的变化。在这里，我们测量了（一种双鞭毛海洋藻类）在垂直和水平刚性边界附近的游动轨迹。我们观察到，在垂直壁附近，急转弯的概率大大增加，导致壁附近平均游动速度、角速度和旋转扩散率的分布（以前没有研究过的量）作为距离和游动方向的函数发生显著变化。这些不能用标准的流体力学模型满意地解释。对单个细胞轨迹的详细检查表明，前鞭毛与壁的接触触发了两个鞭毛行为的复杂变化，这些变化不能包含在机械模型中。我们基于个体的模型使用测量的游动速度、角速度和旋转扩散率分布来预测细胞浓度，与实验吻合得很好。实验和模型在垂直平面中的细胞悬浮液中重复，该平面由水平壁上边界。由于旋向性，预计在壁下会有细胞积累，但由于细胞壁的相互作用，这种积累会被大大放大。这些发现可能有助于预测和控制由旋向性和细胞壁接触介导的细胞分布。
