Bacteria in aquatic environments often interact with particulate matter. A key example is bacterial degradation of marine snow responsible for carbon export from the upper ocean in the biological pump. The ecological interaction between bacteria and sinking particles is regulated by their encounter rate, which is therefore important to predict accurately in models of bacteria-particle interactions. Models available to date cover the diffusive encounter regime, valid for sinking particles larger than the typical run length of a bacterium. The majority of sinking particles, however, are small, and the encounter process is then ballistic rather than diffusive. In the ballistic regime, the shear generated by the particle's motion can be important in reorienting bacteria and thus determining the encounter rate, yet the effect of shear is not captured in current encounter rate models. Here, we combine analytical and numerical calculations to quantify the encounter rate between sinking particles and non-motile or motile microorganisms in the ballistic regime, explicitly accounting for the hydrodynamic shear created by the particle and its coupling with microorganism shape. We complement results with selected experiments on non-motile diatoms. We find that the shape-shear coupling has a considerable effect on the encounter rate and encounter location through the mechanisms of hydrodynamic focusing and screening, whereby elongated microorganisms preferentially orient normally to the particle surface downstream of the particle (focusing) and tangentially to the particle surface upstream of the particle (screening). We study these mechanisms as a function of the key dimensionless parameters: the ratio of particle sinking speed to microorganism swimming speed, the ratio of particle radius to microorganism length, and the microorganism's aspect ratio.

中文翻译：

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细菌与下沉小颗粒之间的相遇率

水生环境中的细菌经常与颗粒物相互作用。一个关键的例子是海洋雪的细菌降解，负责生物泵中上层海洋的碳输出。细菌和下沉颗粒之间的生态相互作用受它们的相遇率调节，因此在细菌-颗粒相互作用模型中准确预测很重要。迄今为止可用的模型涵盖了扩散接触机制，适用于下沉颗粒大于细菌典型运行长度的情况。然而，大多数下沉粒子都很小，因此遭遇过程是弹道而不是扩散。在弹道状态下，粒子运动产生的剪切对于重新定向细菌并因此确定遭遇率非常重要，然而，当前的遭遇率模型没有捕捉到剪切的影响。在这里，我们结合分析和数值计算来量化下沉颗粒与非运动或运动微生物在弹道状态下的相遇率，明确说明由颗粒产生的流体动力剪切及其与微生物形状的耦合。我们通过对非运动性硅藻的选定实验来补充结果。我们发现形状剪切耦合通过流体动力学聚焦和筛选机制对相遇率和相遇位置有相当大的影响，由此细长的微生物优先垂直于颗粒下游的颗粒表面（聚焦）并与颗粒相切颗粒上游的表面（筛分）。