Department of Life Sciences, Imperial College London, London, United Kingdom.
Department of Earth Science and Engineering, Imperial College London, London, United Kingdom.
PLoS Comput Biol. 2022 Jul 27;18(7):e1010291. doi: 10.1371/journal.pcbi.1010291. eCollection 2022 Jul.
Microbes play a primary role in aquatic ecosystems and biogeochemical cycles. Spatial patchiness is a critical factor underlying these activities, influencing biological productivity, nutrient cycling and dynamics across trophic levels. Incorporating spatial dynamics into microbial models is a long-standing challenge, particularly where small-scale turbulence is involved. Here, we combine a fully 3D direct numerical simulation of convective mixed layer turbulence, with an individual-based microbial model to test the key hypothesis that the coupling of gyrotactic motility and turbulence drives intense microscale patchiness. The fluid model simulates turbulent convection caused by heat loss through the fluid surface, for example during the night, during autumnal or winter cooling or during a cold-air outbreak. We find that under such conditions, turbulence-driven patchiness is depth-structured and requires high motility: Near the fluid surface, intense convective turbulence overpowers motility, homogenising motile and non-motile microbes approximately equally. At greater depth, in conditions analogous to a thermocline, highly motile microbes can be over twice as patch-concentrated as non-motile microbes, and can substantially amplify their swimming velocity by efficiently exploiting fast-moving packets of fluid. Our results substantiate the predictions of earlier studies, and demonstrate that turbulence-driven patchiness is not a ubiquitous consequence of motility but rather a delicate balance of motility and turbulent intensity.
微生物在水生生态系统和生物地球化学循环中起着主要作用。空间斑块是这些活动的关键因素，影响着生物生产力、营养循环和营养级之间的动态变化。将空间动态纳入微生物模型是一个长期存在的挑战，特别是在涉及小规模湍流的情况下。在这里，我们结合了对流混合层湍流的完全 3D 直接数值模拟，以及基于个体的微生物模型，以检验关键假设，即旋泳运动和湍流的耦合驱动强烈的微观斑块化。流体模型模拟了由于通过流体表面散热而引起的湍流对流，例如在夜间、秋季或冬季冷却期间或在冷空气爆发期间。我们发现，在这种情况下，由湍流驱动的斑块化是分层的，并且需要高的运动性：在流体表面附近，强烈的对流湍流会破坏运动性，使运动性和非运动性微生物大致均匀地混合。在更大的深度，在类似于温跃层的条件下，高运动性的微生物可以比非运动性微生物集中两倍以上，并且可以通过有效地利用快速移动的流体包来大大提高它们的游动速度。我们的结果证实了早期研究的预测，并表明由湍流驱动的斑块化不是运动性的普遍结果，而是运动性和湍流强度之间的微妙平衡。
