Collaborative Research: Trajectories and spatial distributions of diatoms at dissipation scales of turbulence

在浮游植物所在的上层混合层中，湍流是普遍存在的，也是重要的。在与单个细胞和群体相关的尺度上，消散的湍流侵蚀了细胞周围的分子扩散边界层，并影响到对捕食者-猎物相互作用和聚集等过程至关重要的相遇概率。叠加在周围流体运动上的是细胞由于翻滚和下沉而产生的局部运动。这些运动如何与周围的湍流相互作用还不是很清楚，特别是对于形态多样、不活动的硅藻来说，这些硅藻主导着初级生产力。最近的研究表明，粒子的静水下沉速度和流体的湍流扩散运动不能简单地相加来求解粒子运动；对于负的和近中性的浮力粒子，高湍流强度产生的下沉和上升速度比静水中快好几倍。大气对云中水滴的研究主要集中在加速的惯性机制上，但该提案中的综述表明，在由硅藻的颗粒特征定义的参数空间中，在确定浮游生物-湍流相互作用时，形状可能比惯性重要得多。研究人员建议研究湍流对硅藻沉降轨迹和由此产生的空间分布的影响。利用一种新型的体积粒子成像仪，他们将获得受控湍流下单个细胞和选定硅藻品系的链的3D拉格朗日轨迹。通过针对不同形态和机械特性的物种以及不同生长阶段的养殖，将获得关于物理、形态和生理特性如何影响非移动浮游植物下沉速度和轨迹的新见解。智力优势：下沉是浮游植物的一种重要生命策略，它与环境流的相互作用对浮游植物在混合层中的停留时间以及最终光带浮游植物的生产力和元素向深海的通量具有重要影响。这项研究挑战了关于导致浮游植物在海洋中沉没的机制的普遍观点，并提供了对斯托克斯沉淀物在湍流下沉过程中偏离的机械剖析。通过针对不同形态和机械特性的物种以及不同生长阶段的培养物，将获得新的见解，了解物理、形态和生理特性如何相互作用来确定非移动浮游植物的下沉速度和轨迹，特别是在主导湍流的漩涡及其附近。这项研究的结果还将促进对浮游植物在分辨率较低的分米级到毫米级区域的潜在空间分布的理解，对食草动物的觅食领域和颗粒聚集体的形成具有丰富的意义。广泛的影响：广泛的问题涉及颗粒在湍流中的运动。这项研究的结果将应用于许多水环境，并促进对非球形、低惯性颗粒在湍流中行为的理解，并将应用于其他领域，如大气科学和化学工程。这一提议还将推进湍流的实验室模拟，在现有方法中达成一种新的折衷方案，有望有效地模拟耗散尺度的湍流。VoPI还提供了比计算机密集型全息术或粒子成像测速技术更快速、更有效地识别和跟踪湍流中的粒子的方法。研究人员提议举办一个为期一周的研讨会，将生物海洋学和工程学研究生和博士后聚集在一起，探索与浮游生物与周围水流的微尺度相互作用相关的丰富问题集，以及解决这些问题的方法。两名研究生和两名本科生将得到合作助学金的支持。这项研究的结果将在全国会议上公布，发表在同行评议的期刊上，并纳入私人投资机构教授的研究生和本科生课程。

Turbulence is ubiquitous and important in the upper mixed layer where phytoplankton reside. On scales relevant to individual cells and colonies, dissipating turbulence erodes molecular diffusive boundary layers around cells and affects probabilities of encounter critical for processes such as predator-prey interactions and aggregation. Superimposed on ambient fluid motion are local motions of cells due to tumbling and sinking. How these motions interact with ambient turbulence is not well understood, particularly for the morphologically diverse, non-motile diatoms that dominate primary productivity. Recent studies show that still-water sinking velocities of particles and turbulent diffusive motions of fluids cannot simply be added together to solve for particle motions; for negatively and nearly neutrally buoyant particles, high turbulence intensity produces several times faster sinking and rising velocities than seen in still water. Atmospheric studies of water droplets in clouds have focused on inertial mechanisms of acceleration, but the review in this proposal suggests that--in the parameter space defined by the particle characteristics of diatoms--shape may be much more important than inertia in determining plankton-turbulence interactions. The investigators propose to study effects of turbulence on settling trajectories and resultant spatial distributions of diatoms. Employing a novel volumetric particle imager, they will obtain 3D Lagrangian trajectories of individual cells and chains of selected strains of diatoms under controlled turbulence. By targeting species of different morphologies and mechanical properties as well as cultures at different growth stages, new insights will be gained on how physical, morphological and physiological properties affect sinking speeds and trajectories of non-motile phytoplankton.Intellectual Merit:Sinking is an important life strategy of phytoplankton, and its interaction with ambient flows holds significant implications for the residence time of phytoplankton in the mixed layer and ultimately for phytoplankton productivity in the photic zone and fluxes of elements to the deeper ocean. This study challenges common views on mechanisms that cause phytoplankton sinking in the ocean and offers a mechanistic dissection of departures in turbulent sinking from Stokes settling. By targeting species of varied morphologies and mechanical properties, as well as cultures at different growth stages, new insights will be gained into how physical, morphological and physiological properties interact to determine sinking speeds and trajectories of non-motile phytoplankton, particularly in and near the vortices that dominate turbulent flows. Results from this study will also advance understanding of underlying spatial distributions of phytoplankton in the poorly resolved decimeter to millimeter domain, with abundant implications for the foraging fields of grazers and formation of particle aggregates.Broader Impacts :Wide arrays of problems involve particle motions in turbulence. Results from this study will apply to many aquatic environments and advance understanding of the behavior of non-spherical, low-inertia particles in turbulent flows, with applications to other fields such as atmospheric sciences and chemical engineering. This proposal will also advance the laboratory simulation of turbulence with a novel compromise among existing approaches that promises efficient simulation of dissipation-scale turbulence. The VoPI also provides more rapid and efficient identification and tracking of particles in turbulent flows than has been feasible with computer-intensive holography or particle-imaging velocimetry. The investigators propose a week-long workshop that will bring together biological oceanography and engineering graduate students and postdocs to explore the rich set of questions associated with microscale interactions of planktonic organisms with ambient flows, and approaches to address them. Two graduate students and two undergraduate students will be supported by the collaborative grant. Results from this study will be presented at national meetings, published in peer-reviewed journals and incorporated into graduate and undergraduate courses taught by the PIs.