Collaborative Proposal: Form and function of phytoplankton in unsteady, low Reynolds-number flows

低雷诺数(Re)下的小尺度流动动力学对于浮游植物细胞在营养物质的输送、食草动物的感官检测和物理接触、细菌种群在“藻圈”或紧邻浮游植物细胞周围的区域积累以及作为终止水华的机制的细胞本身的凝聚方面是重要的。在自然界中，大多数浮游植物经历不稳定的流动，即由于湍流的间歇性和草食动物不连续的、空间分布的泵送，细胞附近的速度随时间变化。这种不稳定性以前在浮游生物的模型或测量中没有考虑到。此外，将相关的低Re非恒定流效应模型从应用数学和工程应用到生态应用，还存在着长达十年和一个世纪的滞后。工程模型表明，由于空间上广泛的流动扰动或尾迹的形成历史，非定常效应对中等规模生物群的非定常运动应该是重要的。这个项目将解决这些影响。非游动浮游植物，特别是硅藻，将被用作出现重要的非恒定流行为的最简单的情况。这项研究活动将包括一个多层次的教育计划，针对研究生研究助理、本科生研究实习生、本科生海洋科学专业和高中教师。低Re行为提供了不同寻常的机会来体验数学、物理和生物学是如何不可分割地催化对与直觉背道而驰的现象的理解。这项活动还将包括与世界专家在剑桥(T.J.佩德利)和哥本哈根(T.Kiorboe；Amp；A.W.Visser)就生物流动相互作用开展的国际合作。该活动的总体目标是加速从建模者到测量者再到信息用户的理解流动。投射美国国家标准的教材将在与高中教师的暑期密集研讨会上编写，并在网上提供。对浮游植物的非定常流动效应将通过基于奇点解的显式模型和数学模型来预测(这两种模型都代表了自然界)和数学模型，其中包括在低Re处的近场和在一定Re范围内的远场。奇点解允许显式处理复杂细胞形状的作用。放大的模拟模型将被放置在一个大型Couette容器中，以更好地可视化研究和教学工作的行为。自然尺度但简化的非定常流动将在含有活浮游植物的较小容器(嵌套的反向旋转圆柱体，两个圆柱体之间有海水)中产生，并将通过放大粒子成像测速仪(PIV)进行量化。图像分析将用于测量浮游植物的平移、旋转和弯曲变形。这些研究将测试从一般论题中得出的各种假设，即细胞形状和机械特性与非恒定流相互作用，产生细胞或链及其周围流体的潜在增强适合性的相对运动。一个基本的假设是，不稳定的流体运动会与细胞的弯曲相互作用，从而产生流体和浮游植物的相对运动。一个非常令人兴奋的前景是，已知在低Re时出现的周期性不稳定可能会使灵活的有机体充当“自组织引擎”--通过弹性来利用衰变的湍流产生的能量，从而相对于流体运动。本文对非恒定流中被动弯曲结构的研究将有助于理解柔性附肢在游泳中的应用。这项工作可能会大大有助于将功能与用于鉴定化石标本的微型浮游生物的形状和脊椎联系起来。通过包括低Re时相关的非稳定流体运动，这项研究还将提供许多大型浮游植物、无脊椎动物和鱼类幼体以及其他小型浮游动物所占据的10-1000毫米大小的活浮游生物的形态和功能之间更牢固的联系。

Small-scale flow dynamics at low Reynolds numbers (Re) are important to phytoplankton cells in delivery of nutrients, sensory detection by and physical encounter with herbivores, accumulation of bacterial populations in the "phycosphere" or region immediately surrounding phytoplankton cells and coagulation of cells themselves as a mechanism terminating blooms. In nature most phytoplankton experience unsteady flows, i.e., velocities near the cells that vary with time due to the intermittency of turbulence and to discontinuous, spatially distributed pumping by herbivores. This unsteadiness has not previously been taken into account in models or measurements with plankton. Moreover, there have been decade- and century- long lags in moving relevant models of unsteady flow effects at low Re from applied mathematics and engineering to ecological applications. Engineering models show unsteady effects due to the history of formation of spatially extensive flow perturbations or wakes should be important to unsteady motions of moderately small biota. This project will address these affects. Non-swimming phytoplankton, and in particular diatoms, will be used as the simplest case where important unsteady flow behaviors should arise. This research activity will include a multi-level educational program, aimed at graduate research assistants, undergraduate research interns, undergraduate marine sciences majors and high-school teachers. Low-Re behaviors afford unusual opportunities to experience how mathematics, physics and biology inseparably catalyze understanding of phenomena that run counter to intuition. This activity will also include international collaborations with world experts on organism-flow interaction in Cambridge (T.J. Pedley) and Copenhagen (T. Kiorboe & A.W. Visser). The overall goals of the activity are to accelerate the flow of understanding from modelers to measurers to users of the information and back again. Educational materials that project U.S. national standards will be developed during intensive summer workshops with the high-school teachers and be made available on the web. Unsteady flow effects on phytoplankton will be predicted with explicit models based on singularity solutions (that involve the useful simplification that force is applied to the fluid at a small number of points) and mathematical models that include both the near field at low Re and the far field over a range of Re, both representative of nature. Singularity solutions allow explicit treatment of the role of complex cell shapes. Scaled-up analog models will be placed in a large Couette vessel to better visualize behaviors for both the research and teaching efforts. Natural-scale, but simplified, unsteady flows will be produced in smaller Couettes (nested, counter-rotating cylinders with seawater in the gap between the two cylinders) containing live phytoplankton and will be quantified by magnifying, particle-imaging velocimetry (PIV). Image analysis will be used to measure translation, rotation and flexural deformation of the phytoplankton. These studies will test various hypotheses derived from the general thesis that cell shapes and mechanical properties interact with unsteady flows to produce potentially fitness-enhancing, relative motions of the cell or chain and its surrounding fluids. A basic hypothesis is that unsteady fluid motion will interact with bending of cells to produce relative motion of fluid and phytoplankter. A very exciting prospect is that periodic instabilities known to arise at low Re may allow flexible organisms to act as "self-organizing engines" - through elasticity to harness energy from decaying turbulence and thereby move relative to the fluid. It is also expected that this study of passively bending structures in unsteady flows will help to understand the use of flexible appendages in swimming. The work is likely to aid significantly in associating functions with the shapes and spines of microplankton that are used in the identification of fossil specimens. By including relevant, unsteady fluid motions at low Re, the study will also provide firmer linkages between form and function in living plankton in the size range from 10 - 1000 mm that many large phytoplankton, invertebrate and fish larvae and other small zooplankton occupy.