Aerospace and Mechanical Engineering, University of Notre Dame, Notre Dame, IN 46556, USA.
Proc Natl Acad Sci U S A. 2012 Mar 6;109(10):3856-61. doi: 10.1073/pnas.1116210109. Epub 2012 Feb 21.
Microorganisms play pivotal functions in the trophic dynamics and biogeochemistry of aquatic ecosystems. Their concentrations and activities often peak at localized hotspots, an important example of which are pycnoclines, where water density increases sharply with depth due to gradients in temperature or salinity. At pycnoclines organisms are exposed to different environmental conditions compared to the bulk water column, including reduced turbulence, slow mass transfer, and high particle and predator concentrations. Here we show that, at an even more fundamental level, the density stratification itself can affect microbial ecology at pycnoclines, by quenching the flow signature, increasing the energetic expenditure, and stifling the nutrient uptake of motile organisms. We demonstrate this through numerical simulations of an archetypal low-Reynolds-number swimmer, the "squirmer." We identify the Richardson number--the ratio of buoyancy forces to viscous forces--as the fundamental parameter that quantifies the effects of stratification. These results demonstrate an unexpected effect of buoyancy on low-Reynolds-number swimming, potentially affecting a broad range of abundant organisms living at pycnoclines in oceans and lakes.
微生物在水生生态系统的营养动态和生物地球化学中起着关键作用。它们的浓度和活性通常在局部热点达到峰值，其中一个重要的例子是密度跃层，由于温度或盐度的梯度，水的密度在该区域随深度急剧增加。在密度跃层中，与主体水柱相比，生物体暴露在不同的环境条件下，包括减少湍流、缓慢的物质转移以及高浓度的颗粒和捕食者。在这里，我们表明，在更基本的层面上，密度分层本身可以通过抑制流动特征、增加能量消耗和抑制运动生物的养分吸收来影响密度跃层处的微生物生态学。我们通过对典型低雷诺数游泳者“蠕动者”的数值模拟来证明这一点。我们确定了理查森数（浮力与粘性力的比值）作为量化分层影响的基本参数。这些结果表明，浮力对低雷诺数游泳有意外的影响，可能会影响到海洋和湖泊中大量生活在密度跃层处的丰富生物。
