Top-Down Regulation of Picophytoplankton in the Sargasso Sea: Application of a Reciprocal Transplant / Dilution Approach

这项研究的智力价值在于扩展了我们对海洋浮游植物的生物学和生态学的理解，浮游植物是一组微生物，在世界海洋中发生的光合作用总碳固定中占很大比例。浮游植物作为开放海洋生态系统中主要初级生产者的重要性已得到证实。然而，调控这些种群的分布和数量的因素仍然知之甚少。研究人员将探索在特定背景下浮游植物种群自上而下（食草动物介导）调节的动态：马尾藻海中微蓝藻原绿球藻（但不是聚藻球菌）夏季地下最大值的维持。这一现象代表了一个相对简单和可预测的模型系统，在这个模型系统中可以检验关于海洋浮游植物一般调节的假设。最近的研究结果表明，尽管原绿球藻数量丰富，但与水面上较小的种群相比，水面下最大的原绿球藻生长（和被放牧）相当缓慢。为了了解造成这一明显矛盾的因素，该项目将结合实地和实验室研究来描述和比较原绒球菌及其原生动物食草动物在这两个不同深度的相互作用，以及与不形成这种地下最大值的聚藻球菌的相互作用。更广泛的影响包括研究生和本科生的培训。此外，鉴于浮游植物作为海洋微生物食物网基础的主要生产者的重要性，本项目的结果应该为描述和模拟更广泛的海洋生态系统提供信息，并最终了解其在全球碳循环中的作用。

The intellectual merit of the research is to extend our understanding of the biology and ecology of marine picophytoplankton, a group of microbes that are responsible for a large proportion of the total photosynthetic carbon fixation that occurs in the world's oceans. The importance of picophytoplankton as the dominant primary producers in open-ocean ecosystems is well-established. However, the factors that regulate the distribution and abundance of these populations remain poorly understood. The investigators will explore the dynamics of top-down (grazer-mediated) regulation of picophytoplankton populations in a specific context: the maintenance of summertime subsurface maxima in the pico-cyanobacterium Prochlorococcus (but not Synechococcus) in the Sargasso Sea. This phenomenon represents a relatively simple and predictable model system within which to test hypotheses about the regulation of oceanic picophytoplankton in general. Recent results suggest that despite their abundance, Prochlorococcus in the subsurface maxi-mum are growing (and being grazed) rather slowly, as compared to the smaller population at the surface. In order to understand the factors responsible for this apparent paradox, this project will use a combination of field and laboratory studies to characterize and compare the interactions between Prochorococcus and its protozoan grazers at these two contrasting depths, and in relation to Synechococcus, which forms no such sub-surface maximum. The broader impacts include training for graduate and undergraduate students. In addition, given the significance of picophytoplankton as primary producers at the base of oceanic microbial food webs, the results of this project should inform efforts to describe and model the broader oceanic ecosystem, and ultimately to understand its role in the global carbon cycle.