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

这项研究的学术价值是扩大了我们对海洋浮游植物的生物学和生态学的理解，海洋浮游植物是一群微生物，它们负责世界海洋中发生的总光合作用碳固定的很大比例。微藻浮游植物作为公海生态系统中主要初级生产者的重要性是众所周知的。然而，调节这些种群的分布和丰度的因素仍然知之甚少。研究人员将在一个特定的背景下探索自上而下(捕食者介导的)微藻种群调节的动态：在马尾藻海的微蓝细菌原氯球菌(但不是聚球藻)中维持夏季亚表层最大值。这一现象代表了一个相对简单和可预测的模型系统，在这个系统中可以检验关于海洋浮游微藻总体调控的假说。最近的结果表明，与地表较小的种群相比，地下最大层中的原氯球菌生长(和被放牧)相当缓慢。为了了解造成这一明显矛盾的因素，该项目将使用现场和实验室研究相结合的方法来描述和比较原脉络球菌与其原生动物食草动物在这两个对比深度的相互作用，以及与聚球藻的相互作用，聚球藻没有形成这种次表层最大值。更广泛的影响包括对研究生和本科生的培训。此外，鉴于微藻浮游植物作为海洋微生物食物网基础初级生产者的重要性，该项目的结果应有助于对更广泛的海洋生态系统进行描述和建模，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.