Role of Growth Rate and Grazing Mortality in Regulating the Dynamics of Photosynthetic Prokaryotic Picoplankton in the Sargasso Sea

两种密切相关的单细胞蓝藻，原绿球藻和聚藻球菌，在海洋初级生产总量中占很大一部分。虽然这两种微浮游生物在海洋的大部分地区都是普遍存在的，数量也很丰富，但它们在时间和空间上的分布往往是截然不同的。这种模式在西北马尾藻海的季节循环过程中尤为明显，在夏季（在高度分层、低营养条件下）原绿球藻占主导地位，而在冬季（在充分混合的条件下）通常占主导地位。这两类生物的总生物量大致保持不变，这一事实使人们认为，这些生物共同填补了这种环境中的某种“浮游植物生态位”。对原绿球藻和聚球菌分布的这些不同模式的解释通常是在生长速度调节的背景下进行的：假设一个或另一个群体的优势反映了该群体在当时普遍的环境条件下更快的生长。然而，自然种群的丰度必须反映生长和死亡两项之间的相对平衡，并且没有牢固的理论要求生长速度和常存量是相关的。事实上，迄今为止为数不多的同时检测原绿球藻和聚球藻生长速度的研究并没有发现这种相关性。这些结果表明，除生长速率差异限制外，其他因素对海洋环境中这两种浮游植物类群的相对丰度有重要的调节作用。本项目旨在研究马尾藻海中原绿球藻和聚藻球菌的生长速度、放牧死亡率和相对丰度之间的关系，在这两个群体丰度的明确季节性循环过程中。在四次研究巡航中，将应用一套重叠和互补的方法来测量自然浮游生物种群的这些过程。具体而言，将解决以下问题：原绿球藻和聚球藻相对丰度的季节变化是否与这些种群的相对生长和放牧死亡率的变化相关？微型食草动物的丰度或活动是否有明显的季节性变化？这些变化是否会影响浮游植物的季节循环？原生动物群落对原绿球藻和聚球藻丰度的功能反应是否存在可检测的差异？这些差异会影响浮游植物的季节循环吗？基于细胞周期的原位生长速率估计值是否与稀释实验估计值一致？基于荧光标记猎物的放牧死亡率估计与这些实验的估计一致吗？揭示这两种浮游植物群体（及其总生物量）的相对优势被调节的机制，将有助于更好地理解更普遍但更不易处理的浮游植物群落整体结构调节问题。

Two closely related unicellular cyanobacteria, Prochlorococcus and Synechococcus, are responsible for a significant fraction of total oceanic primary production. Although these two picoplankton groups are ubiquitous and abundant in most parts of the ocean, their distribution in time and space is often contrasting. This pattern is particularly evident over the course of the seasonal cycle in the NW Sargasso Sea, where Prochlorococcus dominates during the summer (under highly stratified, low nutrient conditions), and Synechococcus typically dominates during the winter (under well-mixed conditions). The fact that the summed biomass of these two groups remains approximately constant has led to the suggestion that together these organisms fill some sort of "picophytoplankton niche" in this environment. Explanations for these contrasting patterns of Prochlorococcus and Synechococcus distribution are most often framed in the context of growth rate regulation: dominance by one or the other group is assumed to reflect faster growth by that group under the environmental conditions prevailing at the time. However, the abundance of a natural population must reflect the relative balance between both growth and mortality terms, and there is no firm theoretical requirement that growth rate and standing stock be correlated. In fact, the few studies to date that have simultaneously examined the growth rates of both Prochlorococcus and Synechococcus in the field have found no such correlation. These results suggest that factors other than differential growth rate limitation are important in regulating the relative abundance of these two picophytoplankton groups in oceanic environments.This project seeks to examine the relationship between growth rate, grazing mortality, and relative abundance of Prochlorococcus and Synechococcus, over the course of the well-defined seasonal cycle in the abundance of these two groups in the Sargasso Sea. A suite of overlapping and complementary approaches for measuring these processes in natural picoplankton populations will be applied during four research cruises. Specifically, the following questions will be addressed: Are seasonal changes in the relative abundance of Prochlorococcus and Synechococcus correlated with changes in the relative growth and grazing mortality rates of these populations? Are there significant seasonal changes in micro-grazer abundance or activity? Do such changes influence the seasonal cycle of picophytoplankton standing stock? Are there detectable differences in the functional response of the protozoan community to Prochlorococcus and Synechococcus abundance? Do these differences influence the seasonal cycle of picophytoplankton? Do cell cycle-based estimates of in situ growth rate coincide with estimates from dilution experiments? Are estimates of grazing mortality based on fluoresecently-labeled prey consistent with estimates from these experiments?Unraveling the mechanisms by which the relative dominance of these two picophytoplankton groups (and their summed biomass) is regulated would lead to a better understanding of the more general, but much less tractable, issue of regulation of overall phytoplankton community structure.