The impact of iron availability on the ecology of Prochlorococcus populations in the Sargasso Sea

原绿球藻是微小的（1 - 2 μ m直径）自养蓝细菌，并且是世界上数量上最占优势的光合物种。它们普遍存在于亚热带和热带海洋环流中，构成了广阔的海洋区域。由于其丰富和分布，它们是许多海洋系统中的关键生物，在影响全球气候的总初级生产力和养分循环中占很大比例，是许多海洋生态系统的基础。原绿球藻直到最近才为科学所知，1998年Penny奇泽姆（MIT）发现了它，彻底改变了我们对海洋光合作用的理解。由于这一相对较新的发现，我们只有一个初步的了解的地球化学功能和生态作用的天然原绿球藻种群在开放的海洋。当水柱高度分层时，原绿球藻细胞占主导地位，这种情况在夏天季节性地发生在马尾藻海（亚热带北大西洋）的百慕大地区。在这种高度分层的沃茨中，在阳光照射区的底部（透光层）和营养层的顶部有一个高叶绿素的特征层。这一层，被称为深叶绿素最大值，是占主导地位的原绿球藻。在热带和亚热带沃茨。分子和生理学研究已经确定了两种原绿球藻生态型，称为“高光”和“低光”生态型。弱光生态型主要分布在下透光层，而强光生态型主要分布在上透光层，但并不局限于这一区域。该建议旨在确定这种季节性积累的原因和两种生态型的垂直分布，从而帮助确定这种数量优势种的生态作用。铁是光合作用细胞的基本需求，铁的分布和浓度在海洋系统中通常是稀疏和低的，已被证明会显着影响浮游植物的分布和群落组成。最近，一个子表面的铁浓度最低已被发现开发在百慕大地区的马尾藻海在夏季，这个最低是符合事件与DCM和低光原绿球藻生态型的积累。这种情况已被证明，即使在表面的铁浓度保持高的发展。本提案的目的是确定是否低铁浓度是决定低光生态型的原绿球藻细胞，在百慕大地区的马尾藻海在夏季数量占主导地位的丰度的主要因素。这将通过参加大西洋探索者号R/V在马尾藻海百慕大区域的若干次航行来实现，这些航行的时间安排与次表层铁最低值和原绿球藻细胞积累的早期、中期和晚期发展阶段相吻合。在这些巡航期间，将进行水柱剖面测量，以将原绿球藻的生态分布和生理学与可用铁浓度相关联。将在受控的光照和温度条件下进行甲板上培养实验，并将铁人工添加到从DCM和地表水中采集的水样中（在与铁剖面相同的位置采样）。然后将监测群落的反应，以检测浮游植物对添加的铁的反应，并确定哪些物种对添加的铁作出反应。表层浮游植物群落和DCM群落的不同响应将决定系统的限制因子。在夏季和两个季节进行类似的实验将证实这些特征的时间发展。

Prochlorococcus spp. are tiny (1-2 um diameter) autotrophic cyanobacteria and are the most numerically dominant photosynthetic species in the world. They are ubiquitous in the subtropical and tropical ocean gyres that comprise vast areas of the open ocean. Due to their abundance and distribution, they are key organisms in many marine systems, and are responsible for a large proportion of gross primary productivity and nutrient cycling, which affect the global climate and are the basis of many marine ecosystems. Prochlorococcus was unknown to science until recently, when its discovery by Penny Chisholm (MIT) in 1998 revolutionised our understanding of marine photosynthesis. As a result of this relatively recent discovery, we have only a rudimentary understanding of the biogeochemical functions and ecological roles of natural Prochlorococcus populations in the open ocean. Prochlorococcus cells dominate when the water column is highly stratified, a situation that occurs seasonally in the summer in the Bermuda region of the Sargasso Sea (subtropical North Atlantic). In such highly stratified waters, there is a characteristic layer of high chlorophyll at the base of the sunlit zone (euphotic layer) and the top of the nutracline. This layer, termed the deep chlorophyll maximum, is dominated by Prochlorococcus spp. in tropical and subtropical waters. Molecular and physiological studies have identified two Prochlorococcus ecotypes, known as 'high-light' and 'low-light' ecotypes. The low-light ecotypes dominate the lower euphotic zone, whereas the high-light ecotypes dominate the upper euphotic zone, but are not exclusively restricted to this region. This proposal aims to determine the reason for this seasonal accumulation and the vertical distribution of the two ecotypes, and so help to define the ecological role of this numerically dominant species. Iron is a fundamental requirement for photosynthetic cells and iron distributions and concentrations, which are often sparse and low in oceanic systems, have been shown to significantly affect phytoplankton distribution and community composition. Recently, a sub-surface minimum in iron concentration has been found to develop at the Bermuda region of the Sargasso Sea in the summer, and this minimum is co-incident with both the DCM and the accumulation of the low-light Prochlorococcus ecotype. This situation has been shown to develop even when iron concentrations at the surface remain high. The objective of this proposal is to determine whether a low iron concentration is the main factor in determining the abundance of the low-light ecotype of Prochlorococcus cells that are numerically dominant in the Bermuda region of the Sargasso Sea in the summer. This will be achieved by participation in a number of cruises aboard the R/V Atlantic Explorer in the Bermuda region of the Sargasso Sea, which are timed to coincide with the early, mid and late stages of development of both the sub-surface iron minimum and the accumulation of Prochlorococcus cells. During these cruises, water-column profile measurements will be made to correlate Prochlorococcus ecotype distribution and physiology with available iron concentration. On-deck incubation experiments will be performed under controlled light and temperature conditions, and iron will be artificially added to water samples taken from the DCM and the surface water (sampled at the same location as iron profiles). The resulting response of the community will then be monitored to detect a response of the phytoplankton to added iron, and to identify which if any species respond to the addition. The different responses of phytoplankton communities at the surface and DCM will determine the limiting factors of the system. Conducting similar experiments during the summer and over two seasons will confirm the temporal development of these features.