Paleoecological Analysis of Oceanic N2 Fixing Cyanobacteria Using a Molecular Biological Approach

目前的证据表明，海洋浮游蓝藻对N2的固定可能是控制海洋生产力变化的关键因素，从而控制了古生态时间尺度上的二氧化碳封存。然而，目前缺乏直接评估在这样的时间尺度上N2固定是否或如何与海洋生产力共变的方法。迄今为止，我们对古生态趋势的了解，大部分来自于对上覆水域浮游生物种群沉积到海底的物质的地球化学分析。从水柱中脱落的细胞中的DNA可以在地质意义重大的时间尺度（1 K- 1 OOK年）内保存在沉积物中，因此可能为水柱种群的身份及其代谢潜力提供更直接的记录。固氮酶（N2固定）酶系统的基因已经在多种原核生物中得到很好的表征。研究人员开发了从海洋表层沉积物中有效提取和扩增DNA的方法。蓝藻DNA可以从现有的水生沉积物中回收，并且可以从其他生理类型的N2固定物中分离、鉴定和区分nifH序列。PI将探索利用这种方法分析和重建固氮蓝藻的古生态趋势的能力，方法是从已知水柱中含有固氮蓝藻的区域的较深沉积物层中提取DNA。在这些地层中不可能有活的蓝藻，它们应该代表沉积物中较早沉积的物质。作为对这一命题的检验，作者将检查来自3个地点的沉积物，这些地点已知浮游固氮蓝藻已经增殖，并通过它们的色素特征记录在沉积物中。其中包括加拿大实验湖区的一个湖泊系统，波罗的海的戈特兰盆地，以及地中海的沉积物。关键的初步挑战将是：1)确定随着沉积物深度和时间的增加，蓝藻DNA的保存程度；2)能够提供某种水平的基因在不同层位之间发生的相对定量。如果成功，PI将把这种方法应用于海洋系统的陆架沉积物，在这些系统中，在冰期/间冰期时间尺度上水柱氮固定的强烈趋势已经被推断出来，例如阿拉伯海，从而为可能直接影响气候趋势的微生物种群动态提供重要的直接见解。此外，这些研究将为古生态时间尺度上其他生物地球化学相关功能酶的分析提供一个模型系统。该项目的目的是发展和采用分子方法来描述古代微生物群落的生物地球化学方面。具体目的是检查蓝藻种群的发展，这将反映在地质时间尺度上N2固定条件的存在和缺失。这种方法将为微生物在历史背景下的功能作用提供独特的数据，从而为地球上N2固定的历史提供新的信息。

Capone 9982721Current evidence suggests that N2 fixation by marine planktonic cyanobacteria may be a key factor controlling variations in oceanic productivity and, therefore, of C02 sequestration over paleoecological time scales. However, methods are currently lacking to evaluate directly whether or how N2 fixation has covaried with oceanic productivity over such time scales. Much of our knowledge of paleoecological trends to date derives from the geochemical analysis of material sedimented to the sea floor from planktonic populations in overlying waters. DNA in cells falling out of the water column can be preserved in sedimentary deposits over geologically significant (I K- I OOK years) time scales and may therefore provide a more direct record of the identity of water column populations and their metabolic potentials. The genes of the nitrogenase (N2 fixing) enzyme system have been well characterized for a wide variety of prokaryotes. The investigator has developed methods to efficiently extract and amplify DNA from surficial marine sediments. Cyanobacterial DNA can be recovered from current aquatic sediments and nifH sequences isolated, identified, and distinguished from the nifH sequences of other physiological types of N2 fixers.The PI will explore the ability to use this approach to analyze and reconstruct paleoecological trends in N2 fixing cyanobacteria by extracting DNA from deeper sediment horizons underlying areas known to harbor N2 fixing cyanobacteria in the water column. Living cyanobacteria would not be expected in these horizons and should represent material earlier laid down in the sediments. As a test of this proposition, the author will examine sediments from 3 sites where planktonic N2 fixing cyanobacteria are known to have proliferated and have been recorded in the sediments through their pigment signatures. These include a lake system in the Experimental Lake Region of Canada, the Goteland basin of the Baltic Sea, and in Mediterranean sapropel deposits. Key preliminary challenges will be 1) to determine the extent of preservation of cyanobacterial DNA with increasing sediment depth and, therefore, time and 2) to be able to provide some level of relative quantitation of gene occurrence among horizons.If successful, the PI would be to apply this method to shelf sediments of marine systems where strong trends in water column N2 fixation over glacial/ interglacial time scales have been inferred such as the Arabian Sea - thereby providing important direct insights on microbial populations dynamics which may directly affect climatological trends. Furthermore, these efforts would provide a model system for the analysis of other functional enzymes of biogeochemically relevance on paleoecological time scales.The objective of this project is to develop and employ a molecular approach for describing biogeochemical aspects of ancient microbial communities. The specific aim is to examine the development of cyanobacterial populations that would reflect the presence and absence of N2 fixing conditions over geological time scales. This approach would provide unique data on the functional role of microorganisms in a historical context, and thus will provide new information on the history of N2 fixation on Earth.