Collaborative: Assessing the Genetic Potential and Physiological Controls of N2 Fixation in Two Temperate, N-Limited Estuaries

9408471淡水和海洋初级生产力往往受到N的有效性的限制，然而这两个领域中固定N的种群的特征有很大的不同。固氮异晶蓝藻是氮限制淡水湖泊和河流的显著浮游特征。在N-有限的海洋水域中，如果氮气发生固定，通常仅限于底栖生物环境。海洋和淡水系统的氮气固定剂分布有显著差异的原因尚不清楚，但可能是由于盐度、湍流或其他因素等物理化学限制，也可能反映了流入海洋领域的异地种子种群的来源、多样性和表达(活动)。淡水中氮气固定生物的优势与河口和海洋中明显缺乏氮气固定生物之间的对比，为拟议的研究提供了基础。我们的主要目标是确定和表征氮气固定基因的来源、多样性和分布，这些基因与环境控制纽兹河河口沿盐度梯度的固氮活性有关。本文提出的方法超越了对氮气固定物活性存在或不存在的常规分析和解释，其重点是这种活性的遗传潜力与调节氮气固定装置的物种选择和表达的环境因素之间的基本关系。这项研究的一个新的方面是，这项研究将考察生态选择因素对微生物多样性的重要性，涉及自然水域如何、为什么以及何时通过发展重氮营养群落和固氮活性来响应氮限制。

9408471 Paerl Freshwater and marine primary production are often limited by the availability of N, yet the characteristics of N2 fixing populations in the two realms differ substantially. N2 fixing heterocystous cyanobacteria are prominent planktonic features of N limited freshwater lakes and rivers. In N-limited marine waters, if N2 fixation occurs at all, it is usually restricted to benthic environments. The reasons for the striking differences in N2 fixer distributions between marine and freshwater systems are not known, but could be due to physico-chemical constraints such as salinity, turbulence, or other factors, or they may reflect sources, diversity and expression (activity) of allochthonous seed populations advected into the marine realm. The contrast between the predominance of N2 fixing organisms in freshwater, and the apparent dearth of N2 fixing organisms in estuaries and oceans, provides the basis for the proposed research. Our primary objective is to identify and characterize the sources, diversity and distribution of N2 fixing genes in relation to environmental control of nitrogen fixation activity along a salinity gradient in an N-limited estuary of the Neuse River. The approach proposed here goes beyond the conventional analysis and interpretation of the presence or absence of the activity of N2 fixers by focusing on the fundamental relationships between the genetic potential for this activity and the environmental factors regulating species selection and expression of the N2 fixation apparatus. %%% A novel aspect of this research is that this investigation will examine the importance of ecological selection factors on microbial diversity with respect to how, why and when natural waters can respond to N limitation with development of a diazotrophic community and N2 fixing activity.