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

9410325 Zehr淡水和海洋初级生产常常受到氮供应的限制，但这两个领域的固氮种群特征有很大不同。固氮异囊蓝藻是限氮淡水湖泊和河流的突出浮游特征。在氮含量有限的海水中，如果发生氮固定，通常仅限于底栖生物环境。海洋和淡水系统之间N2固定物分布显著差异的原因尚不清楚，但可能是由于物理化学限制，如盐度、湍流或其他因素，或者它们可能反映了平流到海洋领域的外来种子种群的来源、多样性和表达（活性）。淡水中固氮生物的优势与河口和海洋中固氮生物的明显缺乏之间的对比，为提出的研究提供了依据。我们的主要目的是在Neuse河河口沿盐度梯度确定与固氮活性环境控制有关的固氮基因的来源、多样性和分布特征。本文提出的方法超越了对N2固定物活性存在与否的传统分析和解释，重点关注该活性的遗传潜力与调节物种选择和N2固定装置表达的环境因素之间的基本关系。本研究的一个新颖之处在于，本研究将研究生态选择因素对微生物多样性的重要性，涉及天然水体如何、为什么以及何时通过重氮营养群落的发展和固氮活性来响应氮限制。

9410325 Zehr 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.