POWRE: Phylogeny of Heterocystous Cyanobacteria and Evolution of nitrogen Fixation Genes

巴纳姆9973340蓝细菌是一种多样的产氧光合细菌，几乎栖息在每一个主要的水生和陆地生物群落-沙漠，淡水和海洋，包括温泉和亚北极和南极地区。蓝细菌在贫瘠的土壤和养分枯竭的地区特别重要，是通过固氮过程提供固定氮的主要来源。一些物种与各种植物物种形成共生关系，并直接提供氮供植物吸收。 蓝细菌有着古老的历史，可以追溯到近35亿年前。 他们广泛多样化，成为地球上最成功和生态上最重要的生物体之一，在血统的长寿和对地球早期环境的影响方面。 人们普遍认为，蓝细菌负责通过光合作用将地球的厌氧大气转化为有氧大气-这是导致地球上多种生命进化的关键事件。 虽然通过光合作用固定二氧化碳在生物进化的早期就出现了，但不知道固氮何时首次发生。由于固氮酶被氧气不可逆地灭活，因此有人提出固氮是一个古老的过程，在地球大气层成为有氧之前就存在了。 然而，这一假说与固氮作用在不同谱系的活细菌（包括蓝细菌）中的随机发生模式不一致，而不是根据古老起源所预期的普遍发生模式。 在蓝细菌中，大约有九个进化谱系。 其中之一，异型固氮蓝藻具有异型胞细胞-一种独特的适应空间排除氧气。蓝细菌的异形胞谱系非常适合于固氮（nif）基因进化的初步研究。核糖体DNA序列（23 S核糖体RNA基因）将用于确定异囊谱系的进化历史，作为评估三个固氮基因之一nifD的DNA序列进化的独立框架。最终，其他固氮基因和其他蓝藻谱系将进行检查。

Barnum 9973340Cyanobacteria are a diverse group of oxygen-producing, photosynthetic bacteria that inhabit virtually every major aquatic and terrestrial biome - desert, freshwater and marine, including hot springs and sub-Arctic and Antarctic regions. Cyanobacteria are particularly important in poor soils and nutrient-depleted regions and are the major suppliers of fixed nitrogen through the process of nitrogen fixation. Some species form symbiotic relationships with various plant species and directly provide nitrogen for uptake by plants. Cyanobacteria have an ancient history dating back almost 3.5 billion years. They diversified extensively to become some of the most successful and ecologically significant organisms on Earth, with respect to longevity of the lineage and impact on the Earth's early environment. It is widely believed that cyanobacteria were responsible for the conversion of the Earth's anaerobic atmosphere to an aerobic one via oxygenic photosynthesis - a pivotal event that led to the evolution of diverse life on Earth. While carbon dioxide fixation, through photosynthesis, arose early in the evolution of organisms, it is not known when nitrogen fixation first occurred. Since nitrogen fixation enzymes are irreversibly inactivated by oxygen, it has been proposed that nitrogen fixation is an ancient process that existed before the Earth's atmosphere became aerobic. However, this hypothesis is inconsistent with the random pattern of occurrence of nitrogen fixation among various lineages of living bacteria (including the cyanobacteria), rather than a pattern of universal occurrence that is expected on the basis of an ancient origin. Within the Cyanobacteria, there are approximately nine evolutionary lineages. One of these, the heterocystous nitrogen-fixing cyanobacteria possesses heterocyst cells - a unique adaptation for spatially excluding oxygen. The heterocystous lineage of cyanobacteria is well suited for a pilot study on the evolution of nitrogen fixation (nif) genes. Ribosomal DNA sequences (23S ribosomal RNA genes) will be used to determine the evolutionary history of the heterocystous lineage, as an independent framework to evaluate the DNA sequence evolution of one of the three nitrogen fixation genes, nifD. Ultimately, additional nitrogen fixation genes and other cyanobacterial lineages will be examined.