被稱(chēng)為UCYN-A的迄今尚未培養(yǎng)出的固氮“藻青菌”廣泛分布在世界海洋中,。元基因組分析表明，它沒(méi)有為光合機(jī)構(gòu)中產(chǎn)生氧氣的光合體系II復(fù)合物（該復(fù)合物幫助它在日光下固氮）編碼的基因,，也沒(méi)有固碳基因,。
現(xiàn)在,，利用大規(guī)模并行配對(duì)端焦磷酸測(cè)序技術(shù)，研究人員確定了完整的UCYN-A基因組,。原來(lái),，它是一種特別簡(jiǎn)單的生物，缺少很多核心代謝通道,，嚴(yán)重依賴(lài)于其他生物獲取有機(jī)碳,、甚或有機(jī)含氮化合物,。雖然該基因組與葉綠體和內(nèi)共生體在結(jié)構(gòu)上有相似性，但對(duì)自然種群所做的實(shí)驗(yàn)迄今未能發(fā)現(xiàn)與其他微生物的任何共生關(guān)系,。（生物谷Bioon.com）
生物谷推薦原始出處：
Nature 464, 90-94 (4 March 2010) | doi:10.1038/nature08786
Metabolic streamlining in an open-ocean nitrogen-fixing cyanobacterium
H. James Tripp1, Shellie R. Bench1, Kendra A. Turk1, Rachel A. Foster1, Brian A. Desany2, Faheem Niazi2, Jason P. Affourtit2 & Jonathan P. Zehr1
1 Ocean Sciences Department, University of California, Santa Cruz, 1156 High Street, Santa Cruz, California 95064, USA
2 454 Life Sciences, a Roche Company, 20 Commercial Street, Branford, Connecticut 06405, USA
Nitrogen (N2)-fixing marine cyanobacteria are an important source of fixed inorganic nitrogen that supports oceanic primary productivity and carbon dioxide removal from the atmosphere1. A globally distributed2, 3, periodically abundant4 N2-fixing5 marine cyanobacterium, UCYN-A, was recently found to lack the oxygen-producing photosystem?II complex6 of the photosynthetic apparatus, indicating a novel metabolism, but remains uncultivated. Here we show, from metabolic reconstructions inferred from the assembly of the complete UCYN-A genome using massively parallel pyrosequencing of paired-end reads, that UCYN-A has a photofermentative metabolism and is dependent on other organisms for essential compounds. We found that UCYN-A lacks a number of major metabolic pathways including the tricarboxylic acid cycle, but retains sufficient electron transport capacity to generate energy and reducing power from light. Unexpectedly, UCYN-A has a reduced genome (1.44?megabases) that is structurally similar to many chloroplasts and some bacteria, in that it contains inverted repeats of ribosomal RNA operons7. The lack of biosynthetic pathways for several amino acids and purines suggests that this organism depends on other organisms, either in close association or in symbiosis, for critical nutrients. However, size fractionation experiments using natural populations have so far not provided evidence of a symbiotic association with another microorganism. The UCYN-A cyanobacterium is a paradox in evolution and adaptation to the marine environment, and is an example of the tight metabolic coupling between microorganisms in oligotrophic oceanic microbial communities.
