Metabolic streamlining in an open-ocean nitrogen-fixing cyanobacterium

A lean, mean microbe The as-yet uncultivated nitrogen-fixing cyanobacterium known as UCYN-A is widely distributed in the world's oceans. Metagenomic analysis has shown it to lack genes for the oxygen-producing photosystem II complex of the photosynthetic apparatus — which helps it fix nitrogen in the daylight — and for carbon fixation. Now using massively parallel paired-end pyrosequencing technology, the complete UCYN-A genome has been determined. It emerges as a remarkably simple organism, lacking many core metabolic pathways and depending heavily on other organisms for organic carbon and even organic nitrogen-containing compounds. Though the genome has structural similarities to chloroplasts and endosymbionts, experiments on natural populations have so far not detected any symbiotic relationships with other microbes. UCYN–A is a recently discovered nitrogen-fixing cyanobacterium with unusual metabolic features. The complete genome of this uncultivated organism is now presented, revealing a photofermentative metabolism and dependency on other organisms for essential compounds. Nitrogen (N 2 )-fixing marine cyanobacteria are an important source of fixed inorganic nitrogen that supports oceanic primary productivity and carbon dioxide removal from the atmosphere 1 . A globally distributed 2 , 3 , periodically abundant 4 N 2 -fixing 5 marine cyanobacterium, UCYN-A, was recently found to lack the oxygen-producing photosystem II complex 6 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 operons 7 . 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 prov