Abstract

Biological nitrogen (N 2 ) fixation is important in controlling biological productivity and carbon flux in the oceans. Unicellular N 2 -fixing cyanobacteria have only recently been discovered and are widely distributed in tropical and subtropical seas. Metagenomic analysis of flow cytometry–sorted cells shows that unicellular N 2 -fixing cyanobacteria in “group A” (UCYN-A) lack genes for the oxygen-evolving photosystem II and for carbon fixation, which has implications for oceanic carbon and nitrogen cycling and raises questions regarding the evolution of photosynthesis and N 2 fixation on Earth.