The biological reduction of N2 is catalyzed by nitrogenase, which is irreversibly inhibited by molecular oxygen. Cyanobacteria are the only diazotrophs (nitrogen-fixing organisms) that produce oxygen as a by-product of the photosynthetic process, and which must negotiate the inevitable presence of molecular oxygen with an essentially anaerobic enzyme. In this review, we present an analysis of the geochemical conditions under which nitrogenase evolved and examine how the evolutionary history of the enzyme complex corresponds to the physiological, morphological, and developmental strategies for reducing damage by molecular oxygen. Our review highlights biogeochemical constraints on diazotrophic cyanobacteria in the contemporary world.
Bibliographical noteFunding Information:
Many thanks to John Reinfelder for help with the MINEQL+ calculations of iron speciation in Table 1, to Felisa Wolfe and Costantino Vetriani for generating and analyzing the phylogenetic tree (Fig. 3), Y. Ziv for graphic help, and J. Kasting and anonymous reviewers for critical comments on the manuscript. Support for this research was provided by Rutgers University as a postdoctoral research fellowship (to IBF), and by the National Science Foundation through a Biocomplexity grant to PGF (OCE 0084032).
- Nitrogen fixation