Molecular Aspects and Oxygen Relations of Nitrogen Fixation in Cyanobacteria

Abstract

The biological reduction of nitrogen is catalyzed by nitrogenase enzyme which is irreversibly inhibited by molecular oxygen. Cyanobacteria are nitrogen-fixing organisms that produce oxygen as a by-product of the process of photosynthesis, and which must negotiate the presence of molecular oxygen with an essentially anaerobic enzyme. This chapter draws together an international group of leading cyanobacterial investigators’ experience and excitement to include a state-of-the-art review of the area and addresses problems around cyanobacterial life. On our planet, for quite a long time there have been cyanobacteria. They are mostly widespread, making them ideal model organisms for the study of microbial biogeography as problems of evolution. Aerobic nitrogen fixation is localized in heterocysts and these heterocysts protect nitrogenase from inactivation by atmospheric oxygen. Cyanobacteria contribute greatly to the primary production of oceans and are one of the most important groups that release molecular nitrogen. We confirm that their pigmentation is due to the strength and sometimes even to the color of the light available. Others display remarkable life tolerance under anaerobic conditions; several forms of cyanobacteria thrive at extreme temperatures; other organisms can endure adverse conditions for long periods in salinity and pH, including when growth conditions are not acceptable. Many forms of cyanobacteria are simple to grow in the laboratory, and those in axenic culture have been collected and examined. Cyanobacteria are the most broadly dispersed community of photosynthetic prokaryotes present in virtually every region of the world and play an important role in the nitrogen and carbon cycle of the planet. Research efforts in the past 10 years have revealed a range of O2 sensitivity of nitrogenase in different strains of cyanobacteria and a variety of adaptations for the protection of nitrogenase from damage by both atmospheric and photosynthetic sources of O2. The most complex and apparently most efficient mechanisms for the protection of nitrogenase are incorporated in the heterocysts, the nitrogen-fixing cells of cyanobacteria. Several genetic studies have shown that the controls of heterocyst development and nitrogenase synthesis are closely interrelated and that the expression of nitrogen fixation (nif) genes is regulated by pO2. The incremental transition from decreasing environment to oxidizing atmosphere has become a turning point in the Earth’s evolutionary past which has made possible the conditions for present life types.