Carbon cycling by cellulose-fermenting nitrogen-fixing bacteria

Abstract

The most abundant organic materials on Earth are plant polysaccharides such as cellulose and hemicelluloses. Inasmuch as vast quantities of these polymers are present in anaerobic environments (e.g., in soils and sediments), anaerobic microorganisms that ferment plant polysaccharides play a central role in carbon cycling on the planet as a source of CO 2 and, indirectly, of CH 4. Cellulose-fermenting bacteria from soil and pond sediment were isolated in a medium (incubated in N 2) which lacked a source of combined nitrogen. The isolates had the ability to utilize atmospheric N 2 as the nitrogen source for cell growth. Nitrogenase (the enzyme which catalyzes the reduction of N 2 to ammonia) was demonstrated by means of the acetylene reduction test in these isolates and in several previously described anaerobic cellulolytic bacteria isolated from various natural environments. Thus, cellulose-fermenting bacteria that fix N 2 may be widespread and may play a role in nitrogen cycling as well as in carbon cycling on a global scale. Knowledge of the physiology and ecology of these organisms is crucial to detailing the mechanisms producing local sources and sinks of atmospheric gases, interpreting data obtained using space-based sensors, and understanding the effects of atmospheric warming on fermentations as major sources of CO 2 and CH 4.