Perspectives

Abstract

A chronology for the evolution of major processes in nitrogen metaboolism, culminating in the capacity to fix N₂, is developed in this section. It is based on a consideration of the composition of the primordial atmosphere and on a logical sequence of evolutionary challenge and response, proceeding through the following successive stages 1. initial utilization of ammonia, 2. oxidation of ammonia to nitrate, 3. conversion of nitrate to N₂ and N₂O, as well as reconversion of nitrate to ammonia, and 4. fixation of N₂ to ammonia (Table 3-1). Evolution of nitrogenase in free-living organisms is discussed first, followed by proposals for associative and obligatory symbioses.