Some aspects of the biology of nitrogen-fixing organisms

Abstract

Eukaryotic organisms do not fix nitrogen. Animals generally have no need to do so because of their complex food-acquisition and waste-disposal systems. Plants, by using carbon polymers for structural purposes, minimize their need for nitrogen. When very nitrogen-limited, to enter into symbiosis with nitrogen-fixing microorganisms may be the most controllable method for eukaryotes to obtain fixed nitrogen. Filamentous, heterocystous nitrogen-fixing cyanobacteria may be better adapted to a free-living than to a symbiotic existence, because of their complexity. In symbioses, their photosynthetic machinery becomes redundant and the need to differentiate heterocysts as well as derepress nif genes may be a disadvantage. This could in part account for the greater success of symbioses involving the structurally simpler genera Frankia , Rhizobium and Bradyrhizobium . Nitrogen fixation by legume nodules can be controlled by varying the oxygen supply. This control may be effected by a variable diffusion resistance, enabling oxygen required for ATP synthesis to be matched to available photosynthate. Such a resistance, which is probably located in the nodule cortex, may also be used to reduce nitrogen fixation in the presence of combined nitrogen and could also facilitate rapid responses to other forms of stress. Alternative resistances to gaseous diffusion may operate when water supplies are restricted. Rhizobium and Bradyrhizobium follow different patterns of differentiation into nitrogen-fixing bacteroids. These patterns are coupled with retention or loss of viability and with significant or no natural enrichment of the bacteroids with 15 N respectively. The basic patterns of each type are subject to host-modification. Recent studies on structures of primitive legume nodules show some parallels both with actinorhizas and with nodules on Parasponia induced by Bradyrhizobium . In particular, distribution of rhizobia in nodule tissues is intercellular and infection threads are formed only when bacteria ‘enter’ host cells; there is no intracellular ‘bacteroid’ stage. These threads are retained in the active nitrogen-fixing cells. Many legumes and some actinorhizas are not infected via root hairs. Therefore two of the stages often considered typical of the development of effective legume nodules, i.e. ‘release’ of bacteria into vesicles bounded by peribacteroid membrane and infection through root hairs, can be omitted; these omissions may be of use in attempts to transfer nodulating ability to new genera.