Abstract
Free-living or asymbiotic N2-fixing bacteria are very widely distributed taxonomically, geographically and ecologically. Before considering a number of N2 fixation systems we will review the current list of N2-fixing bacteria and the habitats in which they are observed to occur. Since N2 fixation is most frequently limited by the availability of energy, usually in the form of photo-synthetic products, the degree of coupling between the N2-fixing system and the source of photosynthate is clearly critical. We will discuss systems in which this degree of coupling is relatively loose. Other papers in this symposium deal with photosynthetic bacteria and with some root-associated bacteria, systems in which coupling to photosynthate is relatively close. This paper will therefore make merely brief references to such systems.
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