The Family Azotobacteraceae
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The Family Azotobacteraceae JAN HENDRIK BECKING
The family Azotobacteraceae is represented by two genera, Azotobacter (Beijerinck, 1901a, 1901b) and Azomonas (Winogradsky, 1938). The removal of the genera Beijerinckia and Derxia from the Azotobacteraceae was based on rRNA cistron analysis (De Smedt et al., 1980) and rRNA cistron similarities as observed in DNArRNA hybridization experiments (De Vos et al., 1985). Such experiments showed that Beijerinckia and Derxia are not closely related to the genera Azotobacter and Azomonas, but that they belong to other subdivisions or groups, i.e., the alpha and the beta subclasses, respectively of the Proteobacteria (see The Proteobacteria: Ribosomal RNA Cistron Similarities and Bacterial Taxonomy in the second edition). On the other hand, investigations on rrna similarities of various gram-negative bacteria based on tm(e) values of dna-rrna hybrids showed that Azotobacter and Azomonas are closely related to one another and to the Pseudomonas fluorescens rRNA branch. The latter branch or group belongs to the Superfamily I in the nomenclature of De Ley and coworkers (see The Proteobacteria: Ribosomal RNA Cistron Similarities and Bacterial Taxonomy in the second edition; De Smedt et al., 1980; and De Vos et al., 1985) or the gamma group/ subdivision or subclass of the purple bacteria as defined by Woese et al. (1985a, 1985b) or the Proteobacteria, as defined by Stackebrandt et al. 1988. Members of the Azotobacteraceae are primarily characterized as nonsymbiotic (i.e., free-living), aerobic, heterotrophic bacteria whose main property is the ability to fix molecular (atmospheric) nitrogen in a nitrogen-free or nitrogenpoor medium with an organic carbon compound (preferentially sugars, alcohols, or organic acids) as energy source. However, dinitrogen fixation is not unique to this family, as this property can be observed in quite a number of other unrelated bacteria (see The Genera Azospirillum and Herbaspirillum along with The Rhizobia in the second edition). Moreover, some representatives of this family can produce associative growth This chapter was taken unchanged from the second edition.
(which is different from symbiotic growth) with higher plants. For these reasons, representatives of this family are usually called nonsymbiotic nitrogen-fixers.
General Habitats Representatives of the Azotobacteraceae are regular inhabitants of soil, including aerially transported dust, of water habitats, and of plant surfaces such as the external environment of roots (rhizosphere) and leaves (phyllosphere). Azotobacter chroococcum and A. vinelandii also occur in marine habitats. Some species occur in much larger numbers in the rhizosphere of higher plants than in the soil itself, and it has been shown in some cases that this associative growth is beneficial for the plant because fixed nitrogen becomes available to the plant (Döereiner, 1966, reference is not an exact match1968). Some investigators (see “Applications”) report the same for Azotobacter
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