The Genera Azoarcus, Azovibrio, Azospira and Azonexus

Abstract

The genera Azoarcus, Azospira, Azovibrio and Azonexus harbor mostly nitrogen-fixing Betaproteobacteria. In their first taxonomic description, several strains which were later assigned to the genera Azovibrio, Azospira (Reinhold-Hurek et al., 1993) and Azonexus (Hurek et al., 1997b) were included in the genus Azoarcus sensu lato. Therefore, they are discussed in one chapter. Except for one strain, all these diazotrophic bacteria had been isolated from a similar source, roots of a pioneer grass in Pakistan or fungal resting stages from the same field; they had many physiological features in common, and they were phylogenetically related. Therefore, three groups of bacteria were included into Azoarcus although they were located on the Azoarcus rRNA branch at low Tm(e) values (Reinhold-Hurek et al., 1993) or showed a low 16S rDNA similarity (Hurek et al., 1997b). Isolation of additional strains, which made possible a better phenotypic description, and sequencing of almost complete 16S rDNA sequences allowed the reassessment of the taxonomic structure of Azoarcus sensu lato. The unnamed groups C and D (Reinhold-Hurek et al., 1993) obtained the rank of different genera Azovibrio and Azospira, respectively (Reinhold- Hurek and Hurek, 2000); Azoarcus sensu lato group E (Hurek et al., 1997b) was proposed as Azonexus (Hurek et al., 1997b). Strains of these genera deserve special recognition as endophytes of grass roots or as degraders of aromatic compounds. The Genus Azoarcus Introduction The genus Azoarcus harbors two ecologically different groups of bacteria: 1) soil-borne strains which can degrade aromatic hydrocarbons under denitrifying conditions, and 2) bacteria associated with grass roots epi- or endophytically, which apparently do not survive well in root-free soil. They will be referred to as soil-borne or plant-associated species, respectively, in this chapter. Azoarcus was originally described with two valid species of grass-associated diazotrophs (Reinhold-Hurek et al., 1993). However, a growing number of species of soil-borne strains has been added in recent years.