EVOLUTIONARY RELATIONSHIPS AMONG AMMONIA-OXIDIZING AND NITRITE-OXIDIZING BACTERIA

Comparative 16S rRNA sequencing was used to evaluate phylogenetic rela tionships among selected strains of ammonia- and nitrite-oxidizing bac teria. All characterized strains were shown to be affiliated with the proteobacteria. The study extended recent 16S rRNA-based studies of ph ylogenetic diversity among nitrifiers by the comparison of eight strai ns of the genus Nitrobacter and representatives of the genera Nitrospi ra and Nitrospina. The later genera were shown to be affiliated with t he delta subdivision of the proteobacteria but did not share a specifi c relationship to each other or to other members of the delta subdivis ion. All characterized Nitrobacter strains constituted a closely relat ed assemblage within the alpha subdivision of the proteobacteria. As P reviously observed, all ammonia-oxidizing genera except Nitrosococcus oceanus constitute a monophyletic assemblage within the beta subdivisi on of the proteobacteria. Errors in the 16S rRNA sequences for two str ains previously deposited in the databases by other investigators (Nit rosolobus multiformis C-71 and Nitrospira briensis C-128) were correct ed. Consideration of physiology and phylogenetic distribution suggeste d that nitrite-oxidizing bacteria of the alpha and gamma subdivisions are derived from immediate photosynthetic ancestry; Each nitrifier ret ains the general structural features of the specific ancestor's photos ynthetic membrane complex. Thus, the nitrifiers, as a group, apparentl y are not derived from an ancestral nitrifying phenotype.