THE PHYLOGENY OF AUTOTROPHIC AMMONIA-OXIDIZING BACTERIA AS DETERMINEDBY ANALYSIS OF 16S RIBOSOMAL-RNA GENE-SEQUENCES

Partial sequences of the 16S ribosomal RNA genes of eleven autotrophic ammonia-oxidizing bacteria were determined by PCR amplification from small amounts of heat-lysed biomass followed by direct sequencing of P CR products. The sequences were aligned with those of representative P roteobacteria and phylogenetic trees inferred using both parsimony and distance matrix methods. This confirmed that the autotrophic ammonia- oxidizers comprise two major lines of descent within the Proteobacteri a. Nitrosomonas Spp., Nitrosococcus mobilis, and strains of Nitrosovib rio, Nitrosospira and Nitrosolobus were located in the beta-subdivisio n. The recovery of Nitrosococcus oceanus strains as a deep branch in t he gamma-subdivision supported the RNA catalogue data which had indica ted that the genus Nitrosococcus is polyphyletic. The autotrophic ammo nia-oxidizing bacteria of the beta-Proteobacteria formed a coherent gr oup which is interpreted as representing a single family. Within this clade, the genera, Nitrosovibrio, Nitrosospira and Nitrosolobus exhibi ted very high levels of homology in their 16S ribosomal RNA gene seque nces and can be accommodated within a single genus. Separation of thes e genera is currently based entirely on gross morphological difference s and these can now be considered more appropriate for the identificat ion of species within this group. It is therefore proposed that Nitros olobus, Nitrosovibrio and Nitrosospira strains be reclassified in a si ngle genus for which the name Nitrosospira has priority.