Phylogeny and evolution of the family Ectothiorhodospiraceae based on comparison of 16S rRNA, cbbL and nifH gene sequences

The occurrence of genes encoding nitrogenase and ribulose-1, 5-bisphosphate carboxylase/ oxygenase (RubisCO) was investigated in the members of the family Ectothiorhodospiraceae. This family forms a separate phylogenetic lineage within the Gammaproteobacteria according to 16S rRNA gene sequence analysis and mostly includes photo- and chemoautotrophic halophilic and haloalkaliphilic bacteria. The cbbL gene encoding the large subunit of 'green-like' form I RubisCO was found in all strains, except the type strains of Alkalispirillum mobile and Arhodomonas aquaeolei. The nifH gene encoding nitrogenase reductase was present in all investigated species of the phototrophic genera Ectothiorhodospira, Halorhodospira and Thiorhodospira, but not of the genus Ectothiorhodosinus. Unexpectedly, nifH fragments were also obtained for the chemotrophic species Thioalkalispira microaerophila and Alkalilimnicola halodurans, for which diazotrophic potential has not previously been assumed. The cbbL-, nifH- and 16S rRNA gene-based trees were not highly congruent in their branching patterns since, in the 'RubisCO' and 'nitrogenase' trees, representatives of the Ectothiorhodospiraceae are divided in a number of broadly distributed clusters and branches. However, the data obtained may be regarded as evidence of the monophyletic origin of the cbbL and nifH genes in most species within the family Ectothiorhodospiraceae and mainly corresponded to the current taxonomic structure of this family. The cbbL phylogeny of the chemolithoautotrophic sulfur-oxidizers Thioalkalivibrio nitratireducens and Thioalkalivibrio paradoxus and the nitrifier Nitrococcus mobilis deviated significantly from the 16S-rRNA gene-based phylogeny. These species clustered with one of the duplicated cbbL genes of the purple sulfur bacterium Allochromatium, vinosum, a member of the family Chromatiaceae.