Phylogenetic position of three strains of green sulfur bacteria

Abstract

) are aphysiologically homogeneous group forming a sepa-rate phylogenetic branch isolated from other bacteria(phylum Chlorobi). Until recently, their taxonomy wasbased on phenotypic characteristics (morphologicaland biochemical properties) [1, 2]. Comparative analy-sis of the sequences of the 16S rRNA genes and the fmogenes, encoding FMO protein, which occurs exclu-sively in Chlorobiaceae representatives, led to substan-tial reorganization of the taxonomy of this group ofbacteria [3]. As a result, strains previously assigned toone species may have been transferred to several differ-ent species and even genera.The culture collection of the Department of Micro-biology, Moscow State University, includes severalstrains of green sulfur bacteria previously assigned toChl. limicola forma thiosulfatophilum (strains C, X,and L, or KM MGU 319, 320, and 321, respectively).These have been used in studies of various aspects ofmetabolism of green sulfur bacteria [4–8]; therefore,determination of their phylogenetic positions bychemotaxonomic and molecular biological methods istopical.Using the method described in [9], we determinedthe fatty acid compositions of strains C, X, and L(table); such data are rather informative but scarce forgreen sulfur bacteria. Strains L, X, and C were found tocontain a considerable amount of palmitoleic (16:1 ω7),palmitic (16:0) and myristic (14:0) acids, whereas the18:1 acids (oleic and cis-vaccenic) were virtuallyabsent. Such a fatty acid pattern is characteristic ofChlorobiaceae representatives [2, 3, 10].In order to perform comparative analysis of 16SrRNA genes, their fragments were amplified usingstandard bacterial primers as described in [11]. As aresult, sequences of more than 1300 nucleotides weredetermined for strains C, X, and L (and deposited inGenBank: Chb. parvum strain L, EF560699; Chb. lim-naeum strain X, EF560700; and Chb. limnaeumstrain C, EF560701).According to the preliminary phylogenetic screen-ing performed with BLAST (http://www.ncbi.nlm.nih.gov/blast), all the strains studied belonged to thesubdivision of green sulfur bacteria. For more detailedphylogenetic analysis, the 16S rRNA gene sequences ofthe strains studied and reference bacterial species werealigned using the CLUSTLW v1.75 program. Phyloge-netic trees were constructed using different algorithmsimplemented in the TREECONW software package[12]. In the constructed phylogenetic tree (Fig. 1), allthree strains studied fell into phylogenetic cluster 4,