Phylogeny and diversity of Achromatium oxaliferum

Achromatium oxaliferum was first described in 1893 by Schewiakoff as an unu sually large bacterium living in freshwater sediments. Up to now no pure cu lture is available. Physical enrichments of achromatia collected from the a cidic Lake Fuchskuhle, which houses a peculiar, smaller variety, and the ne utral Lake Stechlin were investigated by the cultivation-independent rRNA a pproach. PCR in combination with cloning and sequencing was used for the re trieval of 24 partial and 4 nearly full-length 16S rRNA sequences that form ed two distinct phylogenetic clusters. Fluorescence-in-situ-hybridization ( FISH) with four 16S rRNA-targeted oligonucleotide probes unambiguously assi gned the different-sequences to either regular, large A. oxaliferum cells o r to the smaller Lake Fuchskuhle population, tentatively named "A. minus". The two Achromatium sp. 16S rRNA sequence clusters form a stable deep branc h in the gamma subclass of the class Proteobacteria. The closest cultivated relatives are Chromatium vinosum, Rhabdochromatium marinum and Ectothiorbo dospira halophila with 16S rRNA similarities of 86.2 to 90.5%. Profound dif ferences in the population structure of achromatia were revealed in the two lakes by FISH. In one sample from Lake Stechlin three genotypes could be v isualized, and 49% of the cells were assigned to A. oxaliferum clone AST01, 28% to Achromatium sp. genotype AFK192/AFK433 and 23% to Achromatium sp. g enotype AFK192/AST433. In contrast, 4 morphologically and phylogenetically homogeneous population of "A. minus". was present in Lake Fuchskuhle.