Natural communities of Achromatium oxaliferum comprise genetically, morphologically, and ecologically distinct subpopulations

The diversity and ecology of natural communities of the uncultivated bacter ium Achromatium oxaliferum were studied by use of culture-independent appro aches. 16S rRNA gene sequences were PCR amplified from DNA extracted from h ighly purified preparations of cells that were morphologically identified a s A. oxaliferum present in freshwater sediments from three locations in nor thern England (Rydal Water, Jenny Dam, Hell Kettles). Cloning and sequence analysis of the PCR-amplified 16S rRNA genes revealed that multiple related but divergent sequences were routinely obtained from the A. oxaliferum com munities present in all the sediments examined. Whole-cell in situ hybridiz ation with combinations of fluorescence-labelled oligonucleotide probes rev ealed that the divergent sequences recovered from purified A. oxaliferum ce lls corresponded to genetically distinct Achromatium subpopulations. Analys is of the cell size distribution of the genetically distinct subpopulations demonstrated that each was also morphologically distinct. Furthermore, the re was a high degree of endemism in the Achromatium sequences recovered fro m different sediments; identical sequences were never recovered from differ ent sampling locations. In addition to ecological differences that were app arent between Achromatium communities from different freshwater sediments, the distribution of different subpopulations of Achromatium in relation to sediment redox profiles indicated that the genetically and morphologically distinct organisms that coexisted in a single sediment were also ecological ly distinct and were adapted to different redox conditions. This result sug gests that Achromatium populations have undergone adaptive radiation and th at the divergent Achromatium species occupy different niches in the sedimen ts which they inhabit.