PHYLOGENETIC AFFILIATION AND ULTRASTRUCTURE OF UNCULTURED MAGNETIC BACTERIA WITH UNUSUALLY LARGE MAGNETOSOMES

Natural enrichments of magnetic bacteria from the Itaipu lagoon near R io de Janeiro were dominated by coccoid-to-ovoid morphotypes that prod uced unusually large magnetosomes. To determine the phylogenetic posit ion of these unusual microorganisms, 16S rRNA genes were retrieved fro m bacteria magnetically separated from sediment of the Itaipu lagoon b y in vitro amplification and cloning of PCR products into a plasmid ve ctor. Partial sequencing of the obtained clones revealed two clusters of closely related sequences affiliated to a distinct lineage consisti ng exclusively of magnetic bacteria within the alpha-subclass of Prote obacteria. For a detailed phylogenetic analysis. several almost comple te sequences of the 16S rRNA genes were determined. One representative clone of each cluster provided a PCR template for the in vitro transc ription of group-specific polynucleotide probes complementary to a var iable region of the 16S rRNA molecule. At least three different morpho types of magnetic bacteria were reliably identified by post-embedding hybridization of ultra-thin sections, Electron microscopic analyses of hybridized cells enabled for the first time a detailed description of the morphological variety and ultrastructure of phylogenetically iden tified, uncultured magnetic bacteria, Two distinct coccoid bacteria we re identified by the transcript probe complementary to the 16S I RNA s equence mabrj 12, whereas the probe complementary to the sequence mabr j58 allowed the identification of an ovoid morphotype that displayed m agnetosomes with the largest volumes observed to date.