Unexpected diversity in populations of the many-celled magnetotactic prokaryote.

The many-celled magnetotactic prokaryote (MMP) is an uncultivated, highly motile aggregate of 10-30 cells containing numerous chains of greigite (Fe(3)S(4)) magnetosomes. It is unique to marine environments and is abundant in slightly sulfidic sediments of the Little Sippewissett salt marsh (Falmouth, MA). We sequenced 16s rDNA genes from a natural population of MMP and found five lineages separated by at least 5% sequence divergence. Fluorescent in situ hybridization probes for three of these lineages showed significant variation in their relative abundances across a seasonal cycle in marsh productivity. The MMP should therefore be considered a separate genus in the delta-proteobacteria rather than a single species as previously thought. All cells in each aggregate express identical SSU rRNAs, suggesting that the aggregates are composed of a single MMP phylotype. This observation supports a model of the MMP as comprised of clonal cells which reproduce by binary fission of the aggregate.