During the late summer to early fall, Horsetooth Reservoir in Fort Collins, CO, USA is fully stratified and exhibits seasonally high fluxes of iron, manganese, and metal-rich particles into the water column. Particles were collected from the mid-region of the hypolimnion and examined for metal content. Nucleic acids extracted from the particles were used to construct bacterial and archaeal 16S rDNA clone libraries. Surprisingly, 50% of cloned bacterial genes were closely related to a coherent cluster within Candidate Division OP10. To our knowledge, this is the first report of an environmental gene clone library that exhibits a dominance of OP10-related clones. Several other sequences, many with long branch lengths, clustered within eight separate bacterial divisions and the diatom chloroplasts. Most of these divisions are commonly found in freshwater environments. However, gene sequences from characterized metal-oxidizing or metal-reducing bacteria were not identified. The archaeal gene clone libraries contained diverse sequences, most with close homology to previously characterized gene clones of methanogens or uncultivated Crenarchaeota from soil and lacustrine environments. This study identified a unique environment where OP10 bacteria are potentially abundant. Furthermore, we demonstrated that the metal-rich particles from this reservoir support a diverse and interesting community of microorganisms.