ENVIRONMENTAL MICROBIOLOGY Pond Sediment Magnetite Grains Show a Distinctive Microbial Community H.-K. Song & S. Sonkaria & V. Khare & K. Dong & H.-T. Lee & S.-H. Ahn & H.-K. Kim & H.-J. Kang & S.-H. Lee & S. P. Jung & J. M. Adams Received: 27 June 2014 /Accepted: 23 December 2014 # Springer Science+Business Media New York 2015 Abstract Formation of magnetite in anaerobic sediments is thought to be enhanced by the activities of iron-reducing bacteria. Geobacter has been implicated as playing a major role, as in culture its cells are often associated with extracel- lular magnetite grains. We studied the bacterial community associated with magnetite grains in sediment of a freshwater pond in South Korea. Magnetite was isolated from the sedi- ment using a magnet. The magnetite-depleted fraction of sediment was also taken for comparison. DNA was extracted from each set of samples, followed by PCR for 16S bacterial ribosomal RNA (rRNA) gene and HiSeq sequencing. The bacterial communities of the magnetite-enriched and magnetite-depleted fractions were significantly different. The enrichment of three abundant operational taxonomic units (OTUs) suggests that they may either be dependent upon the magnetite grain environment or may be playing a role in magnetite formation. The most abundant OTU in magnetite- enriched fractions was Geobacter , bolstering the case that this genus is important in magnetite formation in natural systems. Other major OTUs strongly associated with the magnetite- enriched fraction, rather than the magnetite-depleted fraction, include a Sulfuricella and a novel member of the Betaproteobacteria. The existence of distinct bacterial com- munities associated with particular mineral grain types may also be an example of niche separation and coexistence in sediments and soils, which cannot usually be detected due to difficulties in separating and concentrating minerals. Keywords Geobacter . Sulfuricella . Magnetite . Operational taxonomic units . 16S bacterial ribosomal RNA Introduction Magnetite is a naturally occurring magnetic mineral that is widely distributed around the world [1, 2]. While there is no doubt that its formation follows on from iron reduction (from Fe III to Fe II) by bacteria, it has been unclear whether the next stage—combination of Fe II with more Fe III in sediment to produce magnetite—mostly occurs biotically associated with bacterial cells or abiotically without needing biological assis- tance [3, 4]. In recent years, a number of possible candidate bacteria for magnetite formation in sediments have been identified. When Electronic supplementary material The online version of this article (doi:10.1007/s00248-014-0562-7) contains supplementary material, which is available to authorized users. H.<K. Song : S. Sonkaria : K. Dong : J. M. Adams (*) Department of Biological Sciences, Seoul National University, Gwanak-Gu, Seoul 151, Republic of Korea e-mail: foundinkualalumpur@yahoo.com V. Khare Institute of Advanced Machinery and Design, Seoul National University, Gwanak-Ro 1, Gwanak-Gu, Seoul 151-744, Republic of Korea H.<T. Lee : S.<H. Ahn Mechanical and Aerospace Engineering, Seoul National University, Gwanak-Ro 1, Gwanak-Gu, Seoul 151-744, Republic of Korea H.<K. Kim Celemics, Inc. 612 Avison Biomedical Research Center, Yonsei Medical Center, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-752, Republic of Korea H.<J. Kang : S.<H. Lee School of Civil and Environmental Engineering, Yonsei University, Seoul 120-749, South Korea S. P. Jung Department of Environmental and Energy Engineering, Chonnam National University, Gwangju 500-757, Republic of Korea Microb Ecol DOI 10.1007/s00248-014-0562-7