Greigite-producing Magnetotactic Bacteria in the Freshwater Moat of Xi'an City, China

Magnetotactic bacteria (MTB) are a group of morphologically and phylogentically heterogenous prokaryotes which usually mineralize intracellular membrane-enclosed and nano-sized magnetic crystals of magnetite (Fe3O4). In this study, we investigated the MTB diversity in a freshwater moat of Xi'an city, northwest of China based on transmission electron microscopy (TEM),16S rRNA gene analysis, and fluorescence in situ hybridization. TEM observation revealed different morphologies of MTB cells including cocci, spirilla, helical to vibrioid bacteria, and rod-shaped bacteria. Among these MTB, novel rod-shaped magnetotactic bacteria, which synthesized solely irregular rectangular greigite (Fe3S4), or both irregular rectangular greigite magnetosomes and bullet shaped magnetite magnetosomes in an individual cell, were identified for the first time in a freshwater environment with a large number of populations. Phylogenetic analysis of 16S rRNA genes and fluorescence in situ hybridization revealed that these specific MTB belong to sulfate reducing bacteria in the Deltaproteobacteria. Compositions of magnetosome crystals were analyzed by energy dispersive X-ray spectroscopy, high resolution TEM, and fast Fourier transform patterns analyses. Although it is unknown at this stage the biologically controlled mineralization mechanism within these greigite-producing MTB, we hypothesize that these MTB may have two different magnetosome gene clusters and distinct mineralization mechanisms. The presence of greigite-producing MTB in freshwater environments implies that these bacteria can play important roles in the iron and sulfur biogeochemical cycling in habitats, and magnetosome greigite, if preserved, can be potential carriers of magnetic signals in sediments.