Magnetospirillum bellicus sp. nov., a Novel Dissimilatory Perchlorate-Reducing Alphaproteobacterium Isolated from a Bioelectrical Reactor

Previously isolated dissimilatory perchlorate-reducing bacteria (DPRB) have been primarily affiliated with the Betaproteobacteria. Enrichments from the cathodic chamber of a bioelectrical reactor (BER) inoculated from creek water in Berkeley, CA, yielded a novel organism most closely related to a previously described strain, WD (99% 16S rRNA gene identity). Strain VDY^T has 96% 16S rRNA gene identity to both Magnetospirillum gryphiswaldense and Magnetospirillum magnetotacticum, and along with strain WD, distinguishes a clade of perchlorate-reducing Magnetospirillum species in the Alphaproteobacteria. In spite of the phylogenetic location of VDY^T, attempted PCR for the key magnetosome formation genes mamI and mamL was negative. Strain VDY^T was motile, non-spore forming, and, in addition to perchlorate, could use oxygen, chlorate, nitrate, nitrite, and nitrous oxide as alternative electron acceptors with acetate as the electron donor. Transient chlorate accumulation occurred during respiration of perchlorate. The organism made use of fermentation end products, such as acetate and ethanol, as carbon sources and electron donors for heterotrophic growth, and in addition, strain VDY^T could grow chemolithotrophically with hydrogen serving as the electron donor. VDY^T contains a copy of the RuBisCo cbbM gene, which was expressed under autotrophic but not heterotrophic conditions. DNA-DNA hybridization with strain WD confirmed VDY^T as a separate species (46.2% identity), and the name Magnetospirillum bellicus sp. nov. (DSM 21662, ATCC BAA-1730) is proposed.Dissimilatory perchlorate-reducing bacteria (DPRB) use perchlorate as a terminal electron acceptor during respiration, reducing it completely to chloride. As a consequence, bioremediation of perchlorate has been identified as the most effective means of treating this harmful contaminant (10), which, due to historically unregulated release into the environment, has become widespread (13, 20, 41). Fortunately, DPRB are ubiquitous and can be readily isolated from a variety of environments (1, 10, 11, 39, 44), and a key gene in the pathway, the chlorite dismutase (cld) gene, has been broadly detected (6). Much has been revealed about the biochemistry and genetics of microbial perchlorate reduction through the study of several model organisms, including Dechloromonas aromatica and Dechloromonas agitata, by a variety of groups (5, 6, 8, 9, 17, 28, 29, 34, 35, 38, 47, 51, 56, 57).Less is known about the variation in physiology between these organisms or the evolution of the perchlorate reduction metabolism, highlighting a need for further isolation and characterization of pure cultures. The lack of congruence between phylogenetic trees of cld and the 16S rRNA gene among tested DPRB suggests that the metabolism may be the result of horizontal gene transfer (6). Given that various elements of the pathway may be mobile, it is not unreasonable to expect that organisms with a wide phylogenetic diversity could acquire the ability to reduce perchlorate. As more varied enrichment conditions are tested (2, 39), sometimes as a result of novel bioreactor development for perchlorate treatment (38, 40, 45), the true phylogenetic diversity of DPRB is becoming apparent, supporting the hypothesis that the metabolism may be widespread within the tree of life, similar to other respiratory processes, such as the reduction of sulfate, Fe(III), and nitrate.Although perchlorate has been primarily regarded as an anthropogenic contaminant, a variety of studies have indicated that perchlorate occurs naturally (29-31, 34), which provides a possible explanation for the selective pressure behind the evolution of perchlorate reduction genes. As more is understood about the chlorine redox cycle on earth, knowledge about the diversity of organisms capable of interacting with the various oxyanions of chlorine is becoming more important. Here, we report the characterization of a unique DPRB in the Alphaproteobacteria. Strain VDY^T was isolated from the surface of a working electrode in an active perchlorate-reducing bioelectrical reactor (BER) that was inoculated with water from Strawberry Creek on the University of California, Berkeley, campus (40). This is only the second described DPRB in the Alphaproteobacteria, the other being the closely related strain WD (26), and these strains compose a unique clade of perchlorate-reducing organisms in the genus Magnetospirillum.