Growth of magnetotactic sulfate-reducing bacteria in oxygen concentration gradient medium Christopher T. Lefe `vre, 1 * Paul A. Howse, 2 Marian L. Schmidt, 3 Monique Sabaty, 1 Nicolas Menguy, 4 George W. Luther III 5 and Dennis A. Bazylinski 2 ** 1 CNRS/CEA/Aix-Marseille Universit  e UMR7265 Institut de biosciences et biotechnologies Laboratoire de Bio energ etique Cellulaire, Saint Paul lez Durance, 13108, France. 2 School of Life Sciences, University of Nevada at Las Vegas, Las Vegas, NV, 89154-4004, USA. 3 Department of Ecology and Evolutionary Biology, University of Michigan, Ann Arbor, MI, USA. 4 Institut de Min eralogie, de Physique des Mat  eriaux et de Cosmochimie, Sorbonne Universit  es, Universit  e Pierre et Marie Curie, UMR 7590 CNRS, Institut de Recherche pour le D eveloppement UMR 206, Museum National d’Histoire Naturelle, Paris Cedex 05, 75252, France. 5 School of Marine Science and Policy, University of Delaware, 700 Pilottown Rd. Lewes, DE, 19958, USA Summary Although dissimilatory sulfate-reducing bacteria (SRB) are generally described as strictly anaerobic organisms with regard to growth, several reports have shown that some SRB, particularly Desulfovi- brio species, are quite resistant to O 2 . For example, SRB remain viable in many aerobic environments while some even reduce O 2 to H 2 O. However, repro- ducible aerobic growth of SRB has not been unequiv- ocally documented. Desulfovibrio magneticus is a SRB that is also a magnetotactic bacterium (MTB). MTB biomineralize magnetosomes which are intracel- lular, membrane-bounded, magnetic iron mineral crystals. The ability of D. magneticus to grow aerobi- cally in several different media under air where an O 2 concentration gradient formed, or under O 2 -free N 2 gas was tested. Under air, cells grew as a microaero- philic band of cells at the oxic–anoxic interface in media lacking sulfate. These results show that D. magneticus is capable of aerobic growth with O 2 as a terminal electron acceptor. This is the first report of consistent, reproducible aerobic growth of SRB. This finding is critical in determining important eco- logical roles SRB play in the environment. Interest- ingly, the crystal structure of the magnetite crystals of D. magneticus grown under microaerobic condi- tions showed significant differences compared with those produced anaerobically providing more evi- dence that environmental parameters influence mag- netosome formation. Introduction Dissimilatory sulfate-reducing bacteria (SRB) are gener- ally considered to be strict anaerobes regarding growth (Rabus et al., 2013). Widespread in marine and fresh- water sediments, they constitute a morphologically and metabolically diverse group of prokaryotes, phylogeneti- cally belonging to both the Bacteria and Archaea domains, and able to use a wide variety of organic com- pounds. SRB obtain energy for cell synthesis and growth by coupling the oxidation of these organic com- pounds or molecular hydrogen (H 2 ) to the reduction of sulfate (SO 22 4 ) to sulfide (H 2 S, HS 2 ) (Rabus et al., 2013). Most known SRB belong to the Deltaproteobacteria class of the Proteobacteria phylum in the domain Bacte- ria. Those in the genus Desulfovibrio were described as strictly anaerobic bacteria since their discovery more than 100 years ago (Beijerinck, 1895). D. vulgaris and D. desulfuricans were used for many years as model organisms to study sulfate reduction (Voordouw and Wall, 1993). Dissimilatory SRB are often present in bio- topes where they are exposed to oxic conditions and some have even been shown to be metabolically active in microaerobic environments (Hardy and Hamilton, 1981; Battersby et al., 1985; Sass et al., 1996; 1997; Krekeler et al., 1997; Sass et al., 1998; Lobo et al., 2007) showing that these bacteria are quite resistant to O 2 (Cypionka et al., 1985; Sass et al., 1996; 1997; 1998; Krekeler et al., 1997). Some SRB not only survive exposure to O 2 for at least days, but some even reduce O 2 to H 2 O (Dannenberg et al., 1992), coupling aerobic respiration to ATP formation (Dilling and Cypionka, 1990; *For correspondence. *E-mail christopher.lefevre@cea.fr; Tel. 133 4 42 25 32 93. **E-mail dennis.bazylinski@unlv.edu; Tel. 101-702-895-5832; Fax 101 702 895 3956. V C 2016 Society for Applied Microbiology and John Wiley & Sons Ltd Environmental Microbiology Reports (2016) 8(6), 1003–1015 doi:10.1111/1758-2229.12479