Magnetococcus marinus gen. nov., sp. nov., a marine, magnetotactic bacterium that represents a novel lineage (Magnetococcaceae fam. nov., Magnetococcales ord. nov.) at the base of the Alphaproteobacteria

Magnetotactic bacteria are a morphologically, metabolically and phylogenetically disparate array of bacteria united by the ability to biomineralize membrane-encased, single-magnetic-domain mineral crystals (magnetosomes) that cause the cell to orientate along the Earth’s geomagnetic field. The most commonly observed type of magnetotactic bacteria is the ubiquitous magnetotactic cocci, which comprise their own phylogenetic group. Strain MC-1^T, a member of this group, was isolated from water collected from the oxic–anoxic interface of the Pettaquamscutt Estuary in Rhode Island, USA, and cultivated in axenic culture. Cells of strain MC-1^T are roughly spherical, with two sheathed bundles of flagella at a single pole (bilophotrichous). Strain MC-1^T uses polar magnetotaxis, and has a single chain of magnetite crystals per cell. Cells grow chemolithoautotrophically with thiosulfate or sulfide as the electron donors, and chemo-organoheterotrophically on acetate. During autotrophic growth, strain MC-1^T relies on the reductive tricarboxylic acid cycle for CO₂ fixation. The DNA G+C content is 54.2 mol%. The new genus and species Magnetococcus marinus gen. nov., sp. nov. are proposed to accommodate strain MC-1^T ( = ATCC BAA-1437^T  = JCM 17883^T), which is nominated as the type strain of Magnetococcus marinus. A new order (Magnetococcales ord. nov.) and family (Magnetococcaceae fam. nov.) are proposed for the reception of Magnetococcus and related magnetotactic cocci, which are provisionally included in the Alphaproteobacteria as the most basal known lineage of this class.