Bazylinski, Dennis A.

Significance A wide range of organisms sense Earth’s magnetic field for navigation. For some organisms, like magnetotactic bacteria, magnetic particles form inside cells and act like a compass. However, the origin of magnetotactic behavior remains a mystery. We report that magnetotaxis evolved in bacteria during the Archean, before or near the divergence between the Nitrospirae and Proteobacteria phyla, suggesting that magnetotactic bacteria are one of the earliest magnetic-sensing and biomineralizing organisms on Earth. The early origin for magnetotaxis would have provided evolutionary advantages in coping with environmental challenges faced by microorganisms on early Earth. The persistence of magnetotaxis in separate lineages implies the temporal continuity of geomagnetic field, and this biological evidence provides a constraint on the evolution of the geodynamo.

A marine, magnetotactic bacterium, designated strain MMS-1T, was isolated from mud and water from a salt marsh in Woods Hole, Massachusetts, USA, after enrichment in defined oxygen-concentration/redox-gradient medium. Strain MMS-1T is an obligate microaerophile capable of chemoorganoheterotrophic and chemolithoautotrophic growth. Optimal growth occurred at pH 7.0 and 24–26 °C. Chemolithoautotrophic growth occurred with thiosulfate as the electron donor and autotrophic carbon fixation was via the Calvin–Benson–Bassham cycle. The G+C content of the DNA of strain MMS-1T was 47.2 mol%. Cells were Gram-negative and morphologically variable, with shapes that ranged from that of a lima bean to fully helical. Cells were motile by means of a single flagellum at each end of the cell (amphitrichous). Regardless of whether grown in liquid or semi-solid cultures, strain MMS-1T displayed only polar magnetotaxis and possessed a single chain of magnetosomes containing elongated octahedral crystals of magnetite, positioned along the long axis of the cell. Phylogenetic analysis based on 16S rRNA gene sequences indicated that strain MMS-1T belongs to the family Rhodospirillaceae within the Alphaproteobacteria , and is distantly related to species of the genus Magnetospirillum . Strain MMS-1T is therefore considered to represent a novel species of a new genus, for which the name Magnetospira thiophila gen. nov., sp. nov. is proposed. The type strain of Magnetospira thiophila is MMS-1T ( = ATCC BAA-1438T = JCM 17960T).

The crystal structure of biomineralized magnetic nanocrystals depends on environmental and genetic factors.

Summary A magnetotactic bacterium, designated strain LO‐1, of the Nitrospirae phylum was detected and concentrated from a number of freshwater and slightly brackish aquatic environments in southern Nevada. The closest phylogenetic relative to LO‐1 is Candidatus Magnetobacterium bavaricum based on a 91.2% identity in their 16S rRNA gene sequence. Chemical and cell profiles of a microcosm containing water and sediment show that cells of strain LO‐1 are confined to the oxic–anoxic interface and the upper regions of the anaerobic zone which in this case, occurred in the sediment. This microorganism is relatively large, ovoid in morphology and usually biomineralizes three braid‐like bundles of multiple chains of bullet‐shaped magnetosomes that appeared to be enclosed in a magnetosome membrane. Cells of LO‐1 had an unusual three‐layered unit membrane cell wall and contained several types of inclusions, some of which are sulfur‐rich. Strain LO‐1 is motile by means of a single bundle of sheathed flagella and exhibits the typical ‘wobbling’ motility and helical swimming (‘flight’) path of the magnetotactic cocci. This study and reports from others suggest that LO‐1‐like organisms are widespread in sediments of freshwater to brackish natural aquatic environments.