Magnetite from magnetotactic bacteria: Size distributions and twinning

We studied intracellular magnetite particles produced by several morphologi cal types of magnetotactic bacteria including the spirillar (helical) fresh water species, Magnetospirillum magnetotacticum, and four incompletely char acterized marine strains: MV-1, a curved rod-shaped bacterium; MC-1 and MC- 2, two coccoid (spherical) microorganisms; and MV-4, a spirillum. Particle morphologies, size distributions, and structural features were examined usi ng conventional and high-resolution transmission electron microscopy. The v arious strains produce crystals with characteristic shapes. All habits can be derived from various combinations of the isometric {111}, {110}, and {10 0} forms. We compared the size and shape distributions of crystals from mag netotactic bacteria with those of synthetic magnetite grains of similar siz e and found the biogenic and synthetic distributions to be statistically di stinguishable. In particular, the size distributions of the bacterial magne tite crystals are narrower and have a distribution asymmetry that is the op posite of the nonbiogenic sample. The only deviation from ideal structure i n the bacterial magnetite seems to be the occurrence of spinel-law twins. S parse multiple twins were also observed. Because the synthetic magnetite cr ystals contain twins similar to those in bacteria, in the absence of charac teristic chains of crystals, only the size and shape distributions seem to be useful for distinguishing bacterial from nonbiogenic magnetite.