Environmental Factors Affect Magnetite Magnetosome Synthesis in Magnetospirillum magneticum AMB-1: Implications for Biologically Controlled Mineralization

Abstract

It is widely believed that magnetotactic bacteria (MTB) form membrane-enveloped magnetite crystals (magnetosomes) under strict genetic control. In this study, the Magnetospirillum magneticum strain AMB-1 was cultured in the same growth medium, but under four different growth conditions: Anaerobic static, aerobic static, aerobic 80-rpm rotating, and aerobic 120-rpm rotating to investigate possible environmental influences on magnetite magnetosome formation. Integrated analyses, using transmission electron microscopy, X-ray diffraction and rock magnetism, indicate that, from the anaerobic static to aerobic 120-rpm rotating culture, the formed magnetite magnetosomes became more equidimensional, smaller in grain size, and higher in crystal twinning frequency. Magnetic properties of magnetite magnetosomes such as coercivity, remanence coercivity, remanence ratio and Verwey transition temperature systematically decreased from 22.0 mT to 5.2 mT, 31.3 mT to 9.3 mT, 0.45 to 0.31, and 108 K to 98 K, respectively. Comparison of additional anaerobic 120-rpm rotating cultures with anaerobic static cultures showed that the effect of rotating, at least up to 120 rpm, on the cell growth and magnetite magnetosome formation is weak and negligible. Given that all samples were prepared and measured in the same way, the changes in physical and magnetic properties indicate that environmental factors (oxygen) affected the biomineralization of magnetite magnetosomes in magnetotactic bacteria, which supports the previous findings. In all experiments, only magnetite with the geometry of truncated octahedron was formed within magnetosomes, which suggests that the mineral phase and crystal habit remain to be genetically controlled. These results also imply the physical and magnetic properties of magnetite magnetosomes may, to some extent, reflect the external growth environments.