Iron Transport and Magnetite Crystal Formation of the Magnetic Bacterium Magnetospirillum gryphiswaldense

Abstract
Magnetic bacteria are found in various morphologies as cocci, vibrios, spirilli and rods in aquatic mud layers. Magnetite (Fe₃O₄) is stored in phospholipid vesicles as bullet-shaped, hexagonal or cubooctahydral crystals. Size and form of these crystals are species-specific and precisely controlled. The microaerophilic Magnetospirillum gryphiswaldense forms in a near to linear chain up to 60 cubooctahedral, single domain magnetite crystals of 42-45nm diameter, which generates a magnetic dipole. Six proteins were detected in SDS-gels of the special phospholipid vesicles, which envelope the magnetite crystals. These proteins are probably involved in iron transport (FeII or/and FeIII), in nucleation catalysis, in redox or/and pH control. Iron uptake and dynamics of magnetite crystal formation were estimated simultaneously by change of light scattering of the cells within homogeneous magnetic fields. A very efficient, energy-dependent uptake of FeIII in presence of spent, iron-deficient growth medium was found. Iron uptake was tightly coupled to magnetite crystal formation and magnetisation. When FeIII was added to iron-starved cells under inducing conditions, FeIII was immediately transported into the cells and superparamagnetic crystals of less than 20nm were formed first within 30 min.