BACMAG · Biological and physico-chemical control of intracellular magnetite synthesis in magnetotactic bacteria: an interdisciplinary approach

Final Activity Report Summary - BACMAG (Biological and physico-chemical control of intracellular magnetite synthesis in magnetotactic bacteria: an interdisciplinary approach)

Magnetotactic bacteria (MTB) are microorganisms that have the ability to navigate along geomagnetic field lines owing to the presence of magnetosomes, which are intracellular organelles comprising membrane-enveloped crystals of a magnetic material, as mentioned by Bazylinski and Frankel in 2004. The unique crystalline and magnetic properties of magnetosomes have brought them into the focus of multidisciplinary interest as they are used in biotechnological applications (Lang et al., 2007), or as biomarkers for life on Mars (McKay et al., 1996). Under microoxic conditions the magnetotactic bacterium magnetospirillum gryphiswaldense biomineralises up to 100 cubooctahedral magnetite (Fe3O4) crystals per cell, which is accompanied by the intracellular accumulation of tremendous amounts of iron, ranging up to 4 % of the dry weight as noted by Schüler and Baeuerlein in 1998. This indicates that MTB use very efficient systems for uptake, transport and precipitation of iron, which, however, remain poorly understood. On the basis of Mössbauer spectroscopic and biochemical analyses, we were able to propose a mechanism for magnetite formation, in which iron required for magnetite biomineralisation was processed directly, throughout cell membranes, to the magnetosome membrane without iron flux via the cytoplasm, suggesting that pathways for magnetite formation and biochemical iron uptake were distinct. Magnetite formation occurred via membrane-associated crystallites, whereas the final step of magnetite crystal growth was possibly spatially separated from the cytoplasmic membrane. Moreover, by inducing magnetite nucleation and growth in resting, iron-starved cells of magnetospirillum gryphiswaldense we followed the dynamics of magnetosome development. By studying the properties of the crystals at several steps of maturity we observed that freshly induced particles lacked a well-defined morphology. More surprisingly, even though the mean particle size of mature magnetosomes was similar to that of magnetosomes formed by constantly growing and iron-supplemented bacteria, we found that other physical properties, such as crystal size distribution, aspect ratio and morphology significantly differed. Through correlation of these results with measurements of iron uptake rates we suggested that the expression of different faces of the crystals was favoured for different growth conditions. These results implied that the biological control over magnetite biomineralisation by magnetotactic bacteria could be disturbed by environmental parameters. More specifically, the morphology of magnetite crystals was not exclusively determined by biological intervention through vectorial regulation at the organic boundaries or by molecular interaction with the magnetosome membrane, but also by the rates of iron uptake. This insight might contribute to improved definition of biomarkers as well as to improved understanding of biomineralising systems. Another objective of the proposal was to develop an alternative to the bacterial production of magnetosomes via biomimetic approaches, in order to obtain significant quantities of high quality magnetic nanoparticles with application in biotechnologies and nanotechnologies. The biomimetic approach aimed at mimicking the bacterial biomineralisation pathway in vitro. In MTB, a number of magnetosome proteins with putative functions in the biomineralisation of the nanoparticles were identified using genetic and biochemical approaches. The initial results indicated that some of these proteins had an impact on nanomagnetite properties in vitro; however, the clear specificity of those proteins remained to be determined.

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