Botanical iron minerals: correlation between nanocrystal structure and modes of biological self-assembly

Plants, like animals, use and store iron in their cells. Yet, the composition and structure of the plant-iron biominerals, constituting the inorganic cores of phytoferritin, have remained unknown. Transmission electron microscopy (TEM) and diffraction studies of subcellular phytoferritin, extracted from disrupted plant cells, indicate that phytoferritin occurs as crystalline magnetite (Fe3O4), epsilon -Fe2O3 and hematite (alpha -Fe2O3), with typical sizes of single crystallites in the I - 50 nm range and agglomerate grain sizes up to 4 mum. The three dimensional agglomerates are built with different biomineral nanocrystals in three distinct modes of biological self-assembly: 1) ordered magnetite; 2) semi-ordered mixture of magnetite and epsilon -Fe2O3; and 3) random hematite. These self-assemblies correspond to prior TEM reports of crystalline, paracrystalline and amorphous phytoferritin arrangements in sectioned cell samples. A fourth plant-iron biomineral, tentatively assigned as calcium ferrate hexahydrate, has a morphology and diffraction patterns distinct from the phytoferritin aggregates. We do not attribute the plant iron observed in this study to be the results of atmospheric pollution.