Effects of Environmental Growth Conditions on the Composition and Morphology of Bacterial Magnetosome Crystals and on the Subsequent Dissolution and Preservation of Magnetofossils

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Magnetotactic bacteria (MB) are a metabolically and morphologically diverse group of gram-negative prokaryotes in the Domain Bacteria. Cells of MB biomineralize intracellular membrane-bounded crystals of the magnetic minerals magnetite (Fe3O4) and/or greigite (Fe3S4) via a biologically-controlled mineralization process. These structures, called magnetosomes, cause cells to align along geomagnetic field lines as they swim and appear to work in conjunction with aerotaxis in aiding cells in locating and maintaining an optimal position in vertical chemical (e.g. oxygen)/redox gradients. Based on their high cell numbers in natural habitats and the biogeochemical transformations they catalyze, MB are a significant environmental bacterial group that potentially have major roles in the biogeochemical cycling of Fe, S, N, and C. When MB cells die and lyse, their magnetosome crystals are presumably released into the surrounding environment and eventually end up in sediments where they appear to be preserved for some time as putative "magnetofossils". Magnetofossils have been found in a large number of recent marine and lacustrine sediments as well as in some ancient sediments and meteorites (i.e., Mars meteorite ALH84001) where they have also been used as evidence for the past presence of MB, as indicators of life, and as indicators of recent/ancient environmental conditions. The use of these crystals as magnetofossils is based on a number of chemical, crystallographic, and magnetic criteria while their use as environmental indicators is based on a small number physiological experiments performed on only a few species. Little is known about the conditions under which MB synthesize Fe3O4 and under which magnetosome Fe3O4 crystals are dissoluted or transformed to other minerals when they are released into the environment. It is presently clear, however, that high concentrations of O2 inhibit magnetite synthesis in MB and cause the oxidation of magnetite magnetofossils and that Fe3O4 magnetosomes can be synthesized anaerobically by some MB.The major goal of the research outlined in this proposal is to determine whether different morphological types of Fe3O4 crystals can be reliably used either as magnetofossils (evidence for life) or as environmental indicators. To do this, we will: 1) examine the environmental conditions under which a relatively large number of strains of MB that synthesize Fe3O4 of different crystal morphologies; and 2) examine the conditions under which the Fe3O4 crystals are supposedly dissoluted, transformed, and/or undergo reductive diagenesis (e.g., investigate the effect of reducing agents, siderophores etc.). Research results from this work should advance knowledge in Microbiology, Geology, Chemistry, Environmental Sciences, and Astrobiology. They should also advance discovery and learning for the general public who are somewhat familiar with Martian meteorite ALH84001 and the evidence for life on ancient Mars. This is especially important now: recent studies and debates, some viewed by the public, involving meteorites and ancient rocks from Earth show that we need clear unambiguous biomarkers as evidence for life in fossils on Earth and well as in extraterrestrial habitats and materials. In addition, Fe is a key element to life and is often a limiting nutrient particularly in marine systems. Results from this research may reveal previously unknown aspects of Fe cycling (e.g., microbial Fe3O4 reduction and oxidation) in such habitats and the role of specific bacteria in this cycling.