Chemical signatures of magnetite produced by iron-metabolizing bacteria

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Magnetite crystals are frequently found in ancient marine sedimentary rocks, such as banded iron formations (BIF) or carbonate platforms, some as old as 3.8 billion years. Magnetite can be produced through abiotic or biological pathways. The activity of iron-metabolizing bacteria has been proposed to be responsible for precipitation of magnetite crystals throughout much of the geological record. However, physical characterizations alone have not provided an unambiguous determination for the biological origin of natural magnetite samples. Therefore, it is imperative to discern the mineralogical and chemical properties of biogenic magnetite, or biomagnetite, from abiotic magnetite. In this regard, we propose to characterize the trace element composition of biomagnetite produced by (1) Fe(III)-reducing and (2) Fe(II)-oxidizing bacteria. The Fe(III)-reducing Shewanella oneidensis strain MR-1 and the Fe(II)-oxidizing Acidovorax sp. strain BoFeN1 will be cultivated in media with well-defined chemical composition. The partitioning of trace elements during biomagnetite syntheses, i.e. the specific pattern of trace element incorporation into the biomagnetite, will then be quantified and compared with abiotic magnetite and the previously studied magnetite crystals produced by magnetotactic bacteria. The goal is to uncover different partitioning patterns between the different conditions of magnetite precipitation, and as such, determine a potential biosignature of bacterial metabolic activity. We further propose to experimentally examine the fate of this potential biosignature during diagenesis to mimic magnetite fossilization. Finally, we will compare our experimental results with the chemical composition of natural magnetite samples collected from Precambrian BIF and carbonates with the ultimate aim being to ascertain the presence of magnetite-producing bacteria in Earths ancient history.