Trace Element Partitioning as a Geochemical Biosignature of Biogenic Magnetite Formed by Iron-Reducing Bacteria

Biogenic magnetite nanoparticles produced by microorganisms are ubiquitous in modern environments and are also thought to be abundant in ancient sediments such as banded iron formations and paleosols, given the early emergence of iron-metabolizing microbes. Magnetite can form intracellularly within magnetotactic bacteria (MTB) or extracellularly through dissimilatory iron-reducing bacteria (DIRB). While MTB-derived magnetite exhibits distinctive morphological, chemical, and magnetic biosignatures, identifying DIRB-produced magnetite in ancient sediments remains challenging because its nanometer-sized, aggregated, and often superparamagnetic nature overlaps strongly with abiotic magnetite. In this study, we systematically investigate the behavior of 21 trace elements in biogenic magnetite and abiotic magnetite formed via the transformation of ferrihydrite substrates coprecipitated with different trace-element concentrations. Biogenic magnetite was produced by the DIRB Shewanella oneidensis MR-1, while abiotic magnetite was generated using dissolved Fe²⁺ under comparable conditions. Notably, biogenic magnetite particles were consistently smaller than their abiotic counterparts under same conditions, suggesting that microbial processes impose additional constraints on crystal growth. Additionally, ICP-MS results reveal that most trace elements are preferentially enriched in abiotic magnetite, whereas cobalt (Co) and cadmium (Cd) are consistently enriched in biogenic magnetite, independent of initial trace-element concentrations or washing treatment. In contrast, magnesium (Mg) shows preferential incorporation into abiotic magnetite. The observed differences in trace-element signatures, particularly Co–Cd enrichment in biogenic magnetite and Mg enrichment in abiotic magnetite, provide a promising geochemical indicator for identifying DIRB activity in ancient iron-rich sediments and reconstructing microbial iron cycling in early oceans.