Formation of Biogenic Iron-Oxides (BIOS) and their susceptibility to microbial iron reduction and fermentation under anoxic conditions

• Applicants: Professor Dr. Andreas Kappler; Muammar Mansor, Ph.D.
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 494840258

Biogenic iron oxides (BIOS) are iron-carbon (Fe-C) rich aggregates that are produced by Fe(II)-oxidizing bacteria (FeOB) in suboxic-anoxic environments such as freshwater and marine sediments, paddy soils, aquifers and ancient anoxic oceans. The rate and extent of BIOS degradation by anaerobic microbes is an important constraint on Fe-C transformation in anoxic environments, leading to the release of dissolved Fe2+ and dissolved and gaseous C (e.g., CO2, CH4). BIOS however can be very diverse in terms of composition, structure and reactivity, and it is not well-understood how (a) this diversity is influenced by the identity of FeOB and their growth conditions, (b) this diversity then affects the subsequent rates and extents of microbial Fe(III) reduction and C transformation. In particular, the fractions of BIOS that exists as nanoparticles and colloids (<1,000 nm) are poorly-constrained even though they are likely to be the most reactive fractions. To this end, we developed a research strategy focused on laboratory work using microbial systems. In Work Package (WP) 1, BIOS will be formed by FeOB with known differences in Fe(II) oxidation mechanisms: Rhodopseudomonas palustris TIE-1 (photoautotrophic), enrichment culture KS (a nitrate-reducing consortium) and Acidovorax sp. BoFeN1 (chemodenitrifier). R. palustris TIE-1 will additionally be grown at different Fe2+ concentrations and light flux to determine how environmental parameters may affect the formed BIOS. A suite of techniques will be used to characterize the properties of BIOS: mineralogy (XRD and Mössbauer spectroscopy), organic C amount and functional groups (TOC analyzer and FTIR), Fe redox speciation (ferrozine), and structure and number of cell-mineral aggregates and colloidal Fe-C aggregates (size-fractionation filtration with 5, 0.45 and 0.03 µm filters coupled to single particle ICP-MS with dynamic light scattering and cryo-FIB-SEM). In WP2, the bioavailability of BIOS-associated Fe(III) minerals will be tested by incubation with Fe(III)-reducing bacteria (Shewanella oneidensis MR-1 and Geobacter sulfurreducens) with known differences in their Fe(III) reduction mechanisms (e.g., electron shuttling vs direct cell-mineral contact). In WP3, the bioavailability of BIOS-associated organic C will be tested by incubation with a fermentative consortium enriched from lake sediments. Geochemical analyses (Fe2+, cell numbers, ATP, DOC, volatile fatty acids, gas products) will be performed over time to track Fe and C transformation. In WP4, we will apply multivariate analysis to distinguish the most important factors that affect Fe and C transformation. We expect that nano and colloidal Fe-C aggregates will be the most bioavailable and thus their abundances to be the primary factors in driving Fe-C transformation of BIOS in anoxic environments.

• DFG Programme: Research Grants