Distinguishing Biotic and Abiotic Iron Oxidation at Low Temperatures

Abstract

The rates of microbial and abiotic iron oxidation were determined in a variety of cold (T = 9–12 °C), circumneutral (pH = 5.5–9.0) environments in the Swiss Alps. These habitats include iron–bicarbonate springs, iron–arsenic–bicarbonate springs, and alpine lakes. Rates of microbial iron oxidation were measured up to a pH of 7.4, with only abiotic processes detected at higher pH values. Iron oxidizing bacteria (FeOB) were responsible for 39–89% of the net oxidation rate at locations where biological iron oxidation was detected. Members of putative iron oxidizing genera, especially Gallionella, are abundant in systems where biological iron oxidation was measured. Geochemical sampling suites accompanying each experiment include field data (temperature, pH, conductivity, dissolved oxygen, and redox sensitive solutes), solute concentrations, and sediment composition. Dissolved inorganic carbon concentrations indicate that bicarbonate and carbonate are typically the most abundant anions in these systems. Speciation calculations reveal that ferrous iron typically exists as FeCO3(aq), FeHCO3 +, FeSO4(aq), or Fe2+ in these systems. The abundance of ferrous carbonate and bicarbonate species appears to lead to a dramatic increase in the abiotic rate of reaction compared to the rate expected from chemical oxidation in dilute solution. This approach, integrating geochemistry, rates, and community composition, reveals locations and geochemical conditions that permit microbial iron oxidation and locations where the abiotic rate is too fast for the biotic process to compete.

Microaerophilic Oxidation of Fe(II) Coupled with Simultaneous Carbon Fixation and As(III) Oxidation and Sequestration in Karstic Paddy Soil

“…12 In natural environments, these microorganisms have been shown to actively contribute to net Fe(II) oxidation by up to 89%. 14 This was similar with the maximum biological Fe(II) oxidation of 88% in laboratory-controlled setups under micro-oxic conditions (9-50 μM O2). 12 These reports confirmed the activity and importance of microbial Fe(II) oxidation under micro-oxic conditions.…”

Growth rates for freshwater ferromanganese concretions indicate regional climate change in eastern Canada at the Northgrippian-Meghalayan boundary

“…As aqueous Fe(II) and Mn(II) are transported to the sediment-water interface, Fe(II) is expected to oxidize rapidly by chemical or biologically-mediated mechanisms. The biological mechanism is active when the temperature is above 10 to 15°C ( Davison and Seed, 1983 ; Martin, 2005 ; Millero et al, 1987 ; St Clair et al, 2019 ) and chemical Fe oxidation will dominate at lower temperatures. In contrast, chemical oxidation of Mn(II) is kinetically inhibited at pH below 8 ( Martin, 2005 ) and present-day pH values for these New Brunswick lakes are <7.5 ( Government of New Brunswick, 2020 ).…”

Agent-based modelling of iron cycling bacteria provides a framework for testing alternative environmental conditions and modes of action

“…This shows how a low pH drives the reaction towards Fe 2+ -oxidation. However, at high pH ferrous ions become less available as they are hydrated into other speciations which are more susceptible to abiotic oxidation, for example, ferrous hydroxide [59]:Fe2++2H2normalO→normalFe(normalOH)2+2H+.…”