The oxidation of Fe(II) at circumneutral pH by natural bacterial consortia capable of oxidizing Fe(II) in flocculent mats of bacteriogenic iron oxides (BIOS) was investigated in a series of microcosm experiments under constant pH and pO₂ conditions. The oxidation of Fe(II) was faster in the presence BIOS compared to chemical oxidation in solution with pseudo–first-order rate constants ranging from 0.036 to 0.248 min⁻¹ for BIOS compared to 0.004 min⁻¹ for the chemical control. The Fe(II)-oxidation rates exhibited a kinetic dependency on BIOS concentrations, measured in terms of a pseudo–second-order rate constant at 0.066 Lg⁻¹min⁻¹. From the asymptotic decline in Fe(total) in the microcosms, apparent rate constants for HFO dissolution (zero-order values 0.001 to 0.046 mgL⁻¹min⁻¹) and precipitation (second-order values 0.007 to .297 min⁻¹) were obtained from the BIOS and control microcosms, which yield an apparent equilibrium BIOS solubility of 0.157 mgL⁻¹ (i.e., 2.8 μM). These results reflect the complex interplay of Fe(II) oxidation reactions associated with BIOS including homogenous chemical oxidation in solution, chemical oxidation on HFO surfaces, and bacterially mediated oxidation. Furthermore, BIOS precipitated in response to Fe(II) oxidation exhibit a greater apparent solubility than more stable varieties of HFO. The implication is that the competitive advantage of Fe(II)-oxidizing bacteria over autocatalytic surface chemical Fe(II) oxidation on HFO might be enhanced by a reduction in the total amount of BIOS that is precipitated.

This investigation evaluates spatial relationships between summer (July) groundwater temperatures and Fe(II)/Fe(III) biogeochemical cycling over a five year period in a shallow pristine sand aquifer at Meilleurs Bay near Deep River, Ontario, Canada. A warm subsurface thermal island of 12.5-16.1 °C, compared to background conditions of 10-11 °C, was manifest in contour maps of average groundwater temperature over the study period. The warm zone coincided with an area of convergent groundwater flow, implicating horizontal heat transfer by advective convection as the reason for elevated temperatures. Additionally, high concentrations of dissolved Fe(II) and Fe(III) overlapped the warm thermal island, indicative of increased rates of bacterial Fe(II)-oxidation and Fe(III)-reduction. A depletion in the modal abundance of Fe(II)-bearing minerals, notably amphibole and biotite, inside the area of the warm thermal island was also observed, suggesting enhanced mineral dissolution owing to chemoautotrophic Fe(II)-oxidation coupled to the reduction and fixation of dissolved inorganic carbon as biomass. Throughout the aquifer, redox conditions were poised in terms of Eh and pH close to equilibrium with respect to the Fe(II)/Fe(OH)3 couple, feasibly enabling simultaneous bacterial Fe(II)-oxidation and Fe(III)-reduction with an adequate supply of electron acceptors and donors, respectively. The significance of higher groundwater temperature as a determinant of elevated dissolved Fe(II) and Fe(III) concentrations induced by thermal intensification of microbial biogeochemical activities yielded Pearson product-moment correlations in which temperature alone, as a single independent variable, explains almost 30 to nearly 60 percent of the variation in the measured dissolved Fe(II) and Fe(III) concentrations in the groundwater. These results emphasize the important influence of thermal conditions on biogeochemical processes in aquifers coupled to the development of steep gradients in groundwater quality over short distances in shallow unconfined groundwater systems.

A study was conducted to determine in situ rates of Fe(II) oxidation and Fe(III) precipitation along a 5.0 m reach of a ferruginous groundwater discharge zone under two distinct conditions; (i) the natural state featuring abundant flocculent mats of bacteriogenic iron oxides (BIOS) produced by Fe(II)-oxidizing bacteria, and (ii) after a manual washout of the streambed to remove the microbial mat. Examination of mat samples by differential interference contrast light microscopy revealed tangled meshworks of filamentous Leptothrix sheaths and helical Gallionella stalks intermixed with fine-grained hydrous ferric oxide (HFO) precipitates. The greatest accumulation of BIOS mat was 1.0 m downstream of the groundwater spring. Redox potential (Eh) increased sharply from 200 mV to over 300 mV over the last 2.0 m of the reach. Similarly, dissolved oxygen increased from < 10% saturation to almost 100% saturation over the last 2.0 m of the reach, whereas pH increased from 6.4 to 7.3. Pseudo-first-order rate constants determined on the basis of analytical solutions to sequential partial differential advection-dispersion-reaction equations for the linear Fe(II)→Fe(III)→HFO reaction network yielded in situ Fe(II) oxidation rate constants (k_ox) of 1.70 ± 0.20 min⁻¹ in natural conditions and 0.48 ± 0.14 min⁻¹ after washout. Corresponding Fe(III)-precipitation rates (k_p) before and after washout were 3.45 ± 0.10 min⁻¹ and 0.90 ± 0.01 min⁻¹, respectively. These values for k_ox and k_p are higher than estimates obtained from closed batch microcosm and laboratory experiments, underscoring the crucial dependence of in situ Fe(II) oxidation and Fe(III) precipitation rates on advective and dispersive mass transport. The results also highlight the influence that BIOS microbial mats exert on the reaction kinetics of the multiple heterogeneous reactions contributing not only to Fe(II)/Fe(III) redox transformations in groundwater discharge zones, but also the precipitation of HFO.

A survey of a low-flow wetland drainage channel rich in flocculent bacteriogenic iron oxide (BIOS) deposits was conducted to investigate the in situ kinetic behaviors of bacterial Fe(II) oxidation and its dependence on water flow. On the basis of analytical solutions to the advection-dispersion-reaction (ADR) equation and the channel’s Fe(II) concentration profile, the pseudo-first order rate constant for Fe(II) oxidation ( $k_{\mathit{ox}}$) was 0.78 ± 0.20 min⁻¹. The difference in this estimate from that obtained in a previous study on a lotic groundwater discharge channel with an intact BIOS mat (1.70 ± 0.20 min⁻¹) demonstrates the effects of water flow on Fe(II) oxidation kinetics in circumneutral pH, Fe(II)-rich settings. Importantly, these results also re-emphasize the discrepancies between rates obtained from in situ geochemical data and those of conventional ex situ batch studies in assessing the dependencies of in situ reaction rates on water flow and reactive transport across a range of hydraulic regimes characterizing natural systems.

Although all forms of life are of critical importance biogeochemicalfy, bacteria are especially significant. This uniqueness arises not only from the great diversity of environments populated by bacteria, but also because they perform many biogeochemical transformations that are carried out poorly or not at all by higher organisms. In addition, bacteria exhibit a profound ability to bind substantial quantities of metallic ions. This retention of metals is facilitated by electrostatic interactions with anionic carboxyl or phosphoryl groups in the structural polymers of the cells. The macromolecular constituents in cell walls and external sheaths of bacteria are particularly reactive, so metals tend to concentrate at the cell surface. These cellular structures also tenaciously bind metallic ions during diagenesis and serve as distinct nucleation sites for the formation of authigenic minerals. Evidence of micro-fossils in ancient sedimentary rocks suggests further that bacteria contributed to analogous processes in the past.

Bacterial–mineral aggregates are the products of a tight biogeochemical coupling between microbes and geological media and play an outsized role in governing the composition of natural waters through biogeochemical cycling and mineral formation and dissolution processes. The results of combined batch column settling experiments, volumetric analyses, and microscopic investigations demonstrate that composite bacteriogenic iron oxide aggregates are sensitive to densification in response to hydrodynamic shear, a physical fluid phenomenon that introduces significant alterations to aggregate size and structure, permeability, and settling and transport behaviour. After exposing aggregate suspensions to varying degrees of shear stress, final solids volume fractions decreased by as much as 75% from initial data, while aggregate bulk density saw increases from 999 kg·m^–3 to as much as 1010 kg·m^–3. Inverse modelling of time course data yielded estimates for settling rate constants and initial settling velocities that increased with shear stress application. As well as having implications for aqueous contaminant transport and potential bacterial bioenergetic strategies, these results suggest the preservation potential of microfossils formed from bacterial–mineral aggregates may be significantly reduced with shear-induced alterations, leading to a possible underrepresentation of these microfossils in the sedimentary record and a gap in our understanding of early life on Earth.