The impact of light on microbial iron(II) oxidation in freshwater sediments.
Final Report Abstract
Fe(III) photoreduction is known to be an important source for bioavailable Fe2+ in water bodies such as the ocean. During this research project, we could demonstrate that this process also delivers significant amounts of Fe2+ in upper oxic layers of light-illuminated freshwater and marine sediment. The rate of Fe(III) photoreduction is sufficient to outcompete abiotic Fe(II) oxidation by O2 leading to Fe2+ concentrations in the µM range. Using a combination of different microsensor measurements and spectrophotometric assays, we found that extent and kinetics of light-induced Fe2+ formation highly depends on the kind and availability of organic ligands that can form light-susceptible complexes with Fe(III), such as citrate or certain bacterial siderophores. Both, higher light intensity (photon flux) as well as higher light energy (shorter wavelengths) also increase the efficiency of Fe(III) photoreduction. Most Fe2+ in our experiments formed upon irradiation with energy-rich UV light (350-400 nm) and the threshold for initiating photoreduction of organically complexed Fe(III) was found to be 500 nm. The origin of the complexed Fe(III), however, is of minor importance regarding the formation of photolabile Fe(III)-organic complexes when comparing lab-synthesized ferrihydrite and biogenically produced Fe(III) minerals. In the presence of O2, abiotic Fe(II) oxidation and decreasing Fe2+ concentrations dominate if light energy and availability of organic ligands is limited. As a consequence, H2O2 accumulates in the lower µM range, which can itself abiotically oxidize Fe(II). Photochemically formed Fe2+ can be used as electron donor not only by phototrophic but also microaerophilic Fe(II)-oxidizing bacteria, which are found ubiquitously in freshwater and marine sediment. This was demonstrated by our experiments combining two cultivation approaches using agar-stabilized gradient tubes and liquid culture headspace vials. In summary, our research showed that Fe(III) photoreduction is a so far overlooked source of bioavailable Fe2+ in light illuminated sediments, which changes prevalent redox gradients and fluxes of Fe2+. By the formation of Fe2+ and subsequent consumption by phototrophic or microaerophilic Fe(II)- oxidizing bacteria, Fe(III) photoreduction drives a light-dependent und partly cryptic Fe cycle in freshwater and marine sediment. This highly impacts our view of the classical redox cycling in sediments and adds an important process to our classical view of sedimentary Fe cycling.
Publications
- (2020) Fe(III) photoreduction producing Fe2+aq in oxic freshwater sediment. Environmental Science & Technology 54, 862-869
Lueder, U., Jørgensen, B.B., Kappler, A., Schmidt, C.
(See online at https://doi.org/10.1021/acs.est.9b05682) - (2020) Influence of physical perturbation on Fe(II) supply in coastal marine sediments. Environmental Science and Technology 54, 3209-3218
Lueder, U., Maisch, M., Laufer, K., Jørgensen, B.B., Kappler, A., Schmidt, C.
(See online at https://doi.org/10.1021/acs.est.9b06278) - (2020) Photochemistry of iron in aquatic environments. Environmental Science: Processes & Impacts 22, 12-24
Lueder, U., Jørgensen, B.B., Kappler, A., Schmidt, C.
(See online at https://doi.org/10.1039/C9EM00415G) - (2022) Growth of microaerophilic Fe(II)-oxidizing bacteria using Fe(II) produced by Fe(III) photoreduction. Geobiology
Lueder, U., Maisch, M., Jørgensen, B.B., Druschel, G., Schmidt, C., Kappler, A.
(See online at https://doi.org/10.1111/gbi.12485) - (2022) Influence of light quality, photon flux and presence of oxygen on photoreduction of Fe(III)-organic complexes. Science of the Total Environment 814, 152767
Lueder, U., Jørgensen, B.B., Maisch, M., Schmidt, C., Kappler, A.
(See online at https://doi.org/10.1016/j.scitotenv.2021.152767)
