Mechanism of microbial humic substance electron shuttling to Fe(III) minerals
Final Report Abstract
Fe(III) (oxyhydr)oxides are crucial mineral phases influencing various biogeochemical cycles (C, N, S, etc.) in soils and sediments. Under anoxic conditions, dissimilatory Fe(IN)-reducing bacteria (FeRB) can use Fe(III) minerals as terminal electron acceptor for respiration. The microbial reduction of Fe(III) minerals can result in the dissolution of Fe(III) minerals or in the formation of secondary minerals, therefore further affecting the environmental fate of nutrients, toxic metals, radionuclides and organic contaminants. Over decades, chemically extracted humic substances (HS) have been used in laboratory studies to evaluate the impact of soil organic matter (SOM) as electron shuttles on the rate and extent of microbial Fe(III) reduction. However, our results clearly show that the sodium hydroxide (NaOH) solution with pH 12 applied for the HS extraction directly leaded to the presence of more redox-active functional groups in these chemically-extracted organic molecules compared to water-extracted SOM at pH 7. Therefore, when using HS to represent SOM, the redox activity and potentially also the stimulation effects of SOM on the microbial Fe(III) reduction might be overestimated. Furthermore, although it was already shown that anthraquinone-2,6-disulfonate (AQDS), a model quinone compound, can act as electron shuttle and enable microbial Fe(III) mineral reduction over µm distance, our study demonstrated for the first time that, with AQDS, PPHA or SRNOM as electron shuttle, ferrihydrite or goethite reduction can happen with Shewanella oneidensis MR-1 and Geobacter sulfurreducens over cm distances. Based on our results, we suggest that the mechanism for the cm-long distance electron shuttle is the combination of diffusion of AQDS or OM molecules over small distances from nm to µm with electron hopping from one molecule to another when the distance between these two molecules is less than 2 nm. The higher the concentration of the AQDS/OM, the more important electron hopping becomes compared to diffusion. Since Fe(III) minerals are usually present as poorly soluble phases in the environment, this study helps to improve our understanding about whether and to which extent microbial Fe(III) mineral reduction can happen in soils with abundant OM thus further allowing us to better predict the fate of nutrients, heavy metals and contaminates.
Publications
- (2020) AQDS and Redox-Active NOM Enables Microbial Fe(III)-Mineral Reduction at cm-Scales. Environmental science & technology 54 (7) 4131–4139
Bai, Yuge; Mellage, Adrian; Cirpka, Olaf A.; Sun, Tianran; Angenent, Largus T.; Haderlein, Stefan B.; Kappler, Andreas
(See online at https://doi.org/10.1021/acs.est.9b07134) - (2020) High-pH and anoxic conditions during soil organic matter extraction increases its electron-exchange capacity and ability to stimulate microbial Fe (III) reduction by electron shuttling. Biogeosciences 17, 683-698
Bai, Yuge., Subdiaga, E., Haderlein, S.B.., Knicker, H., Kappler, A.
(See online at https://doi.org/10.5194/bg-17-683-2020)
