The effect of endogeneous soil humic acids on microbial Fe(III) reduction

Humic substances are fraction of soil organic matter which widespread in nature. Humic acids (HAs) are one of the most constituent of humic substances except from fulvic acids and humin. Dissimilatory metal-reducing bacterium, Shewanella oneidensis strain MR-1 utilizes humic acids as electron shuttle for Fe(III) reduction. The presence of HAs humic acids enhances the microbial Fe(III) reduction activities. Reduction of insoluble Fe(III) oxides into soluble Fe(II) increase bioavailability of Fe in soil and could enhance absorption of Fe by plants. Aim of this study is to investigate the effect of endogeneous soil HAs on microbial Fe(III) reduction. HAs were extracted from soil samples collected from 12 locations which comprise of palm oil plantation, paddy field, pineapple plantation, agriculture land and rainforests. The effects of the extracted HAs on Fe(III) reduction activity by S. oneidensis MR-1 were determined by adding the extracted HAs into M1 minimum media supplemented with lactate as the sole electron donor and Fe(III) hydroxyoxides (HFO) as the sole electron acceptor. In the presence of humic acids, microbial Fe(III) reduction significantly increase by functioning as electron shuttle between microorganisms and ferrihydrite anaerobically. FTIR spectrum describe that HAs extracted from 12 locations have similar functional groups but with different strength of bonding. In summary, this study reveal that humic acids is greatly impact on microbial Fe(III) reduction. The mechanism of humic acids as electron shuttle need to be applies in agriculture uses.