Geochemical patterns and microbial contribution to iron plaque formation in the rice plant (Oryza sativa) rhizosphere

• Applicants: Professor Dr. Andreas Kappler; Dr. Caroline Schmidt
• Subject Area: Soil Sciences
• Term: from 2015 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 271022541

Rice is the major food source for more than half of the world population and 80 percent of the worldwide rice cultivation is performed on water logged paddy soils. The establishment of reducing conditions in the soil and across the soil-water interface not only stimulates the microbial production and release of the greenhouse gas methane. These settings also create optimal conditions for microbial iron(III) reduction and therefore saturate the system with reduced ferrous iron. Through the reduction and dissolution of ferric minerals that are characterized by their high surface activity, sorbed nutrients and contaminants (e.g. arsenic in South East Asia) will be mobilized and are thus available for uptake by plants. Rice plants have evolved a strategy to release oxygen from their roots in order to prevent iron toxification in highly ferrous environments. The release of oxygen to the reduced paddy soil causes ferric iron plaque formation on the rice roots and finally increases the sorption capacity for toxic metals. To this date the geochemical and microbiological processes that control the formation of iron plaque are not deciphered. It has been hypothesized that iron(II)-oxidizing bacteria play a potential role in the iron(III) mineral formation along the roots. However, their spatial distribution within the rhizosphere, their abundance pattern as a function of plant growth and their potential to control the local iron redox cycling has never been investigated in detail. The goal of the proposed study is to evaluate the contribution of Fe(II)-oxidizing and thus Fe(III) mineral precipitating bacteria to iron plaque formation on rice roots. To this end, we first intend to map and correlate high resolution (bio)geochemical and iron-mineralogical patterns in the rice plant rhizosphere as a function of rice growth. In a second step, iron(II)-oxidizing and iron(III)-reducing populations will be quantified throughout the established redox gradients at defined vegetative stages of the plant. Thirdly, iron(II) oxidation and iron(III) reduction rates of isolated bacteria from native paddy soil sampled in Vercelli (Italy), will be quantified in growth experiments. And finally, the microbial contribution to iron plaque formation will be evaluated by an experimental series in which the rice plants are cultivated in a sterile geochemically controlled set-up that has been spiked with isolated microaerophilic iron(II)-oxidizing bacteria. The combined dataset will provide insights on the spatially and temporally varying contribution, as well as on the variation of microbial ferric mineral production and dissolution in the rice plant rhizosphere. The acquired knowledge will provide an excellent basis research on contaminant (im)mobilization through iron plaque formation and the potential to apply rice cultivation as food delivering and soil remediation action in parallel.

• DFG Programme: Research Grants