Investigation of iron-reducing and iron-oxidizing bacterial communities in the rice rhizosphere of iron-toxic paddy field: a case study in Burkina Faso, West Africa

Iron (Fe) toxicity in rice is one of the serious problems in some paddy fields in West African areas. Microbial community structures involved in the redox cycle of Fe have not been revealed in the Fe-toxic paddy fields. The present study investigated the bacterial community structure and the abundance of Gallionellaceae, Geobacteraceae, and Anaeromyxobacteraceae, as the representative indicator bacteria of Fe oxidizers and reducers, in the bulk and rhizosphere soils and rice roots of a Fe-toxic paddy field in Burkina Faso (BF)in 2017-2019. Thosein a paddy field in Anjo, Japan (AN) were also analyzed for comparison. The amplicon sequencing analysis revealed that the BF rhizosphere soil was characterized by typical anaerobic bacterial groups like Firmicutes and Deltaproteobacteria, including several potential Fe reducers. The relative abundance of Gallionellaceae, lithotrophic Fe oxidizers, in the BF rice roots was significantly lower than that in the AN rice roots. Quantitative PCR analysis showed that the ratios of Gallionellaceae to Geobacteraceae and to Anaeromyxobacteraceae were higher in the rice roots than in the soils irrespective of the fields. However, the ratios of Gallionellaceae to Geobacteraceae were lower in the BF soils and roots than in the AN soils and roots. The ratios of Gallionellaceae to Anaeromyxobacteraceae in the BF soils were also lower than those in the AN soils. These findings indicated the relative predominance of Geobacter- and Anaeromyxobacter-related Fe reducers over Gallionellaceae-related Fe oxidizers in the rice rhizosphere of the BF field, corresponding well to the circumstances of Fe-toxic soil: higher Fe(II) amounts in the soil. Since Fe(II)-oxidizing activity at rice roots is an important factor as a primary defense system against Fe(II) in the soil solution, the ratios of Gallionellaceae to Geobacteraceae and to Anaeromyxobacteraceae may serve as an indicator of potential Fe(II)-oxidizing activity of rice rhizosphere. Further studies focusing on the activity of Fe oxidizers and Fe reducers at rice roots under effective cultivation practices and in various types of Fe-toxic paddy fields will help to promote a better understanding of the Fe-toxic soil circumstances and to establish sustainable rice cultivation in the Fe-toxic soils.