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Abstract

Dissimilatory iron reduction is of quantitative importance during anaerobic degradation of organic matter in flooded rice field soils. To isolate dissimilatory Fe(III)-reducing microorganisms from rice soil, enrichments were carried out with acetate and ferrihydrite. One of these resulted in the isolation of strain FAc12. This organism grew anaerobically in defined mineral medium with acetate as electron donor and with ferric citrate, ferrihydrite, or nitrate as electron acceptor. Strain FAc12 also grew well aerobically in defined mineral medium with acetate, citrate, glucose, or with complex medium. Comparative sequence analysis of its 16S rRNA gene revealed that strain FAc12 is most closely related to the very recently described Anaeromyxobacter dehalogenans within the order Myxococcales. The overall similarity value between the 16S rRNA gene sequences of strain FAc12 and the type strain of A. dehalogenans (2CP-1) is 99.5%. A. dehalogenans has been reported to be the first facultative anaerobic myxobacterium, while all other members of the Myxococcales were known to be strict aerobes. A. dehalogenans is able to grow by chlororespiration and to utilize nitrate as terminal electron acceptor for growth. Cultivation-independent retrieval of 16S rRNA gene sequences revealed that rice roots are also colonized by various members of this novel subgroup. This information and the metabolic capacity of strain FAc12 allows the assumption that these organisms are physiologically adapted to environments characterized by spatial and temporal fluctuations between oxic and anoxic conditions, as is typically the case for flooded rice soil.