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Since Mn, Fe and As contaminants often coexist in the environment, we hypothesize that the presence of multifunctional bacteria is capable of reducing Mn and Fe oxides and promoting the mobilization and release of arsenic. However, such bacteria have not been reported yet; moreover, the impact of bacteria with the ability to simultaneously reduce Mn and Fe oxides on the formation of high-arsenic groundwater remains unclear. This study aims to address this question. Here, we found that the microbial community in the soils was able to efficiently reduce Mn oxides into Mn(II). An analysis of the microbial community structures of the soil shows that it contained Proteobacteria (41.1%), Acidobacteria (10.9%), Actinobacteria (9.5%) and other less abundant bacteria. Based on this observation, we successfully isolated a novel bacterium Cellulomonas sp. CM1, which possesses both Mn- and Fe-oxide-reducing activities. Under anaerobic conditions, strain CM1 can reduce Mn oxides, resulting in the production of 13 mg/L of Mn(II) within a span of 10 days. Simultaneously, it can reduce Fe oxides, leading to the generation of 9 mg/L of Fe(II) within 9 days when a yeast extract is used as an electron donor. During these reduction reactions, the cells were grown into a density of OD600 0.16 and 0.09, respectively, suggesting that Mn(IV) is more beneficial for the bacterial growth than Fe(III). Arsenic release assays indicate that after 108 days of anoxic incubation, approximately 126.2, 103.2 and 81.5 μg/L As(V) were mobilized and released from three soil samples, respectively, suggesting that CM1 plays significant roles in driving mobilization of arsenic from soils. These findings shed new light on the microbial processes that lead to the generation of arsenic-contaminated groundwater.