Effect of riboflavin on active bacterial communities and arsenic-respiring gene and bacteria in arsenic-contaminated paddy soil

Riboflavin (RF) is a widely distributed micronutrient secreted by many bacteria and can serve as a redox mediator to facilitate extracellular electron transfer. However, studies on the effect of RF on the dynamic shift of active bacterial communities and transcription of arsenate [As(V)]-respiring genes and associated bacteria in arsenic-contaminated paddy fields remain rare. In this study, anaerobic microcosms were established with arsenic-contaminated paddy soil amended with RF and lactate. Arsenic and iron transformation showed that microbial As(V) and Fe(III) reduction took place principally within the first 12 days of incubation, when both occurred simultaneously and were substantially enhanced by RF. Dynamics of the active bacterial community (16S rRNA gene based) revealed that RF mainly increased the relative abundance of Geobacter, Clostridium, and Pelobacteraceae on days 0-12 and that of Dechloromonas on days 12-40, whereas RF had no influence on bacterial diversity. A transcript clone library of arrA gene was constructed to profile the dynamic shift of As(V)-respiring bacteria stimulated by RF, which indicated that As(V)-respiring Geobacter spp. dominated in the RF and lactate microcosms during the incubation period. Quantitative reverse transcription-PCR (RT-qPCR) analyses revealed that the transcription of arrA gene and Geobacter spp. predominantly increased during the first 5 days and was substantially enhanced by RF relative to microcosms with lactate alone. Co-occurrence network analysis indicated a positive correlation between the transcript abundance of arrA gene/Geobacter spp. and dissolved As(V) in the presence of RF and lactate. Our results demonstrate that RF stimulates the activity of indigenous dominant metal-reducing bacteria; moreover, it increases transcription of the As(V)-respiring gene and bacteria to facilitate arsenic reduction and release in arsenic-contaminated paddy soil.