Dynamics of the abundance and structure of metabolically active Clostridium community in response to glucose additions in flooded paddy soils: closely correlated with hydrogen production and Fe(III) reduction

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Microbial Fe (III) reduction is an important biogeochemical process in anaerobic environments, and it can be regulated by adding organic carbon. Many researchers have studied the unique mechanisms and communities of respiratory Fe (III) reducers. However, the contributions and communities of fermentative microbes have rarely been studied. This study aimed to investigate the response of the metabolically active Clostridium community to glucose additions and to assess its contribution to microbial Fe (III) reduction in flooded paddy soils. Paddy soils were amended with three different concentrations of glucose (0, 0.1, and 0.5 mol C kg−1 dry soil) and incubated anaerobically for 40 days. A combination of reverse transcription quantitative PCR (RT-qPCR) and denaturing gradient gel electrophoresis (DGGE) based on reverse transcription PCR (RT-PCR) approaches were used to measure the abundance and structure of the metabolically active Clostridium community. Concurrently, the dynamics of soil pH, hydrogen partial pressure (ppH2), and Fe (II) concentration were investigated to assess their relationship with the metabolically active Clostridium community. Increased glucose concentrations led to a sharp increase in ppH2 and a rapid decline in soil pH. The addition of glucose at a low concentration significantly increased microbial Fe (III) reduction, while a high glucose concentration enhanced the later Fe (II) accumulation. Meanwhile, the glucose additions greatly stimulated the growth of Clostridium and led to a higher Clostridium relative abundance. In addition, this process resulted in significantly differences in the structure of the metabolically active Clostridium community. Most of the metabolically active Clostridium spp., especially in the glucose addition treatments, were closely related to Fe (III)-reducing and H2-producing bacteria. The metabolically active Clostridium community was closely correlated with hydrogen production and microbial Fe (III) reduction in flooded paddy soils amended with glucose. These results reveal that glucose-stimulated Clostridium is responsible for hydrogen production in flooded paddy soils. Moreover, Clostridium spp., as representative of fermentative microbes, play an important role in microbial Fe (III) reduction in flooded paddy soils, with the organic carbon content being the main driving force.