Revealing the underestimated role of Gram-positive bacteria in iron reduction within paddy soils

Iron reduction in paddy fields is crucial for heavy metal migration and nutrient transformation. Gram-negative iron-reducing bacteria like Geobacter and Shewanella have been widely studied due to their strong extracellular electron transfer capabilities. In contrast, Gram-positive bacteria such as Bacillus and Clostridium are often labeled as weak exoelectrogens, leading to their contributions being overlooked. But could their role in iron reduction have been underestimated? To explore this, we enriched paddy soils with glucose, resulting in a predominance of Gram-positive bacteria (e.g., Clostridiaceae and Bacillaceae) and a small presence of Gram-negative reducers (e.g., Geobacteraceae). Remarkably, despite the abundance of Gram-positive iron reducers being 880 times higher in the glucose system, their iron reduction rate was comparable to that of acetate and lactate systems with more Gram-negative bacteria. This suggests a significant contribution by Gram-positive bacteria to iron reduction. Pure culture experiments further revealed that the iron reduction rates of Bacillus and Clostridium are 15.4 and 4.0 times slower, respectively, than a Gram-negative reducer, Geobacter. However, even in natural paddy soil of this study, Gram-positive iron reducers are 9 times more abundant than Gram-negative ones. This gives us an unexpected insight: Considering both abundance and rate, Gram-positive weak exoelectrogens may actually contribute significantly to iron reduction. This study reveals the underestimated role of Gram-positive bacteria in paddy soils, highlighting the unique characteristics of paddy habitats and the need for further research on these weak exoelectrogens.