Rice root exudates affect microbial methylmercury production in paddy soils

Area of Science:

• Environmental Science
• Microbiology
• Agricultural Science

Background:

• Microbial methylmercury (MeHg) production in soil-rice systems is a significant health concern, particularly in Asia.
• The precise mechanisms driving MeHg production in paddy soils remain unclear.
• Previous research focused on soil and microbial factors, but the influence of rice cultivation itself was less understood.

Purpose of the Study:

• To investigate the impact of rice cultivation, specifically rice cultivar differences, on microbial MeHg production in soils.
• To explore the role of root exudate organic matter as a potential electron donor for microbial methylators.
• To elucidate the cultivar-specific mechanisms contributing to MeHg accumulation in rice.

Main Methods:

• Pot and batch incubation experiments were conducted using two rice cultivars (Heigu246 and Neiwuyou8015).
• Soil and rice plant tissues (grain, straw, root) were analyzed for MeHg levels.
• Abundances of mercury methylation genes (hgcA) and concentrations of acetate in root exudates were quantified.

Main Results:

• Cultivation of Heigu246 (H-rice) resulted in significantly higher soil MeHg levels (18-49% higher than control, 23-108% higher than N-rice).
• MeHg accumulation in grain, straw, and root was substantially higher in H-rice compared to N-rice (38-81% higher).
• Root exudate organic matter, particularly acetate (211% higher in H-rice exudate), was identified as a key factor promoting MeHg production by increasing hgcA gene copy numbers.

Conclusions:

• Rice root exudate organic matter, especially acetate, is a critical driver of elevated soil MeHg concentrations during rice cultivation.
• Cultivar-specific root exudates influence microbial MeHg production and subsequent bioaccumulation in rice.
• This study provides new insights into the mechanisms of MeHg risk in contaminated soil-rice systems.