Microbial diversity regulates mercury cycling in paddy soils.

Microbial mercury (Hg) methylation and methylmercury (MeHg) demethylation critically govern MeHg production in paddy soils and its accumulation in rice. However, how the decline in microbial diversity under climate change affects these processes remains unclear. Here, we combined stable isotope tracing with a dilution-to-extinction approach to manipulate microbial diversity across paddy soils with Hg contamination gradient (HX: 165 ng/g, GX: 20,707 ng/g and SK: 659,303 ng/g), investigating its effects on Hg methylation and demethylation. Results showed that diversity loss suppressed methylation (to 0.31-0.82 times the original soil) while enhanced demethylation (to 1.33-7.00 times the original soil) in HX and GX soils, reducing net MeHg production. Conversely, in SK soil, it promoted methylation (to 0.31-0.71 times the original soil) and inhibited demethylation (0.14-0.48 times the original soil), increasing MeHg accumulation. Marginal density curves and linear regression analyses indicate that the regulatory effect of microbial diversity on MeHg production depends strongly on Hg levels, with a significant shift in MeHg concentration across a critical threshold of 30,000 ng/g. Nationwide expanded data further confirmed that diversity loss deceases MeHg production below this threshold but elevates it above. This divergence is attributed to Hg-induced shifts in microbial community structure induced. Our findings highlight the crucial role of microbial diversity in regulating net MeHg production in paddy soils, offering important insights for predicting Hg risks under climate change and ensuring food security.