Clamworm bioturbation reduces mercury methylation through alteration of methylator composition in sediment

Coastal sediment has been recognized as a hotspot of mercury (Hg) methylation and acts as an important reservoir for Hg-methylating microbes. The bioturbation behaviors of benthic organisms can significantly influence sediment properties and potentially affect the mobility and availability of contaminants within the sediment. However, the effects of bioturbation on Hg speciation and disposition in sediment have not been well addressed. This study investigated the influence of clamworm activities on the Hg-methylation process and the composition of methylators in sediment. The results showed that the presence of clamworms greatly suppressed the growth of Hg-methylators and led to a significant decrease in the production rate of methylmercury (MeHg) (from 0.61 to 0.36 ng g^-1 dw d^-1). Metagenomic results indicate that bioturbation significantly decreased the abundance and diversity of putative Hg methylators and altered the dominant contributors to Hg methylation process. Furthermore, clamworm activities influenced the metabolic traits of Hg methylators and shifted the community toward greater oxygen tolerance. Overall, bioturbation by clamworms suppressed the Hg methylation process and increased the abundance of eco-friendly microbiome, which ultimately contributed to making the sedimentary ecosystem more diverse and resilient. These findings highlight the vital role of bioturbation in mitigating MeHg contamination in sediment and provide a deeper understanding of Hg-methylating microbes and the Hg cycling processes in coastal environments.