Abstract
Methylmercury (MeHg) in aquatic systems poses a serious public health risk through bioaccumulation in the aquatic food web. In recent years, MeHg has been observed to increase to concerning levels globally in rivers near cities; however, the causes of this increase are not well understood. Here, we demonstrate the significant role of sewage contamination by analyzing over 1,300 publicly available metagenomes in urban rivers worldwide, and conducting experiments with water samples across China. We find that sewage contamination significantly increases the abundance of mercury (Hg)-methylating microorganisms in urban rivers globally. This increase is primarily attributed to the high abundance of active Hg-methylating microorganisms in sewage, which migrate to rivers via direct discharge or combined sewer overflows (CSOs), becoming key contributors to elevated riverine MeHg levels. Our findings underscore the importance of effectively eliminating Hg-methylating microorganisms from sewage to mitigate the public health risks associated with MeHg in urban rivers.