What causes the urban river to look darker? An underestimated source of sulfide production in methanogenic metabolism

The blackening and increased smelling of waterbodies steadily affect urban aquatic ecology. Sulfide is recognized as the key substance responsible for the darkening of urban rivers. However, the pathway of sulfide production and the underling microbial mechanisms in urban rivers are not fully understood. This study executes a comprehensive approach to investigate mechanism of sulfide production within urban river sediments, integrating field survey, laboratory incubations, and metagenomic sequencing. The results reveal that both sulfide concentrations and sulfidogenic activities in darker river sediments are significantly higher than in lighter rivers. Both the sulfate-reducing bacteria (SRB) and methanogenic communities are closely related to the sulfide content in the sediments. The finding that inhibiting SRB enhanced the potential sulfide production rate suggests the importance of methanogen-derived processes as a sulfide source in sediments. Notably, the abundance of methylated thiol coenzyme M methyltransferase genes increased 53-fold upon after the continuous methionine amendment, confirming that methanogen-derived processes, rather than SRB-derived ones, dominated sulfide production when methylated sulfur compounds are abundant. Overall, this study highlights the potential significance of methanogenesis as a hitherto underestimated sulfide source in urban river sediments, providing valuable insights for optimizing strategies to prevent and mitigate the deterioration of urban aquatic ecosystems.