Elucidating Microbial Pathways of Mercury Methylation During Litter Decomposition

Area of Science:

• Environmental Science
• Microbiology
• Biogeochemistry

Background:

• Tree foliage sequesters atmospheric mercury (Hg).
• Senesced leaf litter in aquatic environments releases Hg and can be microbially methylated into toxic methylmercury.
• The specific microbial communities responsible for Hg methylation during litter decomposition are not fully understood.

Purpose of the Study:

• To identify the dominant microbial groups responsible for mercury methylation during leaf litter decomposition.
• To investigate the role of litter quality in Hg methylation processes.

Main Methods:

• A 28-day microbial inhibition experiment was conducted.
• Two contrasting litter species (pine and maple) were used to simulate decomposition conditions.
• Mercury methylation rates were monitored under anoxic conditions.

Main Results:

• Sulfate-reducing bacteria were identified as the primary microbial methylators of Hg in decomposing leaf litter.
• Methanogens showed a minor role in mediating Hg methylation.
• The study confirmed Hg methylation occurs during anoxic litter decomposition.

Conclusions:

• Sulfate-reducing bacteria are key players in mercury methylation in aquatic ecosystems with accumulating leaf litter.
• Wetlands, ponds, and river pools can be significant hotspots for methylmercury production via sulfate-reduction.
• Methanogenesis may contribute to mercury methylation, but to a lesser extent than sulfate-reduction.