On the Origin and Evolution of Microbial Mercury Methylation

Area of Science:

• Microbial evolution
• Environmental microbiology
• Biogeochemical cycles

Background:

• The origin of microbial mercury methylation, a key process in the global mercury cycle, remains poorly understood.
• Understanding the evolutionary history of mercury methylation is crucial for predicting mercury's fate in the environment.

Purpose of the Study:

• To decipher the evolutionary history of the mercury-methylating gene (hgcAB) and operon.
• To explain the distribution of mercury methylation genes across Bacteria and Archaea.
• To hypothesize the selective pressures driving the evolution and loss of mercury methylation.

Main Methods:

• Genome-resolved phylogenetic analyses were employed to reconstruct the evolutionary history of hgcAB.
• Comparative genomics was used to investigate the distribution of hgc genes in microbial lineages.
• Evolutionary models were applied to infer the roles of vertical inheritance and horizontal gene transfer.

Main Results:

• Phylogenetic analyses suggest ancient origins for the hgc operon, with evidence of both vertical inheritance and horizontal gene transfer.
• The distribution of hgcAB across diverse microbial taxa was elucidated.
• Hypotheses regarding the selective advantages and disadvantages of mercury methylation were formulated.

Conclusions:

• The evolution of mercury methylation likely provided an antimicrobial advantage in early Earth's resource-limited environments.
• The subsequent evolution of mercury detoxification mechanisms (merB) may have reduced the selective advantage of methylation, leading to widespread gene loss.
• This study provides a framework for understanding the complex evolutionary dynamics of microbial mercury methylation.