The exacerbation of mercury methylation by Geobacter sulfurreducens PCA in a freshwater algae-bacteria symbiotic system throughout the lifetime of algae

Area of Science:

• Environmental Science
• Microbiology
• Environmental Chemistry

Background:

• Mercury (Hg) pollution, particularly methylmercury (MeHg) formation, is a significant environmental concern.
• Previous research on Hg methylation primarily focused on aquatic sediments, with limited investigation into algal-bacterial interactions in water layers.

Purpose of the Study:

• To investigate the dynamic effect of the alga Chlorella pyrenoidosa on mercury methylation throughout its life cycle.
• To understand the role of algal organic matter and thiols in exacerbating MeHg production.
• To elucidate the mechanism of Hg methylation in freshwater algae-bacteria symbiotic systems.

Main Methods:

• Culturing Chlorella pyrenoidosa under varying Hg(II) concentrations (1-20 μg/L).
• Co-culturing algae with the methylating bacterium Geobacter sulfurreducens PCA.
• Quantifying MeHg production rates at different algal cell densities and growth stages.
• Analyzing algal organic matter and thiol content.

Main Results:

• Chlorella pyrenoidosa is a non-methylating alga tolerant to Hg stress.
• The presence of live C. pyrenoidosa significantly enhanced Hg methylation by G. sulfurreducens PCA by 62.3-188.3%.
• Increased algal organic matter and thiols correlated with exacerbated MeHg production.
• Dead algae also enhanced methylation, but to a lesser extent (~62.4% lower) than live algae.

Conclusions:

• Freshwater algae can significantly promote mercury methylation by bacteria, contributing to increased methylmercury levels.
• Algal organic matter and thiols play a crucial role in enhancing the methylation process.
• The findings highlight the importance of considering algae-bacteria interactions in understanding mercury cycling in aquatic environments.