Mercury methylation and the microbial consortium in periphyton of tropical macrophytes: effect of different inhibitors.

Macrophyte-associated periphyton is known as a site of Hg accumulation and methylation in tropical environments. Sulfate-reducing bacteria (SRB) is found in periphyton and its role in Hg methylation is acknowledged. However, the contribution of other microorganisms to this process is largely unknown. We tested the effect of inhibitors for different microorganisms on methylmercury (MMHg) formation on distinct macrophyte species from lakes of the Bolivian Amazon basin and in Brazil. We also tested the effect of inhibitors on bacterial secondary activity at two lakes in Brazil. Samples were incubated on-site with (203)Hg and Me(203)Hg was extracted and measured by liquid scintillation. MMHg formation on macrophytes varied among species ranging from 0.2% to 36%. Treatments with specific inhibitors resulted in reduction of MMHg production on most sites and inhibitors. The most successful treatment was the co-inhibition of SRB and methanogens. The inhibitions of algae and fungi activity showed fewer effects on methylation rates at all sites analyzed. Bacterial secondary activity was slightly affected by algae and fungi inhibition, and largely influenced by prokaryotic, SRB and methanogens inhibition. The data suggest that MMHg formation may not be directly performed by all microorganisms in periphyton but depends on complex interactions among them.