Mercury methylation and hydrogen sulfide production among unexpected strains isolated from periphyton of two macrophytes of the Amazon.

The periphyton of macrophytes had previously been identified as important spots for mercury methylation in the Amazon basin, but the microorganisms that facilitate methylation in such compartment are still to be identified. Here, bacteria were isolated from periphyton associated with Eichhornia crassipes and Polygonum densiflorum in Widdel and Pfennig medium and tested for mercury methylation with a stable isotope tracer technique using (198)HgCl, hydrogen sulfide production and molybdate inhibition. Three Pleomorphomona spp., one unidentified Deltaproteobacteria, two Klebsiella spp., and one Tolumonas sp. were isolated. All except Tolumonas sp. were able to methylate mercury (up to 5% of the (198)HgCl added) and produce up to 4 mM of H(2)S, while the Deltaproteobacteria was also able to demethylate methylmercury. Although these bacteria may not be as strong mercury methylators as sulfate-reducing bacteria, they have the potential to contribute to methylmercury accumulation in the system.