Sulfate-Reducing Bacteria in Floating Macrophyte Rhizospheres from an Amazonian Floodplain Lake in Bolivia and Their Association with Hg Methylation

Abstract

ABSTRACT Five subgroups of sulfate-reducing bacteria (SRB) were detected by PCR in three macrophyte rhizospheres ( Polygonum densiflorum , Hymenachne donacifolia , and Ludwigia helminthorriza ) and three subgroups in Eichhornia crassipes from La Granja, a floodplain lake from the upper Madeira basin. The SRB community varied according to the macrophyte species but with different degrees of association with their roots. The rhizosphere of the C 4 plant Polygonum densiflorum had higher frequencies of SRB subgroups as well as higher mercury methylation potentials (27.5 to 36.1%) and carbon (16.06 ± 5.40%), nitrogen (2.03 ± 0.64%), Hg (94.50 ± 6.86 ng Hg g −1 ), and methylmercury (8.25 ± 1.45 ng Hg g −1 ) contents than the rhizosphere of the C 3 plant Eichhornia crassipes . Mercury methylation in Polygonum densiflorum and Eichhornia crassipes was reduced when SRB metabolism was inhibited by sodium molybdate.