S12: Mercury Methylation: Microbial Species and Communities Involved in Hg Transformations (ORNL Hg SFA, Microbial Genetics and Transformations)

The discovery and verification of the mercury methylation genes, hgcAB, has been a seminal discovery by ORNL. This discovery now opens up new avenues of research. We have examined microbial communities involved in mercury methylation in streams receiving Hg from the Y12 plant at Oak Ridge. Microcosm studies suggested active methylation downstream, and was stimulated by the addition of sulfate or the carbon sources lactate, ethanol, methanol, or acetate. Both molybdate and cellobiose inhibited methylation activity. Sediments were analyzed for the microbial community phylogenetically via 454 pyrosequencing of the 16S rDNA gene V4 region. As expected, cellobiose stimulated the Firmicute population which has not been traditionally recognized in methylation while the delta-Proteobacteria decreased, which has been repeatedly implicated in Hg-methylation. However, our recent discovery of the methylation genes led us to predict that Firmicutes and methanogens are capable of this activity. Further inspection of the 16s rRNA sequences revealed that the stimulated Firmicutes did not possess the hgcAB genes and therefore explains the decreased methylation activity. However, analysis did show that the relative abundance of methanogens that possess the hgcAB genes was increased. This suggests that, although the latter population increased in abundance, their methylation activity is low. This was in fact the case when our predictions were tested and revealed that all predictions for methylators and non-methylators were accurate, but that the methylating rate and yield for the Firmicutes and methanogens was far below that of the delta-Proteobacteria.