Biotically mediated mercury methylation in the soils and sediments of Nam Co Lake, Tibetan Plateau.

Previous research found that methylmercury (MeHg) levels in the fish of Nam Co Lake of Tibetan Plateau were remarkably higher than those obtained from Hg-polluted areas, probably indicating a stronger biomagnification and higher MeHg transfer efficient. Until now, little research has been carried out on the distribution of Hg methylators here, which maybe important to explain the higher fish MeHg levels. MeHg concentrations were remarkably different between the soil and sediment samples in both seasons. Illumina MiSeq sequencing of 16S rRNA gene amplicons showed that species richness estimates of the soil and sediment samples were both quite low based on Ace and Chao estimators. Community composition differed between the sediment and bank soil samples. DsrB gene quantities were relatively high, but the hgcA quantities were low, which indicated that most of the SRB found may not be Hg methylators. It is predicted that strains in Ruminococcaceae may be the main Hg methylators in the sediment, whose Hg methylation abilities were lower comparing with those in δ-proteobacteria. The relative abundances of the genera that contain known Hg methylators were all below 0.8%, which may explain the relatively lower levels of MeHg in the sediment of Nam Co Lake compared to other aquatic systems. This may also reflect that Hg methylators were relatively rare among most clades and abiotically regulated Hg methylation may exert relatively more important role here.