Dissolved organic matter and inorganic mercury loadings favor novel methylators and fermentation metabolisms in oligotrophic sediments

1 Advances in genetics have allowed for greater investigation into the complex microbial 2 communities mediating mercury methylation in the environment. In wetlands in particular, 3 dissolved organic matter (DOM) may influence methylmercury production both through direct 4 chemical interactions with mercury and through substrate effects on the environmental 5 microbiome. We conducted microcosm experiments in two chemically disparate wetland 6 environments (unvegetated and vegetated sediments) to examine the impact of DOM from 7 leachate of local vegetation and inorganic mercury loadings on microbial community 8 membership, metagenomic potential, DOM processing, and methylmercury production. We 9 show that while DOM loadings impacted the microbiome in both environment types, sediment 10 with high organic carbon content was more resistant than oligotrophic sediment to changes in 11 microbial community structure and methylmercury production. We identify putative 12 chemoorganotrophic methylators within the class Clostridia as primary community members 13 associated with methylation rates in contrast to previous work focusing on microorganisms 14 involved in sulfur, iron, and methane cycling. Metagenomic shifts toward fermentation, and 15 secondarily iron metabolisms, as well as degradation of complex DOM indicated by fluorescence 16 indices further support the involvement of rarely acknowledged biogeochemical pathways in 17 mercury toxicity. We therefore propose that DOM addition in our system generates 18 methylmercury production either 1) via direct methylation by fermenting bacteria or 2) via 19 enhancing the bioavailability of simple carbon compounds for sulfateand iron-reducing bacteria 20 through breakdown of complex DOM. Our results demonstrate variation in sediment 21 methylmercury production in response to DOM loading across geochemical environments and 22 . CC-BY-NC-ND 4.0 International license peer-reviewed) is the author/funder. It is made available under a The copyright holder for this preprint (which was not . http://dx.doi.org/10.1101/072017 doi: bioRxiv preprint first posted online Aug. 29, 2016;