Trends in dissolved organic matter cycling, sediment microbiomes, and methylmercury production across vegetation heterogeneity in a Great Lakes wetland

1 Recent advances have allowed for greater investigation into microbial regulation of 2 mercury toxicity in the environment. In wetlands in particular, dissolved organic matter (DOM) 3 may influence methylmercury (MeHg) production both through chemical interactions and 4 through substrate effects on microbiomes. We conducted microcosm experiments in two 5 disparate wetland environments (unvegetated and vegetated sediments) to examine the impacts 6 of plant leachate and inorganic mercury loadings on microbiomes, DOM cycling, and MeHg 7 production in the St. Louis River Estuary, which has a legacy of mercury contamination. Overall, 8 our research reveals the greater relative capacity for mercury methylation in vegetated over 9 unvegetated sediments in this environment. Further, oligotrophic unvegetated sediments 10 receiving leachate produced more MeHg than unamended microcosms, pointing to the role of 11 organic matter and vegetation patterns as an important control on MeHg production in these 12 sediments.We also show that while leachate influenced the microbiome in both environment 13 types, sediment with high organic carbon content was more resistant to change than oligotrophic 14 sediment. Our work supports emerging research suggesting that Clostridia may be important 15 methylators in oligotrophic environments. We demonstrate changes in community structure 16 towards Clostridia and metagenomic shifts toward fermentation as well as degradation of 17 complex DOM and MeHg production in unvegetated microcosms receiving leachate. Together, 18 our work shows the importance of wetland vegetation in driving MeHg production in the Great 19 Lakes region and provides evidence that this may be due to both enhanced microbial activity as 20 well as differences in the composition of microbiomes associated with higher DOM levels. 21