Oligotrophic wetland sediments susceptible to shifts in microbiomes and mercury cycling with dissolved organic matter addition

Emily B Graham; Rachel S Gabor; Shon Schooler; Diane M McKnight; Diana R Nemergut; Joseph E Knelman

doi:10.7717/peerj.4575

Oligotrophic wetland sediments susceptible to shifts in microbiomes and mercury cycling with dissolved organic matter addition

PeerJ. 2018 Apr 3:6:e4575. doi: 10.7717/peerj.4575. eCollection 2018.

Authors

Emily B Graham¹, Rachel S Gabor², Shon Schooler³, Diane M McKnight^{4

5

6}, Diana R Nemergut^{4

7}, Joseph E Knelman⁴

Affiliations

¹ Biological Sciences Division, Pacific Northwest National Laboratory, Richland, WA, United States of America.
² School of Environment and Natural Resources, Ohio State University, Columbus, OH, United States of America.
³ Lake Superior National Estuarine Research Reserve, Superior, WI, United States of America.
⁴ Institute for Arctic and Alpine Research, University of Colorado at Boulder, Boulder, CO, United States of America.
⁵ Civil Engineering Department, University of Colorado at Boulder, Boulder, CO, United States of America.
⁶ Environmental Studies Program, University of Colorado at Boulder, Boulder, CO, United States of America.
⁷ Biology Department, Duke University, Durham, NC, United States of America.

Abstract

Recent advances have allowed for greater investigation into microbial regulation of mercury toxicity in the environment. In wetlands in particular, dissolved organic matter (DOM) may influence methylmercury (MeHg) production both through chemical interactions and through substrate effects on microbiomes. We conducted microcosm experiments in two disparate wetland environments (oligotrophic unvegetated and high-C vegetated sediments) to examine the impacts of plant leachate and inorganic mercury loadings (20 mg/L HgCl₂) on microbiomes and MeHg production in the St. Louis River Estuary. Our research reveals the greater relative capacity for mercury methylation in vegetated over unvegetated sediments. Further, our work shows how mercury cycling in oligotrophic unvegetated sediments may be susceptible to DOM inputs in the St. Louis River Estuary: unvegetated microcosms receiving leachate produced substantially more MeHg than unamended microcosms. We also demonstrate (1) changes in microbiome structure towards Clostridia, (2) metagenomic shifts toward fermentation, and (3) degradation of complex DOM; all of which coincide with elevated net MeHg production in unvegetated microcosms receiving leachate. Together, our work shows the influence of wetland vegetation in controlling MeHg production in the Great Lakes region and provides evidence that this may be due to both enhanced microbial activity as well as differences in microbiome composition.

Keywords: Carbon; Fermentation; Fluorescence spectroscopy; Freshwater microbiology; Mercury contamination; Mercury methylation; Microbial community structure; Organic matter chemistry; Sediment microbiome; Wild rice.

Grants and funding

This work was supported by EPA STAR and NOAA NERRS fellowships to Emily B. Graham and a JGI CSP grant to Diana R. Nemergut. The authors received support from the US Department of Energy (DOE), Office of Biological and Environmental Research (BER), as part of Subsurface Biogeochemical Research Program’s Scientific Focus Area (SFA) at the Pacific Northwest National Laboratory (PNNL). PNNL is operated for DOE by Battelle under contract DE-AC06-76RLO 1830. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.