Mercury Methylating Microbial Community Structure in Boreal Wetlands Explained by Local Physicochemical Conditions

Jingying Xu,Moritz Buck,Van Liem-Nguyen,Andrea G Bravo,Stefan Bertilsson,Erik Björn

doi:10.3389/fenvs.2020.518662

Abstract

The potent neurotoxin methylmercury (MeHg) is a major concern due to its negative effects on wildlife and human health. Boreal wetlands play a crucial role in Hg cycling on a global scale, and therefore, it is crucial to understand the biogeochemical processes involved in MeHg formation in this landscape element. By combining high-throughput hgcA amplicon sequencing with molecular barcoding, we reveal diverse clades of potential HgII methylators in a wide range of wetland soils. Among Bacteria, Desulfuromonadota (14% of total reads), Desulfurobacterota_A, and Desulfurobacterota (up to 6% of total reads), previously classified as Deltaproteobacteria, were important members of the hgcA+ microbial community in the studied wetlands. We also identified Actinobacteriota (9.4% of total reads), Bacteroidota (2% of total reads), and Firmicutes (1.2% of total reads) as members of the hgcA+ microbial community. Within Archaea, Methanosarcinales represented up to 2.5% of the total reads. However, up to half of the hgcA+ community could not be resolved beyond domain Bacteria. Our survey also shows that local physicochemical conditions, such as pH, nutrient concentrations, water content, and prevailing redox states, are important for shaping the hgcA+ microbial community structure across the four studied wetlands. Furthermore, we observed a significant correlation between HgII methylation rate constants and the structure of the hgcA+ microbial community. Our findings expand the current knowledge on the hgcA+ microbial community composition in wetlands and the physicochemical factors underpinning spatial heterogeneity in such communities.

Highlights

Wetlands cover 6% of the world’s land surface and contain about 12% of the global carbon pool (Erwin, 2009)
We present a correlation plot between hgcA+ microbial community (Bray–Curtis) dissimilarity and the Euclidean distance of HgII methylation rate constants
Understanding the responses of bacterial communities to environmental factors is a major focus for research in microbial ecology, and this applies to HgII methylating microbial communities

Summary

Introduction

Wetlands cover 6% of the world’s land surface and contain about 12% of the global carbon pool (Erwin, 2009) These systems play an important role in different global biogeochemical cycles, such as that of carbon and mercury (Hg) (Mitra et al, 2003; Tjerngren et al, 2012b). Because wetlands are parts of large watersheds and thereby are hydrologically connected to adjacent systems, there is a high risk for MeHg that is produced in wetlands to be transported to downstream waters. This is critical as MeHg bioaccumulates and biomagnifies along the aquatic food webs, and this poses severe risks to the environment and humans (Boening, 2000). An understanding of the potential MeHg formation in boreal wetlands is crucial for robust risk assessment and rational landscape management

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