Ogorek, J. M. | bioRxiv Science

bioRxiv2020

Mercury methylation trait dispersed across diverse anaerobic microbial guilds in a eutrophic sulfate-enriched lake

Mercury (Hg) methylation is a microbially mediated process that converts inorganic Hg into the bioaccumulative neurotoxin methylmercury (MeHg). Exploring the diversity and metabolic potential of the dominant Hg-methylating microorganisms can provide insights into how biogeochemical cycles and water quality parameters underlie MeHg production. However, our understanding of the ecophysiology of methylators in natural ecosystems is still limited. Here, we used shotgun metagenomics paired with biogeochemical data to identify likely hotspots for MeHg production in a lake with elevated sulfate levels and characterize the microbial methylators and the flanking microbial community. Identified putative methylators were dominated by hgcA sequences divergent from those in canonical, experimentally confirmed methylators. Using genome-resolved metagenomics, these sequences were identified within genomes associated with Bacteroidetes and the recently described phylum Kiritimatiellaeota. Over half of the hgcA abundance comes from genomes corresponding to obligately fermentative organisms, many of which have a large number of glucoside hydrolases for polysaccharide degradation. Sulfate-reducing genomes encoding hgcA were also identified, but only accounted for 22% of the abundance of hgcA+ genomes. This work highlights the diverse dispersal of the methylation trait across the microbial anoxic food web.

genomics↗

bioRxiv2020

Genome-resolved metagenomics and detailed geochemical speciation analyses yield new insights into microbial mercury cycling in geothermal springs

Geothermal systems emit substantial amounts of aqueous, gaseous and methylated mercury, but little is known about microbial influences on mercury speciation. Here we report results from genome-resolved metagenomics and mercury speciation analysis of acid warm springs in the Ngawha Geothermal Field (<55 {degrees}C, pH < 4.5), Northland Region, Aotearoa (New Zealand). Our aim was to identify the microorganisms genetically equipped for mercury methylation, demethylation, or Hg(II) reduction to volatile Hg(0) in these springs. Dissolved total and methylated mercury concentrations in two adjacent springs with different mercury speciation ranked among the highest reported from natural sources (250-16000 ng L-1 and 0.5-13.9 ng L-1, respectively). Total solid mercury concentrations in spring sediments ranged from 1273 to 7000 {micro}g g-1. In the context of such ultra-high mercury levels, the geothermal microbiome was unexpectedly diverse, and dominated by acidophilic and mesophilic sulfur- and iron-cycling bacteria, mercury- and arsenic-resistant bacteria, and thermophilic and acidophilic archaea. Integrating microbiome structure and metagenomic potential with geochemical constraints, we constructed a conceptual model for biogeochemical mercury cycling in geothermal springs. The model includes abiotic and biotic controls on mercury speciation, and illustrates how geothermal mercury cycling may couple to microbial community dynamics and sulfur and iron biogeochemistry. IMPORTANCELittle is currently known about biogeochemical mercury cycling in geothermal systems. This manuscript presents an important new conceptual model, supported by genome-resolved metagenomic analysis and detailed geochemical measurements. This work provides a framework for studying natural geothermal mercury emissions globally. Specifically, our findings have implications for mercury speciation in wastewaters from geothermal power plants and the potential environmental impacts of microbially and abiotically formed mercury species, particularly where mobilized in spring waters that mix with surface- or ground-waters. Furthermore, in the context of thermophilic origins for microbial mercury volatilisation, this report yields new insights into how such processes may have evolved alongside microbial mercury methylation/demethylation, and the environmental constraints imposed by the geochemistry and mineralogy of geothermal systems.

microbiology↗