Carbon Amendments Alter Microbial Community Structure and Net Mercury Methylation Potential in Sediments

Abstract

Neurotoxic methylmercury (MeHg) is produced by anaerobic Bacteria and Archaea possessing the genes hgcAB, but it is unknown how organic substrate and electron acceptor availability impacts the distribution and abundance of these organisms. We evaluated the impact of organic substrate amendments on mercury (Hg) methylation rates, microbial community structure, and the distribution of hgcAB+ microbes with sediments. Sediment slurries were amended with short-chain fatty acids, alcohols, or a polysaccharide. Minimal increases in MeHg were observed following lactate, ethanol, and methanol amendments, while a significant decrease (∼70%) was observed with cellobiose incubations. Postincubation, microbial diversity was assessed via 16S rRNA amplicon sequencing. The presence of hgcAB+ organisms was assessed with a broad-range degenerate PCR primer set for both genes, while the presence of microbes in each of the three dominant clades of methylators (Deltaproteobacteria, Firmicutes, and methanogenic Archaea) was measured with clade-specific degenerate hgcA quantitative PCR (qPCR) primer sets. The predominant microorganisms in unamended sediments consisted of Proteobacteria, Firmicutes, Bacteroidetes, and Actinobacteria. Clade-specific qPCR identified hgcA+ Deltaproteobacteria and Archaea in all sites but failed to detect hgcA+ Firmicutes. Cellobiose shifted the communities in all samples to ∼90% non-hgcAB-containing Firmicutes (mainly Bacillus spp. and Clostridium spp.). These results suggest that either expression of hgcAB is downregulated or, more likely given the lack of 16S rRNA gene presence after cellobiose incubation, Hg-methylating organisms are largely outcompeted by cellobiose degraders or degradation products of cellobiose. These results represent a step toward understanding and exploring simple methodologies for controlling MeHg production in the environment.IMPORTANCE Methylmercury (MeHg) is a neurotoxin produced by microorganisms that bioacummulates in the food web and poses a serious health risk to humans. Currently, the impact that organic substrate or electron acceptor availability has on the mercury (Hg)-methylating microorganisms is unclear. To study this, we set up microcosm experiments exposed to different organic substrates and electron acceptors and assayed for Hg methylation rates, for microbial community structure, and for distribution of Hg methylators. The sediment and groundwater was collected from East Fork Poplar Creek in Oak Ridge, TN. Amendment with cellobiose (a lignocellulosic degradation by-product) led to a drastic decrease in the Hg methylation rate compared to that in an unamended control, with an associated shift in the microbial community to mostly nonmethylating Firmicutes. This, along with previous Hg-methylating microorganism identification methods, will be important for identifying strategies to control MeHg production and inform future remediation strategies.

Reduced sulphur sources favour HgII reduction during anoxygenic photosynthesis by Heliobacteria

“…Nutrient cycling and cometabolic Hg transformations stand to be closely linked in Hg methylation hotspots such as periphytic microbial mats (Olsen, Brandt, & Brooks, ), aquatic sediments (Correia & Guimarães, ; Fleming, Mack, Green, & Nelson, ) and waterlogged soils (Eklof et al, ). While our study makes a case for sulphur cycling as a coupling point for competing cometabolic Hg transformations, other nutrients such as iron (Bravo et al, ; Fleming et al, ; Sugio et al, ; Wiatrowski et al, ) and organic carbon (Christensen et al, ; Grégoire et al, ; Grégoire & Poulain, ) that are known to control Hg methylation and reduction have the potential to fulfil a similar role. Curiously, the coupling between other macronutrients under strong microbial control, such as nitrogen, and Hg cycling remains unexplored despite the importance of nitrogen cycling in aquatic and terrestrial ecosystems.…”

Biotic formation of methylmercury: A bio–physico–chemical conundrum

“…Based on a higher number of sequenced microbial genomes now available, Christensen et al () developed a broad range hgcAB primer pair that improved the coverage of prior developed primers by 10% (Bae et al ; Schaefer et al ), and several clade‐specific PCR primers to improve amplification of the various members of the Hg II ‐methylating community (Christensen et al ) (Table ). In sediments, these clade‐specific primers were useful to detect Hg II ‐methylating δ‐Proteobacteria and Archaea but failed to detect Hg II ‐methylating Firmicutes (Christensen et al ).…”

“…Despite the differences in primer pairs used in the studies mentioned earlier (Table ), until now, data globally suggested that in hgcA + δ‐Proteobacteria communities are abundant in surface sediments, but in some sites hgcA + methanogens and other hgcA + uncultivated groups are prevalent (Christensen et al , ; Vishnivetskaya et al ; Bravo et al ; Jones et al ). Among δ‐Proteobacteria , syntrophs and Geobacter spp.…”

“…Natural OM, besides its strong capacity to bind Hg II and influence Hg II speciation, controls microbial activity and Hg II methylation (Graham et al ; Hsu‐Kim et al ). The molecular composition of OM shows a central role in controlling Hg II methylation (Bravo et al ), notably phytoplankton derived OM and fresh humic matter were associated with both high bacterial activity and Hg II methylation rates in sediments (Graham et al ; Schartup et al ; Mazrui et al ; Bravo et al ; Christensen et al ; Herrero Ortega et al ). Also, an increase in nutrients, associated to an enhanced algae biomass production, increased MMHg formation in sediments (Bravo et al ; Herrero Ortega et al ).…”

Mercury-methylating bacteria are associated with copepods: A proof-of-principle survey in the Baltic Sea

“…Recently, clade-specific quantitative PCR (qPCR) assays were developed to quantify the abundance of hgcA gene of the main methylators [10]. Hence, hgcAB and hgcA distribution can be used to predict occurrence of potential Hg methylators in the environment [11]. Understanding hgcAB and hgcA distribution is essential for estimating MeHg production in the water column and biomagnification in food webs [12].…”