Carbon Amendments Alter Microbial Community Structure and Net Mercury Methylation Potential in Sediments
Authors: Christensen Geoff A. , Somenahally Anil C. , Moberly James G. , Miller Carrie M. , King Andrew J. , Gilmour Cynthia C. , Brown Steven D. , Podar Mircea , Brandt Craig C. , Brooks Scott C. , Palumbo Anthony V. , Wall Judy D. , Elias Dwayne A. 
Published: November 2017

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.