Molecular characterization of microbial methylmercury production in Adirondack wetland ecosystems

 

Mercury in rain and snow has created a major contamination problem in the Adirondacks, located in Northern New York State. Our lab is a part of a multidisciplinary team, headed by Charles Driscoll from the University of Syracuse, that studies how mercury that is deposited to the forest canopy accumulates as methylmercury in highly isolated Adirondacks mountain lakes. Within this project we are investigating the processes that control the synthesis and degradation of methylmercury. Our study site is Sunday Lake, located at the western reaches of the Adirondacks, and the wetland surrounding it (see pictures below).

 
Sample collection in Sunday Lake: On the right is a map of our Adirondacks study location. In the center is a view of the study site, Sunday Lake. On the left, consultation during sampling by Riqing Yu, a graduate student in our lab (back center) with our collaborators Mark Hines (right) and graduate student Isaac Adato (left).

To begin to understand how mercury is methylated in Sunday Lake we determine what microorganisms and metabolic processes are responsible for methylmercury formation. We apply molecular and biochemical tools to samples collected within the Sunday Lake ecosystem. Together with our collaborators Mark Hines and Isaac Adato from the university of Massachusetts, Lowell, we discovered that sulfate reducing bacteria (SRB) that live in water saturated zones in the wetland around Sunday Lake can methylate mercury. This discovery was based on the demonstration that (i) stimulating activities of SRB enhanced the potential rates of mercury methylation, and (ii) that SRB were present in the same samples that methylated mercury (see figure below). Our results are summarized in a poster that we recently presented in an international conference.

An image of a DGGE gel of PCR amplification products using sulfate reducing bacteria (SRB) primers specific to the 16S rRNA genes of SRB. Each lane in this gel represents a different taxonomic group of SRB and each band likely originated from one species of SRB. The bends are excised from the gel and sequenced to identify the SRB from which they originated.

We also are interested in the potential of the microbial community in Sunday Lake to reduce mercuric mercury to elemental mercury because this process limits methylation by competing for the same substrate. The elemental mercury that is produced by this reaction is highly volatile and is removed to the atmosphere. We therefore looked for the presence of merA, the bacterial gene that encodes for the mercuric reductase enzyme, in DNA extracts of the microbial biomass from soils and sediments that had been collected in Sunday Lake. Preliminary data suggested to us that microbes that carry these genes are abundant in our samples. This result suggests that methylmercury accumulation in Sunday Lake may be limited by the activity of microbes that volatilize it to elemental mercury.