Mercury Speciation, Retention, and Abundance of Genes Involved with Mercury Methylation in Fertilized Salt Marsh Sediments

Mercury (Hg) is a global pollutant which exists in both aquatic and terrestrial systems in three main forms including elemental Hg, ionic mercury, and methyl mercury, (MeHg), and Hg cycling plays an important role in ecosystems. Great Sippewisset Marsh, (GSM) in Falmouth MA has been chronically treated with fertilizer that contains heavy metals such as Hg in varying concentrations since the early 1970s and provides for an excellent location to study Hg contamination. The overall goal of this study is to analyze the key geochemical and microbial conditions that lead to MeHg production in the presence of the applied fertilizer. Sediment cores were taken in three experimental plots, control, high, and extra high fertilization. Mercury concentrations were determined using cold vapor atomic fluorescence spectroscopy and direct mercury analysis. Total sulfur was determined and quantitative polymerase chain reaction (qPCR) data was generated for the bacterial 16S gene, and dissimilatory sulfite reductase (dsrA), as most sulfate reducing bacteria methylate Hg. Total Hg was higher in the fertilized plots, while total sulfur decreased with increasing fertilization. The average concentration of MeHg was highest in the high fertilized plot while the ratio of MeHg to total Hg was highest in the control plot despite the fact that this plot is only receiving mercury through atmospheric deposition. Bacterial 16S copy number per ng DNA extracted was lowest in the control and fluctuated with depth for control, low, and high fertilized conditions. Higher nitrate concentrations within fertilized treatment plots may play a role in decreasing MeHg production, but more research is needed to determine why MeHg concentrations are decreasing with increased fertilization and to develop a greater sense of the response of the bacterial communities to the applied fertilizer.