Fig. 3. Influence of wetland type on mercury-methyltransferase (HgMT) activity (closed bars) and sulfate reduction rate (closed circles). Error bars are standard error of methyltransferase activity and one quartile for sulfate reduction rate. U, one unit of HgMT activity was defined as the formation of 1 nmol of tetrahydrofolate formed in response to the addition of 1 ng of Hg to the reaction vessel.