Each sample of dry bacterial biomass was suspended in 1ml 70% nitric acid and was heated at 60°C for 16 h. One milliliter of each 100-kDa filtrate was diluted 1:1 in 70% nitric acid and was heated at 60°C for 16 h. After appropriate dilution of the samples, mercury analyses were performed on a Vista-MPX optical emission spectrometer (Agilent, Massy-Palaiseau, France). The ICP-OES system was calibrated by serial dilutions of mercury standards within limits of detection ranging from 50 g/liter to 50 mg/liter. The emission lines used for the analyses were 184.887 and 194.164 nm.