Production of methylated mercury, lead, and cadmium by marine bacteria as a significant natural source for atmospheric heavy metals in polar regions

Abstract

Mixed and pure bacterial cultures of polar origin were incubated in model experiments under polar conditions. The releasing rates of monomethyl and dimethyl mercury (MeHg + and Me 2Hg), trimethyl lead (Me 3Pb +), and monomethyl cadmium (MeCd +) were determined in dependence on the incubation time. This is the first time that methylation of cadmium by bacteria could be shown. The formation of tetramethyl and dimethyl lead (Me 4Pb and Me 2Pb 2+) was also checked but no release of these methylated compounds was observed. The determination of methylated mercury compounds was carried out by using a purge and trap system after derivatisation of monomethyl mercury into the volatile methylethyl mercury compound, subsequent separation by gas chromatography and detection with an atomic fluorescence detector. A differential pulse anodic stripping voltammetric method was applied for the determination of Me 3Pb + and MeCd +, respectively. The mixed bacterial cultures showed production of trimethyl lead and monomethyl cadmium, but no methylated mercury compound was released by these marine species. In contrast to that the isolated pure bacterial cultures released relatively high amounts of dimethyl mercury besides monomethyl mercury, trimethyl lead, and monomethyl cadmium. These methylated heavy metal compounds were preferably formed in the stationary period of bacterial growth. Depth profiles of methylated heavy metal compounds in the Arctic Ocean and the South Atlantic show maximum concentrations in water depths of up to 50 m, often correlating well with the chlorophyll-a content. But also significant concentrations in depths of about 200 m were found, where no chlorophyll-a could be detected. This is an important indication that, at least, at deeper water levels bacteria must be the marine species which mainly contribute to methylated heavy metals. Dimethyl mercury, released by marine bacteria into the polar ocean, is the methylated heavy metal compound which contributes most to the atmospheric heavy metal content in the remote areas of Antarctica and the Arctic due to its high volatility. From measured Me 2Hg concentrations in the surface seawater and the corresponding marine air at polar locations a preliminary atmospheric ocean-atmosphere transfer could be estimated to be 0.21×10 9 g yr −1 and 0.24×10 9 g yr −1 for the Antarctic and Arctic Ocean, respectively.