Production of methylated mercury and lead by polar macroalgae - A significant natural source for atmospheric heavy metals in clean room compartments

During August and September 1995 samples of different species of macroalgae were collected from the Kongsfjord on Spitsbergen. These macroalgae were cultivated in incubation vessels under polar conditions and the releasing rates of methylated heavy metal compounds for mercury, lead and cadmium were determined. The analysis of monomethyl and dimethyl mercury was carried out by a gas chromatographic system with atomic fluorescence detection after conversion of monomethyl mercury into the volatile methylethyl mercury compound. A differential pulse anodic stripping voltammetric method was used for the determination of trimethyl lead and monomethyl cadmium, respectively. The different species of macroalgae showed distinct differences in the releasing rate of the various methylated heavy metals. Laminaria saccharina, for example, was found to produce monomethyl mercury, dimethyl mercury and trimethyl lead but no monomethyl cadmium. Fucus distichus released only the two methylated mercury compounds, whereas Desmarestia aculeata showed only production of dimethyl mercury. The releasing rates of ten different types of macroalgae were determined to be in the range of up to 13 pg for methylated mercury and of up to 110 pg for trimethyl lead per gram of wet alga and day by allowing incubation times of 1-5 days. Because permethylated heavy metal compounds show high volatility as well as low solubility in ocean water, they are easily emitted into the atmosphere. It therefore follows from the results of this work that macroalgae can significantly contribute to the atmospheric heavy metal content, especially of mercury by the production of dimethyl mercury, in the clean room compartments of the polar regions. Methylated compounds have been found in the ocean, up until now, for the heavy metals mercury [1,2], lead [3] and cadmium [4]. It is widely accepted today that the formation of these alkylated heavy metal compounds is carried out by biomethylation in the ocean but there is little evidence for the formation of others than methylated derivatives [5]. Various biological species are capable to carry out the methylation of heavy metals. This was shown for mercury and lead in different model experiments with bacteria and fungi [6-10]. Comparable biomethylation experiments of lead with marine phytoplankton and marine macroalgae were successfully as well [11]. However, only little is known about the question which biological species produces what type of methylated heavy metal compound and whether there exist characteristic fingerprints for the different biological species or not. In addition, there is a lack of knowledge about the formation of methylated heavy metals in the polar oceans even if these remote areas, with their relatively high biological activity, should substantially contribute to the global natural sources of these environmentally important compounds. Because of their volatility these natural methylated compounds are substantially responsible for distinct levels of some heavy metals in the atmosphere.