Three whole plants of the brown seaweed Fucus vesiculosus were collected on the western coast of Sweden and kept in 10-1 bottles with seawater for 48 h, including light and dark periods. Two of the bottles contained 5 ppm mercury (as HgCl2) an 0.3 ppm tin (tributyltin fluoride), respectively. The third bottle did not contain any further additives. The concentration of volatile halocarbons was determined by capillary gas chromatography in subsamples frequently taken from each bottle. In the untreated control sample the polyhalogenated methanes, bromoform, dibromomethane, chlorodibromomethane and diiodomethane, were released in rather high amounts; up to several hundreds of ng per g dry algal tissue. The release of this group of substances, including smaller amounts chloroiodomethane, was increased during the light periods. The monoiodinated alkanes, iodoethane, 1-iodopropane, 1-iodobutane and 2-iodopropane, occurred at much lower concentrations in the culture medium, and the release did not show any pronounced light dependence. Exposure of the algae to mercury(II) ions resulted in a decreased release of most of the substances measured whereas the presence of tributyltin in the seawater seemed to provoke an increased release of some halocarbons. Except for chloroiodomethane, no further increase in halocarbon release was observed after the removal of the algae from the seawater. The results are discussed with regard to possible metabolic pathways and sites of formation.
