Wmer Rein.arch Vol. S. pp. 219 to 225. Pergamon Press 1974. Printed in Great Britain. A BACTERIAL METHYLMERCURY-MINERALIZING ACTIVITY IN RIVER SEDIMENTS GILLES BILLEN,* CLAUDE JOIRIS and ROLAND WOLLAST Laboratorium voor Ekologie en Systematiek. Vrije Universiteit Brussel. Belgium and Laboratoire de Chimie Industrielle. Universit~ Libre de Bruxelles. 1050 Brussels Belgium (Received 6 Auyust 1973) Abstract--A bacterial activity involving mineralization of methylmercury was found in bottom sediments of the river Sambre (Belgium), in a zone highly polluted with inorganic mercury. The possibility, shown by other authors and ourselves, that biological production of toxic methylmercury compounds occurs in such a medium gives a great ecological significance to this finding. Some properties of this mineralizing activity were studied at the community level in the laboratory. The mineralizing capacity of the community can be increased in response to increased concentrations of methylmercury, probably by means of the selection of methylmercury-resistant bacterial species, among which the organisms responsible for the transformation constitute only a part. Because of this adaptation, it is suggested that some equilibrium can be reached between the degrada- tion of methylmercury and its addition to, or its production in, mineral mercury polluted environments. INTRODUCTION Owing to their high toxicity and the ease of their ac- cumulation in food chains, methylmercury compounds in the environment have been of major concern for some years. They can be released in aquatic media as pesticides, or as side products of catalytic processes in industry (Ui, 1969) but can also be produced from mineral mercury through bacterial activity in sedi- ments, as shown by Jensen and Jernel6v (1969). From a soil treated with organomercuric fungicide, Tonomura et al. (1968) isolated a bacterial species able to mineralize these organic compounds. We also found such a mineralizing activity in the sediments of a river polluted with mineral mercury. After incubating the same sediments with mineral mercury under various conditions, we were not able to detect significant meth- ylmercury, contrary to the results of Jensen and Jerne- 16v (1969), although the presence of methylating organisms was easily demonstrated by enrichment procedures (for methane bacteria and for peptoclostri- dia) which gave rise to active mercury-methylating cul- tures (Billen et al., in preparation). We thus formulated the hypothesis that the methylating activity, if any, in the natural bacterial community of the sediments could be counterbalanced by the mineralizing activity of other organisms. The subject of the present paper is the description of this mineralizing activity and some of its ecological * Research-fellow at the Fonds National Beige de la Recherche Scientifique. properties in response to the presence or the produc- tion of methylmercury. MATERIALS AND METHODS Sediments Bottom sediments were collected in the Sambre river with a sterilized field grab sampler from a boat in the middle of the river, 3"5 km below Auvelais (Belgium). The mineral mercury content of the sediments was as high as 70 mgkg -1 dry weight; methylmercury was undetectable. Bacterioloyif:al procedures In order to study as far as possible only ecologically significant phenomena, we made all our experiments handling a global microbial community, without any purification or isolation. Cultures ofthe whole bacterial community of the sedi- ments were prepared as in the following description. The medium consisted of sterilized non-enriched Sambre water. The organic matter present in this water (chemical oxygen demand, as determined according to Rainwater and Thatcher, 1960:104 mg O2 1-t) was enough to allow a bacterial growth up to 5 x 106 bacteria ml- t. The inoculum was prepared by suspending sediments in sterilized Sambre water, treating it with an "Ultra- Turrax" mixer (Gunkel, 1964) in order to separate bac- teria from their solid support, and allowing this to set- tle for 15 min. A few cubic centimetres of the clear solu- tion was then injected into 100 cm 3 of the medium. In- cubations of the cultures were performed at 25"C un- der the conditions shown in Table 1. 219