Toxic effects of pollutants on the mineralization of acetate in methanogenic river sediment.

Abstract

The highly industrialized drainage basin of the river has a total area of 160,000 km 2 before it enters the Netherlands (Van der Weijden and Middelburg 1989). About 58% of the river's load of suspended matter is deposited in sedimentation areas in the Netherlands (Admiraal and Van Zanten 1988) where the river eventually reaches the North Sea. These sediments are heavily polluted with persistent organic compounds and heavy metals and form a major environmental problem in the Netherlands. The Dutch research program on the 'Ecological Rehabilitation of the River Rhine included research on the effects of toxic compounds on the biodegradation in river sediments. The input of organic matter into the sediment is estimated to be I0' kg C yr -I (Admiraal and Van Zanten 1988). High numbers of bacteria are present (Van Beelen and Fleuren-Kemil~ 1989) and oxygen supplied by the overlying water will be rapidly consumed in the top layer of the sediment. The underlying part of the sediment is anoxic and there biodegradation of natural as well as anthropogenic organic substrates is dependent on anaerobic processes. Anaerobic mineralization of organic matter by micro-organisms is an essential component of the carbon cycle. Acetate is a key intermediate in anaerobic degradation of organic matter and a major component of anaerobic waste water. Organic polymers like proteins, carbohydrates, and lipids are hydrolysed to amino acids, sugars and fatty acids. These are converted by acidogenic bacteria which produce hydrogen, acetate, propionate, butyrate, lactate, and alcohols (Gorris 1987). Acetogenic bacteria convert these products to acetate. Hydrogen can be oxidized by hydrogenotrophic acetogens with the use of carbon dioxide to form acetate. Hydrogen and carbon dioxide are also used directly to form methane but about 70% of the methane formation in anaerobic freshwater sediments is derived from acetate (Lovley and Klug 1986). The following reaction is performed: CHsCOOH -> CH 4 + CO 2 AG ~ = -32.5 kJ/mol The energy liberated by this reaction is only 4% of the energy which is liberated upon the aerobic oxidation of acetate (Thauer et al. 1977). Hence the fraction of the acetate which is converted to biomass is small. During the aerobic mineralization of [14C] acetate about 50% of the label is converted to biomass while during the anaerobic mineralization 50% of the label is converted to methane. The mineralization of acetate under methanogenic conditions is performed by a few specialized methanogenic bacteria like Send reprint requests to Dr. P. van Beelen at the above address.