Dissolved humic substances can directly affect freshwater organisms

Direct effects of pure humic substance (BS1 FA) on three different aquatic organisms (Ceratophyllum demersum, Dreissena polymorpha, and Chaetogammarus ischnus) were demonstrated in this study. Exposure to environmentally rele vant concentrations (0.5 mg/L) of this humic substance led to the activatio n of the microsomal and soluble glutathione S-transferase (GST) in Ceratoph yllum demersum. Exposure to 3-chlorobiphenyl showed also an elevation of GS T activity, which is due to the proposed detoxication metabolism of these c hlorinated biphenyl. Coexposure to a mixture of 3-chlorobiphenyl and BS1 FA showed a reduction of enzyme elevation, but still significant over an untr eated control. The direct impact of humic substances seems not to be restri cted to a specific class of organisms.