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 relevant concentrations (0.5 mg/L) of this humic substance led to the activation of the microsomal and soluble glutathione S-transferase (GST) in Ceratophyllum demersum. Exposure to 3-chlorobiphenyl showed also an elevation of GST activity, which is due to the proposed detoxication metabolism of these chlorinated biphenyl. Coexposure to a mixture of 3-chlorobiphenyl and BS1 FA showed a reduction of enzyme elevation, but still significant over an untreated control. The direct impact of humic substances seems not to be restricted to a specific class of organisms.