Toxicity of nitriles and their derivatives on Raphidocelis subcapitata (Algae)

Abstract

Algae are ecologically important organisms in the aquatic food chain and are frequently used in environmental studies for assessing the relative toxicity of various chemicals and/or waste discharges. Currently, however, most standard algal test protocols are based on conventional batch technique (ASTM, 1994; ISO, 1987; US EPA, 1996; OECD; 1984). The European Centre for Ecotoxicology and Toxicology of Chemicals (1996) has concluded that current algal toxicity test protocols are unsuitable for assessing the effects of volatile compounds because the experimental design for these methods may cause considerable losses of volatile organic toxicants and, consequently, underestimations of the toxicity of volatile organic chemicals. Nitriles and their derivatives are common intermediate products from plastic and resin manufactories. Their toxic effects to phytoplankton have rarely been investigated. This study evaluated the toxicity of 12 kinds of nitriles and halogen-substituted nitriles using a closed-system algal toxicity test with no head-space. Raphidocelis subcapitata was selected as the test alga. Toxicity testing was conducted by transferring adequate amounts of algal suspension, dilution water (with growth medium), and toxicants into 300-ml BOD bottles. The BOD bottles were completely filled up with no head-space left. The initial cell density and the exposure time were 15,000 cells/mL and 48 hours, respectively. Light intensity and temperature were kept at 65 mu EM(-2)s(-1) (+/- 10%) and 24 +/- 1 square. The DO level at the beginning of the test was approximately 1 to 3 mg/L. Two response endpoints, i.e., dissolved oxygen production and algal growth rate, were employed to assess the toxicity of the test chemicals. The median effective concentration (EC50) was defined as the toxicant concentration that resulted in 50% reduction with respect to the response endpoints (i.e., ADO, or growth rate). The median effective concentrations (EC50) range from 0.051 to 5843 mg/L. In general, the DO endpoint revealed higher inhibition than that from algal growth rate. The above phenomenon indicates that certain nitrils and their derivatives have stronger toxic effects to algal photosynthesis reactions. Furthermore, halogen-substituted nitriles were found to be extremely toxic to Raphidocefis subcapitata. With increasing numbers of the halogen molecules, stronger toxicity was observed. The bromine substitutent also seems to be more toxic than chlorine substitutent. Quantitative structure-activity relationships (QSARs) were established based on the chemicals'ELUMO values with R 2 values equal to 0.85. Such relationships can thus be used to predict the toxicity of other nitrile compounds.