Impact of volatile aromatic hydrocarbons, alone and in combination, on growth of the freshwater alga Selenastrum capricornutum

A sealed, air-tight exposure system was employed for toxicity testing of volatile aromatic hydrocarbons. Within the sealed flasks, growth of the representative green alga, Selenastrum capricornutum , was inhibited due to the restriction of gas exchange, but inhibition was overcome by the addition of NaHCO 3 (0.4% w/v) to the growth medium. Toxicity was determined as the concentration required to reduce growth by 50% following an 8-day exposure period (EC 50 ). Relative toxicity of 6 aromatic hydrocarbons was, in order of increasing toxicity, benzene (EC 50 =41.0 mg/l), toluene (9.4 mg/l), ethyl-benzene (4.8 mg/l) and p -, m -, o -xylene (between 3.9–4.4 mg/l). Examination of the toxicity of hydrocarbon mixtures revealed an additive toxicological interaction between benzene, toluene and m -xylene, indicating that the toxicity of a mixture can be predicted from the sum of the toxicities of each component. A synergistic interaction was evident between benzene and toluene at higher concentration levels, but the interpretation of synergism is complicated by a rapid increase in sensitivity of algae to greater toluene concentrations.