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

Abstract

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.