Comparison of OECD and radiolabeled substrate methods for measuring biodegradation in marine environments

Two methods for assessing biodegradation in marine environments, the OECD method, and a method using radiolabeled test substrate, were compared utilizing a model aromatic compound, benzoic acid. In samples from a relatively unimpacted estuary, Santa Rosa Sound, definitive biodegradation at 20 mg/liter was not detected by either method. However, the radiolabeled substrate method measured rapid biodegradation of the material at 50 micrograms/liter, which approximates expected environmental concentrations. In the Fraser River Estuary, which receives large discharges of municipal wastewater, biodegradation of benzoic acid was rapid at both 20 mg/liter and 50 micrograms/liter, and was detected by both methods. The results of the study illustrate the utility of radiolabeled substrates for determining the biodegradation of synthetic chemicals. Methods such as the OECD technique are very useful screening tools for assessing the overall potential of a chemical to biodegrade. However, such techniques generally cannot measure biodegradation at realistic concentrations. The use of radioisotopes in biodegradation studies can significantly increase the sensitivity of biodegradation measurements. As a result, realistic estimates of biodegradation can be obtained at concentrations which are often outside the scope of screening methods.