Comparative toxicity of hydrophobic contaminants to microalgae and higher plants

To enable rapid and sensitive screening of phytotoxic compounds in terrestrial system, a 4 day solid-phase microalgal bioassay was developed. Three species of microalgae (Selenastrum capricornutum, Chlorococcum hypnosporum and Chlorococcum meneghini) were chosen to investigate their responses to DDTs (DDT, DDD and DDE) and PAHs (naphthalene, phenanthrene and pyrene) spiked sands. The bioassay results showed that PAHs and DDTs were toxic to microalgae in a 4-day exposure tests but not to seed germination of ryegrass (Lolium perenne). Phenanthrene was the most phytotoxic. Among three investigated endpoints, fluorescence emissions by microalgae were less sensitive than cell density (optical density OD(650)) and chlorophyll a concentration as endpoints. In general, S. capricornutum was the most sensitive species for PAHs (EC(50) for phenanthrene = 9.4 mg kg(-1)), while C. meneghini for DDTs (EC(50) for DDE = 20.0 mg kg(-1)). Comparison of the microalgal tests with US EPA standard seed germination/root elongation test (using Lolium perenne) demonstrated the superior screening potential of phytotoxic hydrophobic compounds using the proposed bioassay. Using OD(650) as the endpoint, EC(10) of selected microalgae for PAHs and DDTs were 0.43-64.3 mg kg(-1) and 0.67-117 mg kg(-1) respectively, which were much lower than the EC(10) of L. perenne for both PAHs (94-187 mg kg(-1)) and DDTs (113-483 mg kg(-1)). The results encourage further studies involving wider types of vascular plants and more comparison with standard phytotoxicity tests from different authorities using contaminated soils to verify the effectiveness of the microalgal bioassay.