Increased input of MTBE into aquatic systems via leaking underground storage tanks and via gasoline from boats and personal watercraft has lead to concerns about its effect(s) on aquatic life. Depending on time of exposure and endpoint measured, the compound is acutely toxic to various aquatic organisms at concentrations of 44 mg/L to >1000 mg/L (invertebrates) and 388 mg/L to >3000 mg/L (vertebrates). Bacterial assays were most sensitive with toxicity to Salmonella typhimurium measured at 7.4 mg/L (48 hours). MTBE does not appear to bioconcentrate in fish and is rapidly excreted or metabolized. Most of the studies measure mortality within 96 hours. To gain a better understanding of the possible developmental toxicity of MTBE, we are employing a sensitive vertebrate assay using embryos of the teleost fish, medaka (Oryzias latipes). Adult breeding pairs are exposed to aqueous baths of MTBE at one of six concentrations (100, 1, 0.1, 0.01 and 0.001 mg/L MTBE, controls). As hydration of fertilized eggs takes place, MTBE will enter eggs, and embryonated eggs are continued to be exposed to aqueous bath until seven days after hatching. Embryos are monitored daily, and the transparent chorion (egg shell) permits direct, non-invasive observation of development. Endpoints will be heart rate, consumption of yolk, time to hatching, survival, and the presence of abnormal morphology of organs and tissues.

The EPA 3-species test using a green algae, a crustacean and a fish species (Selenastrum capricornutum, Ceriodaphnia dubia, Pimephales promelas) was established in our laboratory for routine toxicity testing of water samples. In addition, test protocols have been developed using California resident aquatic species (rotifers (Brachionus calyciflorus), mysid shrimp (Neomsis mercedis)). MTBE and alternative compounds are being tested for their toxicity to these indigenous organisms. In addition, the EPA 3-species test is used to detect potential toxicity of MTBE breakdown products produced by conventional drinking water treatment techniques (in collaboration with T. Young) and MTBE water residues from 2-cycle engine operation (in collaboration with J. Reuter).