Diphenyl ether

Toxicity to aquatic algae and cyanobacteria

Description of key information

Two short-term toxicity studies with Diphenyl Oxide (DPO or Diphenyl Ether) were conducted with the freshwater green algae, Pseudokirchneriella subcapitata (formerly known as Selenastrum capricornutum) following Japanese Ministry of the Environment Guideline (equivalent to OECD Guideline 201) and US EPA guidelines respectively. The acute and chronic toxicity of DPO to green algae was also estimated by a widely accepted quantitative structure-activity relationship (QSAR) model ECOSAR v1.11 as a weight of evidence.

Key value for chemical safety assessment

EC50 for freshwater algae:

0.455 mg/L

EC10 or NOEC for freshwater algae:

0.24 mg/L

Additional information

A phytotoxicity test (key study) was performed at Kureha Special Laboratory Co. sponsored by Japanese Ministry of the Environment, to determine the effect of DPO on the freshwater alga Pseudokirchnerella subcapitata. Culture and test procedures followed those of the Notice On the Test Method Concerning New Chemical Substances (Nov 21, 2003, No. 1121002, Pharmaceutical and Food Safety Bureau, MHLW, No.2, Manufacturing Industries Bureau, METI, No. 031121002, Environ Policy Bureau, MOE) -- Equivalent to OECD 201. Based on initial measured concentrations, the 72-hour ErC50 (growth rate) was 0.580 mg/L with 95% confidence limits of 0.556-0.608 mg/L. The NOEC on growth rate was 0.32 mg/L. The 72-hour EbC50 (biomass) was 0.405 mg/L with 95% confidence limits of 0.392 - 0.417 mg/L. The NOEC on biomass was 0.25 mg/L. Based on arithmetic mean measured concentrations between initial and 72 hour measurement, the 72-hour ErC50 (growth rate) was 0.455 mg/L with 95% confidence limits of 0.436-0.477 mg/L. The NOEC on growth rate was 0.24 mg/L. The 72-hour EbC50 (biomass) was 0.304 mg/L with 95% confidence limits of 0.294 - 0.313 mg/L. The NOEC on biomass was 0.188 mg/L.

A phytotoxicity test (supporting study) was performed at EG&G, Bionomics Marine Research Laboratory (BMRL), Pensacola, Florida, to determine the effect of DPO on the freshwater alga Selenastrum capricornutum. Culture and test procedures followed those of U.S. Environmental Protection Agency (1978). Concentrations >/= 2 .5 ppm of DPO appeared to adversely affect the growth of Selenastrum capricornutum throughout the 96 hours of exposure. Based on decrease of in-vivo chlorophyll a, the estimated 24-hour EC50 was >10 ppm while the calculated 96-hour EC50 was 2.5 ppm with 95% confidence limits of 1.2-5.4 ppm. The calculated 96-hour EC50, based on cell number decrease, was also 2.5 ppm with 95% confidence limits of 1.2-5.3 ppm.After 96 hours of exposure, decrease of in-vivo chlorophyll a was from 15% in cultures exposed to 2.5 ppm to 100% in those exposed to 5 and 10 ppm; in-vivo chlorophyll a increased in cultures exposed to </= 1.2 ppm. Decrease of cell numbers was from 19% in cultures exposed to 2.5 ppm to 100% in those exposed to </=1 .2 ppm.

Quantitative structure-activity relationship (QSAR) predictions were conducted using the latest version of US EPA's ECOSAR (v.1.11, 2012) using input values of 4.21 for log Kow, 27°C for melting point, and 18 mg/L for water solubility in the QSAR for neutral organics. The model predicted a 96-h EC50 on growth rate and a chronic value (geometric mean of NOEC and LOEC) for green algae of 0.75 and 0.72 mg/L, respectively. The model estimation results corroborate the results of the study sponsored by Japanese Ministry of the Environment.