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Toxicity to aquatic algae and cyanobacteria

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In a key, static 72 hour Pseudokirchneriella subcapitata growth inhibition test (OECD 201, KS=1), 10000 cells/mL were exposed to the Water Accommodated Fraction (WAF) of light cracked gas oils (MRD-10-595) at nominal concentrations of 0.05, 0.23, 1.01, 4.56 and 20.5 mg/L. The 72 hour EL50 was 0.51 mg/L and NOELR was 0.05 mg/L based on growth rate (ExxonMobil 2012e).

In a supporting, static 72 hour Pseudokirchneriella subcapitata growth inhibition test (OECD 201, KS=1), 10000 cells/mL were exposed to the Water Accommodated Fraction (WAF) of light cracked gas oils (MRD-10-578) at nominal concentrations of 0.05, 0.23, 1.01, 4.56 and 20.5 mg/L. The 72 hour EL50 was 0.93 mg/L and NOELR was 0.05 mg/L based on growth rate (ExxonMobil 2011d).

In a supporting, static 72 hour Pseudokirchneriella subcapitata growth inhibition test (OECD 201, KS=1), 10000 cells/mL were exposed to the Water Accommodated Fraction (WAF) of light catalytic cracked gas oils (MRD-10-576) at nominal concentrations of 0.10, 0.32, 1.02, 3.28 and 10.5 mg/L. The 72 hour EL50 was 0.53 mg/L based on growth rate. The 72 hour NOELR was 0.1 mg/L (ExxonMobil 2011e).

In a supporting, static 72 hour Pseudokirchneriella subcapitata growth inhibition test (OECD 201, KS=1), 10000 cells/mL were exposed to the Water Accommodated Fraction (WAF) of thermally cracked gas oils (MRD-10-604) at nominal concentrations of 0.32, 1.6, 8.0, 40 and 200 mg/L. The 72 hour EL50 was 8.82 mg/L and the NOELR was 0.32 mg/L based on growth rate (ExxonMobil 2012f).

The aquatic toxicity was estimated using the PETROTOX computer model, which combines a partitioning model (used to calculate the aqueous concentration of hydrocarbon components as a function of substance loading) with the Target Lipid Model (used to calculate acute and chronic toxicity of non-polar narcotic chemicals). PETROTOX computes toxicity based on the summation of the aqueous-phase concentrations of hydrocarbon block(s) that represent a petroleum substance and membrane-water partition coefficients (KMW) that describe the partitioning of the hydrocarbons between the water and organism. The estimated algal 72-hr EL50 value is 0.202 mg/L.