The influence of Corexit 9500A and weathering on Alaska North Slope crude oil toxicity to the ammonia oxidizing bacterium, Nitrosomonas europaea

► Corexit 9500A was not toxic at environmentally relevant concentrations. ► Corexit 9500A increased toxicity of ANSC by increasing the COD of the DEWAF. ► ANSC:Corexit 9500A toxicity was due to a small volatile fraction of the ANSC. ► Weathered WAF and DEWAF was not toxic to N. europaea. ► NE1545 overexpression suggests that aromatic hydrocarbons play a role in toxicity.