Marine Algal Sensitivity to Source and Weathered Oils.

After the Deepwater Horizon oil spill in 2010, toxicity tests were conducted using 4 microalgae (Dunaliella tertiolecta, Skeletonema costatum, Isochrysis galbana, and Thalassiosira pseudonana) and one macroalga (Ectocarpus siliculosus) to study potential impacts on phytoplankton and other primary producers in the Gulf of Mexico and characterize species sensitivity. Tests were performed with Corexit 9500 and fresh source oil and weathered oil samples collected from the field during the Deepwater Horizon oil spill. Because crude oils are mixtures of poorly water‐soluble hydrocarbons, dosing was performed using water‐accommodated fractions (WAFs) and chemically enhanced (CE) WAFs with the addition of dispersant at a 1:20 dispersant:oil ratio using standard toxicity testing protocols. Exposure media were analyzed for volatile organic compounds, parent and alkylated polycyclic aromatic hydrocarbons, and saturated hydrocarbon compounds. Toxicity was reported as no‐observable effect concentration and median effect concentration (EC50) values for average specific growth rate based on nominal percent dilution of stock solution WAFs and sum of dissolved oil toxic units for WAF/CEWAF tests. The macroalga and green alga D. tertiolecta were largely unaffected by any WAF or CEWAFs tested. Isochrysis galbana was found to be the most sensitive species overall with significant growth rate inhibitions for dispersant and all the WAFs/CEWAFs tested. Physically dispersed source oils were generally more toxic than weathered oils. The protectiveness of the chronic toxic units was effective at identifying observed algal growth rate inhibitions across algal species and oil types despite the impact of dispersants. Environ Toxicol Chem 2021;40:2742–2754. © 2021 SETAC [ABSTRACT FROM AUTHOR]