All experiments were conducted on DT cultured at the Huntsman Marine laboratory, and in their exponential growth phase, to ensure the culture was in good health prior to performing the test. Six individual PACs were chosen for the study to represent a diverse spectrum of chemicals found in oil. Exposures were conducted using a novel passive dosing method to ensure a constant exposure concentration was maintained over the entire trial period. “Traditional exposure methods struggle to maintain a constant PAC exposure concentration over time. Using a partitioned-controlled delivery system with silicone O-rings, our lab is able to complete benchtop scale toxicity exposures over an extended period for up to several days while not having to renew test solution,” explained Philibert. 

 A spectrofluorometer at Huntsman Marine was used to confirm the actual exposure concentrations used for each trial. Temperature tests were completed at 15, 20 and 30 °C while photomodification studies examining the effect of UV light compared test solutions exposed to either 0 or 18 hours of simulated natural sunlight in the lab. “The importance of sunlight to modify toxicity was underappreciated before the Deepwater Horizon oil spill in the Gulf of Mexico. Establishing an artificial solar irradiation chamber in the lab has since opened many research opportunities for our toxicology program,” added Philibert. The scientific endpoint for all trials in this study was microalgae culture growth over the course of the 72 hour test period for each replicate, concentration and PAC. 
 

Only two of the PACs tested (fluorenone and 1-methylnaphthalene) caused greater than 50% growth inhibition – an important toxicology statistic referred to as the EC50 – at all three test temperatures while only 1-methylnaphthalene produced results that indicated sensitivity to warmer water temperature. These results mirror prior published studies from the Huntsman Marine lab conducted on lobster larvae. Those studies also found 1-methylnaphthalene toxicity to increase at warmer temperatures, while fluorenone remained unaffected. Exposing the test solutions to UV light had no impact on PAC toxicity with this specific microalgae species. 
 

Oil spill models are dependent on having high quality single PAC toxicity data across a range of species and life stages for calibration and validation purposes. This study contributes to these data needs with the microalgae species DT while also expanding our understanding of the impact of environmental modifiers like temperature and UV light on PAC toxicity. “Our results demonstrated that increases in temperature can impact acute toxicity of some PACs to the microalgae species DT but there was relatively minor impact from exposure to UV light. More research is needed to determine if the effects we observed with DT can be replicated with other marine microalgae species,” concludes Philibert. Having these insights are important to initiate further research on the effects of PAC exposure to additional microalgae species given the increasing variability in surface water temperatures due to climate change.