Effects of nutrient availability on the diatom Phaeodactylum tricornutum in the presence of oil

The Deepwater Horizon oil spill released 4.3 million barrels of crude oil into the Gulf of Mexico (GoM). In the coastal zone of the GoM, phytoplankton are known to be nitrogen limited for most of the year, including the post spill period. Little is known about the phytoplankton response to nutrient availability in the presence of oil. Previous studies show that the diatom Phaeodactylum tricornutum (Pt) is resilient in the presence of oil and it is hypothesized to be an effect of its protective silica frustule. The purpose of this study was to test if nitrate and silica limitation affect the response of Pt in the presence of oil. Diatom cultures were grown in four different f/2 media nutrient treatments for a year: replete, -Si, ¼ N, and combination -Si and ¼ N. Each culture was subsequently inoculated into a control and water accommodated fraction of oil (WAF) treatment for 4 days. Samples were taken daily to monitor photosynthetic efficiency using fluorescence induction and relaxation and PhytoPAM, changes in chlorophyll-a concentrations, concentration of oil in the growth medium, and intracellular and extracellular macromolecular composition. Results showed that photosynthetic efficiency and growth rates were more affected by nitrogen limitation than the presence of oil. Extracellular macromolecular composition showed higher carbohydrate to protein ratios in the nitrogen limited treatments as well. In addition, Pt showed morphological variability with cells linking together in “chains” when Si and N limited for prolonged periods. This study could help us better predict how the GoM phytoplankton may respond to any future oil spills when affected by nutrient stress.