Impact of volatile aromatic hydrocarbons on algal plasma membranes

Crude oil is toxic to phytoplankton. The most toxic components are volatile aromatic hydrocarbons (VAH). However, mechanisms of toxicity at the molecular level are not clear. Plasma membrane composition and physical state are critically important to any cell. Living cells maintain constant membrane fluidity (homeoviscous adaptation) by adjusting membrane lipid composition in response to many environmental stimuli. If toxicity of VAH is due to disruption of optimal physical state of plasma membranes, organisms might maintain efficient membrane function by adjusting fatty acid content of membrane lipids. Detailed analysis of plasma membrane chemistry called for isolation and purification of plasma membranes. We have modified established procedures to successfully obtain sufficient quantities of membranes from three species of marine microalgae: Tetraselmis suecica, Chaetoceros gracilis, and Isochrysis galbana. Following characterization of plasma membrane fatty acid composition under normal conditions, we exposed cells growing in continuous culture to increasing concentrations of VAH (up to lethal levels) each time comparing plasma membrane fatty acid composition to controls. During exposure, ratios of saturated:unsaturated fatty acids as well as absolute levels of saturated fatty acids in plasma membranes increase significantly in a time and dose dependent manner. Additionally, cells adapt to exposure by incorporating longer chain fatty acids into plasma membranes. When VAH are removed, absolute levels of membrane fatty acids return to near pre-exposure levels. These data are consistent with our hypothesis and suggest that VAH act as lipid solvents and increase fluidity of plasma membranes. The fluorescent probe diphenylhexatriene (DPH) provided more direct evidence of membrane fluidity changes resulting from VAH exposure. With increasing concentrations of VAH, membrane fluidity increases in a dose-dependent manner. All these experiments provide evidence showing one mechanism of VAH toxicity is disruption of the structural integrity of plasma membranes and homeoviscous adaptation as a principal mechanism by which algal cells adapt to VAH exposure. A model is introduced illustrating the toxic mechanism and the means by which algal cells adapt to sub-lethal VAH exposure. Also discussed are possible mechanisms by which cells sense membrane fluidity changes that might utilized in signal transduction pathways to respond and adapt to VAH exposure.