Toxicity, morphological changes, and dissipation of oil sands naphthenic acids in Chlamydomonas reinhardtii

Abstract

Naphthenic acids fraction components (NAFCs) are a toxic byproduct of oil sands extraction and refining in the Athabasca region of Alberta, Canada, accumulating with other contaminants in the large volumes of oil sands process water produced. NAFCs are incredibly complex mixtures identified as a contaminant of concern across a wide variety of taxa, but the reasons for their acute and chronic toxicity are poorly understood.

Studies were conducted to help ascertain the toxic effects of NAFCs on the unicellular green algae Chlamydomonas reinhardtii wild type (WT) and two cell wall mutants: CC-400, which retains the innermost and outermost glycoprotein wall layers, and CC-3395, believed to be completely naked. The presence of the cell wall was strongly linked to NAFC toxicity. WT cells were most susceptible to NAFC toxicity (growth reduced at 10 mgL-1, but growing even at 100 mgL-1), followed distantly by CC-400 (decreased growth at 100 mgL-1) while CC-3395 was unaffected.

Microscopy experiments (visible light, confocal laser scanning, and Fourier-transform infrared) and thin-layer chromatography of lipids were carried out to observe physiological effects. Exposure to NAFCs induced changes in cell surface protein structure and protein confirmations in WT and CC-400, and altered the diversity and composition of their phospholipid and lipid pools. CC-3395 had minor changes in phospholipid/lipid pools. Exposed WT cells showed evidence of decreased uptake of environmental macromolecules and palmelloid induction. Exposed CC-400 cells exhibited loss of phospholipids and showed some evidence of altered internal membrane and protein structures. CC-3395 showed increased active transport/export after exposure. All exposed cultures had an increase in the size and roundness of cells, as well as increased presence of vacuoles and granules and indicators of osmotic stress or metabolic leakage. These changes were consistent with surfactant exposure, a theory supported by the differences in toxic impacts between cell lines, as surfactant effects vary greatly between similar species and similar compounds.

Studies were also conducted to determine the potential of NAFC biotransformation by C. reinhardtii. Changes in NAFC composition were observed, and were highly specific to compound class, structure, and algal strain. WT and CC-400 were capable of removing classical O2 NAFCs, while modification of O2S and O3 composition were mediated by all three algae.