Molecular-Level Composition and Acute Toxicity of Photosolubilized Petrogenic Carbon

To examine the molecular-level composition and acute toxicity per unit carbon of the petroleum-derived dissolved organic matter (DOM_HC) produced via photo-oxidation, heavy and light oils were irradiated over seawater with simulated sunlight. Increases in dissolved organic carbon concentrations as a function of time were associated with changes in the DOM_HC composition and acute toxicity per unit carbon. Parallel factor analysis showed that the fluorescent dissolved organic matter (FDOM) composition produced from the heavy oil became more blue-shifted over time, while the light oil produced a mixture of blue- and red-shifted components similar to FDOM signatures. Ultrahigh-resolution mass spectrometry reveals that the composition of the DOM_HC produced from both heavy and light oils was initially relatively reduced, with low O/C. With time, the composition of the DOM_HC produced from the heavy oil shifted to unsaturated, high-oxygen compounds, while that produced from the light oil comprised a range of high O/C aliphatic, unsaturated, and aromatic compounds. Microtox assays suggest that the DOM_HC initially produced is the most toxic (62% inhibition); however, after 24 h, a rapid decrease in toxicity decreased linearly to 0% inhibition for the heavy DOM_HC and 12% inhibition for the light DOM_HC at extended exposure periods.