Aqueous photooxidation of live bacteria

Live bacteria in atmospheric aqueous droplets are exposed to photooxidants such as hydroxyl radicals (·OH), organic triplet excited states (³C*) and singlet oxygen (¹O₂). These photooxidants are produced from photochemical processes involving organic matter present in atmospheric aqueous droplets. ·OH is the photooxidant known to drive many aqueous photochemical processes. Even though the ·OH photooxidation of organic matter in atmospheric aqueous droplets has been widely studied, equivalent investigations on the ·OH photooxidation of bioaerosols are limited. Little is known about the daytime encounters between ·OH and live bacteria in atmospheric aqueous droplets.

We investigated the aqueous ·OH photooxidation of four bacterial strains in microcosms composed of artificial cloud water that simulated the chemical composition of cloud water in South China. The survival rates for the four bacteria strains decreased to zero within 6 hours during exposure to 1 × 10⁻¹⁶ M of ·OH under artificial sunlight. Bacterial cell damage and lysis released biological and organic compounds, which were subsequently oxidized by ·OH. We used ultrafiltration to separate the water-soluble biological and organic compounds into different molecular weight fractions and found that the molecular weights of some of these biological and organic compounds were larger than 50 kDa. The biological and organic compounds were identified as proteinaceous-like and humic-like components by excitation emission matrix fluorescence spectroscopy with parallel factor analysis. High-resolution mass spectrometry measurements revealed that the O/C, H/C, and N/C elemental ratios increased at the initial onset of photooxidation. As the photooxidation progressed, there were little changes in the H/C and N/C, whereas the O/C continued to increase for hours after all the bacterial cells have died. The increase in the O/C was due to functionalization and fragmentation reactions, which increased the O content and decreased the C content, respectively. We observed that fragmentation reactions played particularly important roles in transforming the biological and organic compounds. These fragmentation reactions cleaved the C-C bonds of carbon backbones of higher molecular weight proteinaceous-like matter to form a variety of lower molecular weight compounds, including humic-like components of molecular weight <3 kDa and highly oxygenated organic compounds of molecular weight <1.2 kDa. We also investigated the propensity of the biological and organic compounds from the bacteria to produce ·OH, ¹O₂, and ³C* upon illumination with artificial sunlight. The steady-state concentrations and quantum yields of the three photooxidants produced varied among the different molecular weight-separated fractions due to the diversity of their chemical composition and optical properties. Using a variety of correlation analysis and machine learning techniques, we identified various chemical and optical parameters that correlated particularly well with the steady-state concentrations or quantum yields of the three photooxidants. Overall, our results provided new insights at the process level on the photooxidation of live bacteria in atmospheric aqueous droplets.