Diatomaceous organic matter is overlooked but forms disinfection byproducts of high cytotoxicity during chlorination

Diatomaceous organic matter (DAOM) has the potential to be the main precursor of disinfection byproducts (DBPs) in multiple drinking water sources during diatom blooms. However, characterization of DAOM and subsequent formation of chlorination DBPs, especially at the molecular level, have rarely been studied, let alone the links between DAOM chemodiversity and DBPs' cytotoxicity. Herein three types of DAOM derived from Cyclotella meneghiniana, Synedra ulna, and Fragilaria nanana - which are common species during diatom blooms in drinking water sources - were selected. Cyanobacterial organic matter (CAOM) that originated from the common bloom-forming cyanobacterium Microcystis aeruginosa was also selected to thoroughly compare the differences. Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR/MS) revealed that DAOM contained more lipids, while the content of proteins in CAOM was higher. Compared with CAOM, DAOM had fewer molecules with CHON formulas, but more with CHO formulas, lower molecular weight, fewer aromatic and unsaturated compounds, and higher hydrogen saturation. High-molecular-weight DBPs (more than two carbon atoms) made up the majority of DBPs (67.7-78.7 %). DAOM generated more high-molecular-weight carbonaceous DBPs, but CAOM formed more macromolecular nitrogenous DBPs after chlorination. Chlorination of DAOM mainly occurred through chlorine substitution, and the proportion of precursors associated with substitution reactions in DAOM was larger than that in CAOM. Furthermore, the cytotoxicity of chlorinated DAOM was obviously higher than that of CAOM at the same algal density (about 2.3-3.1 times). This study provides new insights into the formation of DBPs, especially the unknown macromolecular DBPs, and the potential cytotoxicity of DBPs during chlorination of DAOM-containing water.