Molecular characteristics and formation mechanisms of chlorinated disinfection byproducts from diatomaceous organic matter: Deciphering intracellular vs. extracellular contributions via FT-ICR MS and machine learning.

Diatom-derived algal organic matter (AOM), including intracellular organic matter (IOM) and extracellular organic matter (EOM), is an important precursor of disinfection byproducts (DBPs) in drinking water sources during diatom blooms. Clarifying the differences in the toxicity and formation mechanisms of chlorinated DBPs between IOM and EOM is crucial for identifying priority removal targets and optimizing drinking water treatment. However, these differences remain poorly understood. In this study, the toxicity, formation characteristics, and underlying mechanisms of IOM-derived DBPs and EOM-derived DBPs were systematically investigated and compared, with a focus on molecular-level interactions. At the same dissolved organic carbon (DOC) concentration (1.135 mgC/L), chlorinated IOM showed higher cytotoxicity than chlorinated EOM. IOM preferentially formed more nitrogenous DBPs, including (semi-)volatile and macromolecular species, whereas EOM produced more carbonaceous DBPs. Fourier transform ion cyclotron resonance mass spectrometry combined with machine learning revealed distinct molecular signatures of DBP precursors in IOM and EOM. Precursors of high-molecular-weight carbonaceous DBPs had lower unsaturation and oxidation degrees but higher aromaticity, whereas precursors of macromolecular nitrogenous DBPs showed the opposite pattern. Given the substantially higher DOC concentration of IOM than EOM at the same diatom density, IOM should be prioritized for removal. These findings provide molecular-level insight for controlling diatom-related DBP risks in drinking water treatment.