Molecular Drivers of Contrasting Photoreactivity in Extracellular versus Intracellular Organic Matter from Chlorophyta and Cyanobacteria

Algal organic matter (AOM) is a critical precursor for aquatic photochemical reactions, yet how molecular composition relates to the contrasting photoreactivity of extracellular and intracellular organic matter (EOM/IOM) remains insufficiently understood. This study systematically analyzed the photoreactivity of EOM and IOM derived from representative Chlorophyta and Cyanobacteria. Integrating optical spectroscopy and ultrahigh-resolution mass spectrometry (FT-ICR MS), we characterized the molecular features associated with their photosensitization capacity. EOM, dominated by humic-like substances, functions as a more effective photosensitizer with reactive species (RS) quantum yields up to 22.5-fold higher than protein-like IOM. At the taxonomic level, Chlorophyta-EOM outperformed Cyanobacteria-EOM, whereas Cyanobacteria-IOM exhibited pigment-mediated reactivity. NaBH 4 reduction coupled with FT-IR and 2D-COS suggested that EOM photoreactivity was more sensitive to borohydride-sensitive chromophoric moieties, whereas IOM appeared less affected by borohydride treatment. FT-ICR MS revealed EOM contained relatively lignin/CRAM-like and unsaturated molecular features, while irradiated IOM evolved into more saturated and recalcitrant residues, indicative of photochemical aging. These findings provide molecular-level insights into the divergent environmental fates of algal fractions, highlighting EOM as an important contributor to oxidative self-purification, while irradiation of IOM was associated with photochemical aging and the formation of more saturated residual molecular signatures.