Ecological risks of DOM produced during cyanobacterial lysis induced by algicidal bacterial metabolites: Degradation features and toxicity toward algae

Dissolved organic matter (DOM) released during cyanobacterial lysis induced by algicidal bacteria represents a poorly understood byproduct in biological bloom-control strategies, with limited understanding of its environmental behavior and ecological risks. This study characterized DOM generated during the algicidal action induced by sterile extracellular filtrates from Bacillus altitudinis G3 on Microcystis aeruginosa (DOM-G3). We compared it with DOM from healthy algae (DOM-Ma), focusing on differences in structural characteristics, degradation behavior, and biotoxicity under both without and with Cd 2+ -stressed conditions. DOM-G3 exhibited markedly higher aromaticity, hydrophobicity, molecular weight, and proportions of protein- and humic-like substances ( p  < 0.05). These features contributed to its faster photodegradation (53.0%) and higher initial microbial degradation during the first day (64.6% at 25 °C) than DOM-Ma (27.0% and 55.5%, respectively, p  < 0.01). However, after 14 days, DOM-G3 exhibited a slightly lower microbial degradation ratio than DOM-Ma (74.4% vs. 78.9%), indicating a shift toward long-term stability. Bioassays demonstrated that DOM-G3 exerted stronger intrinsic inhibition on M. aeruginosa growth. Under Cd 2+ co-exposure, DOM-G3 initially alleviated metal toxicity, but subsequently caused a 40.6% enhancement in algal mortality, revealing a composition-dependent biphasic effect. In contrast, DOM-Ma consistently mitigated Cd 2+ toxicity and promoted post-stress recovery. These findings provide the first evidence that DOM produced during algicidal bacteria–mediated cyanobacterial lysis possesses distinct structural features and composition-dependent ecological risks, particularly its potential to shift from protective to harmful roles under metal stress. The findings highlight the necessity of incorporating DOM-mediated risks into the environmental assessment and safe application of microbial bloom-control technologies.