Phenyllactic Acid Suppresses Growth and Induces Coordinated Changes in PSII Photochemistry and Energy Dissipation in the Filamentous Cyanobacterium Dolichospermum sp. FACHB-82

Dolichospermum sp. FACHB-82, a representative harmful bloom-forming cyanobacterium, has garnered widespread attention in freshwater ecosystems due to its high environmental adaptability and potential for rapid proliferation. Phenyllactic acid (PLA) is a structurally defined aromatic organic acid with known antimicrobial activity, but its inhibitory effects on FACHB-82 and their photosynthetic apparatus remain poorly understood. In this study, we investigated the concentration-dependent effects of PLA on biomass accumulation, photosynthetic pigment production and photosynthetic activity in FACHB-82. The findings revealed that PLA induced a concentration-dependent biphasic response on biomass accumulation and photosynthetic pigment synthesis of FACHB-82. Chlorophyll a fluorescence analyses revealed that high-concentration PLA exposure disrupted functions of PSII at multiple levels, including reduced PSII performance, enhanced QA− accumulation, accelerated reaction center closure, decreased energetic connectivity, impaired donor side function, and restricted downstream electron transport. Although NPQ was significantly upregulated under high PLA stress, dark-recovery analysis demonstrated that the increased non-photochemical energy dissipation was accompanied by incomplete recovery of PSII photochemical efficiency. Our finding, for the first time, provides physiological insight into the anticyanobacterial activity of PLA for the control of FACHB-82 growth, thus offering a theoretical basis for the control of cyanobacterial blooms.