ABSTRACT: Guanitoxin, a neurotoxin produced by cyanobacteria and the only known naturally occurring organophosphate, poses environmental risks comparable to synthetic organophosphate insecticides. Its effects on aquatic organisms, particularly in combination with compounds sharing similar mechanisms of action, remain poorly understood. This study evaluated the developmental toxicity of guanitoxin, alone and combined with the organophosphate insecticides malathion and trichlorfon, in zebrafish (Danio rerio) embryos and larvae at environmentally relevant concentrations. Developmental, neurobehavioral, and molecular endpoints were assessed. All compounds exhibited dose-dependent toxicity, with mixtures significantly enhancing both lethal and sublethal effects. The trichlorfon + guanitoxin combination was particularly potent, causing embryo coagulation, cardiac arrest, hatching failure, and pronounced pericardial swelling. Sublethal effects were consistently observed across mixtures, including edema, craniofacial malformations, muscle disorganization, and strong hypoactivity. Transcriptomic profiling revealed downregulation of genes critical for muscle contraction, synaptic signaling, energy metabolism and oxidative stress. The highest number of differentially expressed genes was observed in the trichlorfon + guanitoxin mixture (1348 genes). Synergistic interactions were observed, with mixture-induced effects exceeding those of individual compounds. Together, these findings demonstrate that guanitoxin, particularly in combination with synthetic organophosphates, poses a significant ecological risk and highlights the importance of considering mixture toxicity in environmental risk assessments.
