Study on photoactivatable toxicity of phycobiliprotein from Microcystis aeruginosa as potential photoinsecticide

Our objective was to study if the phycobiliproteins of the cyanobacterium Microcystis aeruginosa rich in phycoerythrin can be used as a photosensitive insecticide. Microcystis, the main genus of freshwater algal blooms, is a rich source of phycobiliproteins (MC-PBP), which are the light-harvesting pigment proteins of cyanobacteria. In this study, we tested the photoactive toxicity of MC-PBP with mortality assays and morphological analysis in a Spodoptera frugiperda 21 (SF21) cell line, Caenorhabditis elegans, and Chilo suppressalis larvae. We found that MC-PBP was mainly localized in the lysosome in the SF21 cells. The LC50 on SF21 was 80 μg mL−1 with sunlight treatment of 880 μmol photons m−2 s−1 for 15 min and 1.5 mg mL−1 on Chilo suppressalis larva in fourth instar with the sunlight irradiation of 650 μmol photons m−2 s−1 for 5 h. Morphological observation demonstrated that autophagic apoptosis is the main death way of C. elegans, and midgut and fat body are the target tissues of C. suppressalis larvae induced by photoactive toxicity of MC-PBP. Interestingly, we also found that MC-PBP can degrade a certain amount of microcystin-LR (MC-LR), one variant of cyanobacterial microcystins. This is the first report showing that MC-PBP has potential for development into natural photosensitive insecticides which are environmental friendly but toxic for nematode and insect cells with degradation capacity for MC-LR.