Shanlin Wang,Ping Ding,Siyu Lu,Pian Wu,Xiaoqian Wei,Ruixue Huang,Tianhan Kai
As the secondary metabolites of cyanobacterial harmful algal blooms (Cyano-HABs), microcystins (MCs) were generated under various environmental and cellular conditions. The understanding of the causes of MCs generation is of great interest in the field of water treatment and environmental science. In this work, we studied how Microcystis aeruginosa (FACHB-905) cell densities affect the MCs synthetase genes ( mcy ) expression, microcystin-LR (MC-LR) and quorum sensing molecules (Acyl-homoserine lactones (AHLs)) production. An electrochemical sensor was developed here for sensitive and quantitative detection of MC-LR that cultured at different cell densities. The results showed that mcy expression and MC-LR concentration started to increase when the cell density reached ca. 22 × 10 6 cells/mL, and was significantly increased with increasing cell densities. Moreover, the up-regulation of AHLs with increasing cell densities revealed that MC-LR is quorum sensing-mediated. Our results undoubtedly confirmed that MC-LR was produced in a cell density-dependent way that mimics quorum sensing, and the minimum cell density ( ca. 22 × 10 6 cells/mL) that was required to produce MC-LR was provided and offered a reference standard for the prevention and control of MCs pollution in the actual water environment.
The structural specificity of organic nitrogen sources in modulating cyanobacterial physiology and toxin production remains poorly understood. This study systematically evaluated the bioavailability of exogenous glycine peptides in Microcystis aeruginosa ( M. aeruginosa ) and their regulatory roles in algal growth and microcystins (MCs) synthesis through an integrated physiological and transcriptomic approach. Glycine peptides demonstrated hierarchical bioavailability: glycine (Gly) > Glycylglycine (Gly2) > Tetraglycylglycine (Gly4) > Hexaglycylglycine (Gly6) > Cycloglycylglycine (CGly2). Gly and Gly2 were rapidly assimilated by M. aeruginosa , supporting robust algal growth comparable to the control group with nitrate as nitrogen source, while CGly2 and Gly6 induced nitrogen stress, damaging biomass and photosynthetic pigments. Notably, M. aeruginosa cultured with Gly4 as sole nitrogen source uniquely enhanced intracellular MCs production, and shifted MCs variants toward the highly toxic MC-LR. The shift in MCs variants ratios (MC-RR/MC-LR) reflected nitrogen availability, demonstrating a trade-off between nitrogen storage and toxin potency. Transcriptomic profiling unveiled nitrogen-centric metabolic rewiring, resulting in enhanced nitrogen assimilation, mcy genes for MCs biosynthesis, photosynthetic efficiency, and resource reallocation. While exogenous glycine peptides represent only a minor component of organic nitrogen in natural systems, these findings provide mechanistic insights into how peptide structure can influence cyanobacterial physiology and toxin profiles, contributing to a more nuanced understanding of organic nitrogen utilization by harmful cyanobacteria.
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