Nitrogen-dependent luteolin effect on Microcystis growth and microcystin-pollution risk - Novel mechanism insights unveiled by comparative proteomics and gene expression.

Phytogenic allelochemical luteolin has potential to mitigate Microcystis-dominated cyanobacterial blooms (MCBs), but its algicidal effect against toxigenic Microcystis may be impacted by natural factors, especially nitrogen (N) level in waters. This study innovatively explored N-dependent effect of luteolin on Microcystis growth and its microcystins (MCs) production/release, and elucidated underlying mechanisms from proteomics and gene expression views. Generally, at each N level, rising luteolin dose progressively inhibited Microcystis growth by inhibiting proteins syntheses and genes expression involving light-capturing, photosynthetic electron transfer, Calvin cycle and phosphorus (P) acquisition, according to comparative proteomics and gene expression. At higher luteolin dose and lower N level, Microcystis cell tended to increase microcystins (MCs) production and conservation ability, with the highest increase degree observed at 12 mg/L luteolin and 0.5 mg/L N on day 10, reaching 1.96 and 2.68 folds of luteolin-free control, respectively, but decrease MC-release as extracellular MCs content (EMC), with inhibition ratio of 72.86%, 73.57%, 74.45% and 40.58%, 45.28%, 60.00% at rising N level under 12 mg/L luteolin stress on day 10 and 16, respectively. These enabled cellular defensive response of Microcystis to stronger stress and N limitation. Under luteolin stress, higher N level more strongly up-regulated numerous processes (e.g., oxidoreductase activity, ATP binding and transmembrane transport, oxidative phosphorylation, tricarboxylic acid cycle, fatty acid biosynthesis, glycolysis/gluconeogenesis, pyruvate, amino acids metabolism, metal ion-binding, P acquisition) as compensative protective responses to progressively down-regulated photosynthetic and ribosomal processes at higher N level, thus causing faster Microcystis growth than lower N level. This study provided novel insights for N-dependent effect and mechanisms of luteolin on MCBs mitigation and MCs risk control, and guided algicidal application of luteolin in different eutrophic-degree waters.