Algicidal bacteria induce a molecular stress response in Microcystis aeruginosa and Aphanizomenon gracile leading to physiological alterations and cell death

Great interest has been placed on studying algicidal bacteria to control cyanobacterial outbreaks in freshwater ecosystems. However, the mechanism of changes at the molecular and cellular level in cyanobacteria stressed by algicidal bacteria requires better exploration. Here, the physiological stress of Microcystis aeruginosa and Aphanizomenon gracile strains, two well-known bloom-forming cyanobacteria, was described when exposed to algicidal bacteria Morganella morganii SU7S0818 and Bacillus pumilus SU8S0818 (isolated from a eutrophic reservoir). The results indicated that both bacteria induced a strong algicidal activity, although Aphanizonmenon strains were more sensitive than Microcystis. Studied cyanobacterial strains accumulated high amounts of reactive oxygen species (ROS) contributing to almost complete cell death after six days of co-culture. Gene expression analysis based on the upregulation of the psbA and recA genes for photosynthesis and DNA repair mechanisms, respectively, showed that A. gracile strains responded slower to cellular stress (24-30 h) than M. aeruginosa strains (12 h). Microscopic analyses revealed the increase of cyanophycin granules in affected cyanobacterial cells, which may indicate the development of a survival mechanism under algicidal stress conditions. These findings are critical to understanding and managing the problem of global expansion of harmful algal blooms caused by different species of cyanobacteria.