BACTERIA-MEDIATED ANTAGONISTIC MECHANISMS AND  DEGRADATION OF MICROCYSTINS AS MEASURES OF CONTROLLING  CYANOBACTERIAL BLOOMS 

The occurrence of toxic and nuisance cyanobacterial blooms in freshwater is increasing 
worldwide. Cyanolytic bacteria that co-exist with cyanobacteria play a significant role in 
regulating cyanobacterial blooms and degrading cyanotoxins. Therefore, this study 
hypothesized that cyanolytic bacteria associated with blooms could be isolated and used as 
a promising strategy to control cyanobacterial blooms and cyanotoxins. However, screening 
bacteria and understanding cyanolysis mechanisms and cyanotoxin degradation are crucial 
in developing efficient bacteria-mediated strategies to mitigate cyanobacterial blooms. This 
study aimed to investigate the antagonistic mechanisms of locally isolated cyanolytic 
bacterial strains towards frequently found bloom-forming cyanobacteria and their potential 
to degrade microcystins (MCs) in vitro assays. In total, 24 bacterial strains capable of lysing 
cells of toxic Microcystis aeruginosa were identified, of which 21 strains degraded MCs. 
Two of the most efficient lytic strains, LB-C (Klebsiella pneumoniae LB-C; MZ007852) 
and BG-E (Pseudomonas fluorescens BG-E; MZ007859), isolated from Wilpita wewa, 
Matara, and Bandagiriya wewa, Hambantota, respectively, were screened for the lysis of 
potent bloom-forming filamentous cyanobacteria. BG-E exerted the highest cyanolytic 
activity, 82% and 73%, against filamentous Pseudanabaena sp. LW2 and Pseudanabaena 
lonchoides LW1, respectively, following ten days of inoculation. The underlying 
antagonistic mechanisms of BG-E against the two Pseudanabaena species were 
investigated by time-course experiments and scanning electron microscopy (SEM). BG-E 
showed the highest cell lysis in Pseudanabaena sp. LW2 when treated with bacterial culture, 
whereas its cell-free supernatant was effective against P. lonchoides LW1. Therefore, its 
cyanolytic mode against two Pseudanabaena species appeared to be species-specific. The 
results of time-course experiments showed activation of cellular antioxidants, viz. catalase, 
peroxidase, ascorbic peroxidase, and superoxide dismutase, in both Pseudanabaena species 
in response to the oxidative stress induced by P. fluorescens and its secreted metabolites in 
the cell-free supernatant. Further, the induced stress has significantly (P