The interplay of low HO doses, lytic cyanophage, and Microcystis aeruginosa: Implications for cyanobacterial bloom control and microcystin production/release.

Low HO doses can suppress cyanobacterial blooms without damaging non-target species but enable undesirable regrowth. Besides, the role of cyanophage in preventing regrowth after low HO exposure remains unclear. Applying phages to cyanobacteria pre-exposed to low HO in early growth stages may improve host removal and reduce microcystin (MC) production/release. Lytic cyanophage MDM-1 with a 172 PFU/cell burst size, 2-day short latent period against MCs-producing Microcystis, shows high HO stability. Low HO (1 to 2.5 mg/L) doses significantly (p < 0.05) inhibited Microcystis aeruginosa growth rate, biofilm and MCs concentration reduction in a dose-dependent manner but regrowth occurred at all concentrations. Phage treatment eliminated cells without HO pretreatment within 3 days and reduced MC production. HO-pretreated M. aeruginosa cells altered the phage dynamics, affecting adsorption, latency, production, and cell lysis in response to HO-induced oxidative stress. At 1.5 mg HO/L pretreatment, cells were eliminated with reduced MC production, like untreated cells. HO pretreatment with 2.0 and 2.5 mg/L resulted in an extension of the phage absorption phase and the latent period. This was accompanied by a reduction in lysis efficacy, attributed to the increased ROS production. At 2.5 mg HO/L, 17.10 % of phages remain un-adsorbed, with cell lysis rate dropped from 0.89 d to 0.26 d compared to the untreated control. The highest phage titer (70 %) was obtained with 1.5 mg/HO pretreated cells. This study emphasizes that low-dose HO eliminates Microcystis but severely affects phage lysis and MCs release depending on HO-induced ROS levels. It is a crucial consideration when using phages to control cyanobacterial blooms with HO-induced stress.