Influence of bloom stage on the effectiveness of algicidal bacteria in controlling harmful cyanobacteria: A microcosm study

Cyanobacterial harmful algae blooms (cyanoHABs) pose significant ecological and public health concerns in freshwater ecosystems worldwide. Understanding the dynamics of phytoplankton communities and the efficacy of mitigation strategies is crucial for managing bloom events. This study investigates the impact of bioaugmentation with algicidal bacteria on Microcystis-dominated blooms through laboratory microcosm experiments. Field-collected samples from MID- and POST-summer bloom stages were treated with Morganella morganii, Exiguobacterium acetylicum, and a bacterial consortium including Bacillus pumilus. Phytoplankton composition, microcystin concentrations and genes related to microbial community dynamics (16S, mcyA, nosZ and amoA) were assessed by microscopy, HPLC and qPCR, respectively. Results showed that M. morganii significantly altered the phytoplankton community structure and promoted diatom proliferation in MID-summer microcosms, though treatments were less effective in POST-summer microcosms representing more mature bloom periods. Additionally, algicidal bacteria influenced microcystin levels, with M. morganii and E. acetylicum reducing toxigenic Microcystis genotypes, as indicated by lower mcyA gene copy numbers. Molecular analyses also revealed that algicidal bacterial treatments contributed to shifts in microbial functional genes, including increased denitrification activity linked to nosZ gene abundance. These findings highlight the intricate interplay between algicidal bacteria and microbial communities, where algicidal activity extends beyond direct cyanobacteria suppression to broader ecosystem-level effects. By rebalancing phytoplankton communities toward eukaryotic dominance and reducing toxigenic cyanobacterial genotypes during intense bloom episodes, bioaugmentation with algicidal bacteria emerges as a promising strategy for bloom management and ecosystem restoration.