A combination cyanobacterial treatment approach using hydrogen peroxide and L-lysine successfully improved the removal efficiency of toxic cyanobacteria.

Harmful cyanobacterial blooms have been increasing globally, introducing new challenges for protecting aquatic ecosystems and human health. A combined algaecide treatment, similar to combination antibiotic therapy, may more rapidly and effectively remove cyanobacteria by broad targeting of different growth mechanisms, reducing the recovery of bloom-forming cyanobacteria. To confirm this hypothesis, hydrogen peroxide (10.5 mg/L), L-lysine (8.2 mg/L), and mixed treatment (hydrogen peroxide: 8.4 mg/L; L-lysine: 6.7 mg/L) of both chemicals were examined for cyanobacterial removal in a 7-day mesocosm study. We found that both hydrogen peroxide and L-lysine effectively reduced cyanobacterial abundance to less than 1% at the end of the experiment. Mixed mesocosm phytoplankton communities responded similarly to hydrogen peroxide treatment throughout the sampling period. Microcystis abundance sharply dropped within 24 h (94% reduction) and Microcystis was no longer detected by day 7. Of the three treatments, the mixed treatment had the greatest impact on Microcystis abundance and gene expression. In lysine mesocosms, cyanobacteria were replaced by Chlorophyta, whereas the hydrogen peroxide and mixed treatments led to a shift toward the eustigmatophyte Nannochloropsis. Transcriptomics revealed that heterotrophic bacteria such as Exiguobacterium, which was resilient to hydrogen peroxide and mixed treatments, benefited from increased catalase expression, which helped mitigate oxidative stress and allowed them to dominate in bacterial succession. Lysine toxicity on microbial communities and taxa sensitivity likely stemmed from amino acid homeostasis disruptions as well as abnormal lysine riboswitch and degradation activity. Overall, a combination cyanobacterial treatment approach using hydrogen peroxide and L-lysine successfully improved the removal efficiency of toxic cyanobacteria.