Abstract
Microcystis aeruginosa is a ubiquitous, toxic, bloom-forming cyanobacterium found in freshwater ecosystems worldwide. In this study, we assessed two algaecidal mitigation methods (hydrogen peroxide and l-lysine) for the treatment of two toxic and two non-toxic strains of M. aeruginosa. Whole-transcriptome analysis with total RNA sequencing was used to assess the modes of action of these algaecides. We found both treatments led to increased transcriptional activity for genes associated with sulfur and cysteine pathways, which both support antioxidant enzyme activity. These regulatory mechanisms along with methionine metabolism were more enhanced for l-lysine treated strains, and may demonstrate a lack of l-lysine regulatory mechanisms and potential impact to the biosynthesis of other amino acids as well as intracellular sulfur recycling processes seen in transcription results. Toxic and non-toxic strains displayed a differential regulatory response to reduce oxidative stress levels. Two toxic strains downregulated the expression of microcystin biosynthesis genes and increased the expression of genes encoding antioxidant enzymes. Our results provide further support for the view that microcystins do not offer protection against high levels of oxidative stress. Production of microcystin may interfere with antioxidant enzyme functionality, and additionally, sulfur and cysteine demands for antioxidant enzymes likely outcompete their use for microcystin production under oxidative stress conditions. These findings can aid in the mitigation of toxin-producing M. aeruginosa blooms, suggesting high-dose hydrogen peroxide application has the potential to lower bloom toxicity through fast-acting impairment to microcystin gene synthetase expression as well as other metabolic functioning, likely lowering intracellular microcystin content, which may be released upon cell lysis after treatments. To our knowledge, the expression of microcystin synthetase genes after exposure to l-lysine has never been examined before. Seeing as this is a safe algaecide specifically targeting M. aeruginosa, this was another beneficial finding.