Cyanobacterial sensitivity and response to hydrogen peroxide treatment

Oxidative stress caused by reactive oxygen species like hydrogen peroxide (H2O2) is a critical factor in aquatic ecosystems, often triggered by environmental changes such as high light exposure, pollution, or management strategies like chemical treatments to control harmful blooms. Understanding how cyanobacteria respond to such stress is essential for predicting bloom dynamics, assessing the risks associated with toxin production, and developing effective mitigation strategies. To address these questions, we determined the impact of H2O2 on growth, photosynthesis efficiency, toxin production (cylindrospermopsin and microcystins), and the expression of toxin-related genes (cyr and mcy) of different freshwater cyanobacteria. The selected strains of Raphidiopsis raciborskii, Microcystis aeruginosa and Aphanizomenon flos-aquae, originating from several different localities around the world, represent a diverse range of toxin-producing and non-toxin-producing cyanobacteria.  The relative growth trends reveal significant strain-specific differences, with A. flos-aquae 2012/KM1/D3 exhibiting the steepest decline, suggesting a severe impact on its growth. In contrast, R. raciborskii KLL07 showed the least growth reduction, indicative of greater resilience. PSII activity data further supports these observations, with KLL07 showing a marked increase in PSII efficiency at 24 and 48 hours, whereas strains like CS-506 and 2012/KM1/D3 experienced pronounced declines. These results suggest that KLL07 has a robust adaptive mechanism to maintain photosynthetic efficiency and growth under stress, whereas other strains are more susceptible to damage, possibly due to differences in their genetic or physiological responses. Additional data for the toxic strains R. raciborskii and M. aeruginosa, including toxin gene expression and toxin production, provided further insights into their responses to oxidative stress. Expression of the cyr genes in R. raciborskii during H2O2 exposure showed some fluctuations compared to the control, however, statistical analysis did not reveal any significant differences. In contrast, the results for mcy gene expression in M. aeruginosa revealed a downregulation during H2O2 exposure. Furthermore, exposure to 5 mg/L H2O2 resulted in a significant decrease in the intracellular cyanotoxin content (CYN or MC per mL) for all investigated toxic strains after 48 hours of treatment. These findings highlight that oxidative stress affects not only growth and photosynthesis, but also impacts toxin production and gene expression, with toxic strains showing variable responses in their ability to regulate toxin biosynthesis pathways under stress conditions suggesting potential differences in their adaptive mechanisms and resilience to environmental stressors. This variability could have significant ecological implications, influencing bloom toxicity and treatment in natural ecosystems.   Keywords: hydrogen peroxide, Raphidiopsis raciborskii, Microcystis aeruginosa and Aphanizomenon flos-aquae, growth, PSII efficiency, toxicity   Acknowledgements and Funding  The authors would like to thank the student Robert Maziarz and Semko Walat. Nada Tokodi was supported by postdoctoral funding from Narodowa Agencja Wymiany Akademickiej (NAWA) with ULAM grant number PPN/ULM/2019/1/00219. This publication is based upon work from COST Action CYANOACTION (Nr. CA23160) supported by COST (European Cooperation in Science and Technology).

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Primary Presenter: Nada Tokodi, University of Novi Sad (nada.tokodi@dbe.uns.ac.rs)

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Nada Tokodi, University of Novi Sad  (nada.tokodi@dbe.uns.ac.rs)