Bacterial Contribution to Hydrogen Peroxide Decomposition in Lake Erie Cyanobacterial Harmful Algal Bloom Communities Is Dependent on Bloom Stage

ABSTRACT Cyanobacterial harmful algal blooms (cHABs) are expanding globally and often involve toxin‐producing species such as Microcystis aeruginosa that pose ecological and public health risks. Some cyanobacteria, including some strains of M. aeruginosa , lack antioxidant enzymes such as catalase, making them more reliant on associated bacteria for reactive oxygen species (ROS) decomposition. To evaluate bacterial contribution to oxidative stress defence in Microcystis blooms across stages of bloom development, we conducted a time‐series incubation after a pulse of elevated hydrogen peroxide spanning the 2023 Lake Erie bloom season during pre‐bloom, peak bloom, and bloom decline. Metatranscriptomics was used to assess bacterial gene expression for catalase, superoxide dismutase, and alkyl hydroperoxide reductase across 23 bacterial genera. Bacterial transcription of these enzymes was highest during peak bloom, with catalase eliciting the most robust transcriptional response. Bacterial taxa exhibited unique transcription patterns that fell into four categories: first responders, delayed responders, downregulators, or non‐responders, with the largest and most diversified response during peak bloom. These results support the hypothesis that Microcystis partially outsources oxidative stress defence to bacterial partners and provide new evidence that bacterial contribution depends on bloom stage.