The ability of hydrogen peroxide (H₂O₂) to degrade saxitoxin-, microcystin-, anatoxin-, and non-toxin-producing strains of the harmful cyanobacterium, Dolichospermum

Hydrogen peroxide (H₂O₂) has been used to mitigate cyanobacterial harmful algal blooms (CHABs), yet little is known about how H₂O₂ affects specific CHAB-forming genera as well as cyanotoxins beyond microcystin. This project examined the dose-dependent effects of H₂O₂ on six strains of Dolichospermum spp. including those that produce saxitoxin, anatoxin-a, and microcystin. Beyond toxins, this study quantified changes in photosynthetic efficiency, cell density, H₂O₂ concentration, and N₂-fixation rates. All strains were sensitive to H₂O₂ with responses being dependent on dose (0–30 mg L⁻¹) of H₂O₂, cell density, and strain. At 1 × 10⁵ cells mL⁻¹, 4 mg H₂O₂ L⁻¹ significantly reduced cell density, photosynthetic efficiency, toxins, and N₂-fixation rates of all strains (p < 0.05 for all compared to controls). At 1 × 10⁶ cells mL⁻¹, however, higher doses of H₂O₂ were needed to reduce one or more of the variables, with some strains unaffected by as much as 15 mg L⁻¹, a concentration known to harm zooplankton and invertebrates. While H₂O₂ degraded anatoxin-a at all cell densities and doses, at 1 × 10⁶ cells mL⁻¹ neither microcystin nor saxitoxin were significantly degraded after four days, even by 15 mg H₂O₂ L⁻¹, despite significant reduction in Dolichospermum cell densities. This finding suggests that during dense Dolichospermum blooms, H₂O₂ treatment may destroy cells but may concurrently liberate saxitoxin or microcystin that persists in the water column and enters food webs. Collectively, this study demonstrated that although H₂O₂ can efficiently lyse Dolichospermum cells, doses needed to mitigate dense blooms of all strain types (≥15 mg L⁻¹) may harm non-target organisms and may not effectively degrade saxitoxin and microcystin.