Granular hydrogen dioxide efficacy: Granular hydrogen dioxide selectively controls blue-green algae at lower concentrations. At higher concentrations it controls many algae but is not effective on which group?

• A. Blue-green algae
• B. Green algae
• C. Diatoms
• D. Macroalgae such as Chara or Nitella

Answer: (D)

Granular hydrogen dioxide (hydrogen peroxide) kills cells by oxidation, and its effectiveness depends on the target organism’s biology. Microalgae and cyanobacteria have smaller, simpler cells that are more readily damaged by the oxidant, so they’re controlled at lower concentrations. When concentrations are higher, the product can affect many algae, but larger aquatic macrophytes with tougher tissues—like macroalgae such as Chara or Nitella—are not effectively controlled because their size and structure hinder penetration and damage. That’s why the correct answer is macroalgae such as Chara or Nitella.