There have been growing interests in the biorefining of bioactive compounds from marine microalgae, including pigments, omega-3 fatty acids or antioxidants for use in the nutraceutical and cosmetic sectors. This study focused on the comparative responses of five marine microalgal species from different lineages, including the dinoflagellate Amphidinium carterae, chlorophyte Brachiomonas submarina, diatom Stauroneis sp., haptophyte Diacronema sp. and rhodophyte Rhodella violacea, to exposure during their batch growth to hydrogen peroxide (H 2 O 2 ). A. carterae returned an enhanced signal with the DPPH assay (8.8 µmol Trolox eq/g DW) when exposed to H 2 O 2 , which was associated with reduced pigment yields and increased proportions in saturated C16 and C18 fatty acids. B. submarina showed enhanced antioxidant response upon exposure to H 2 O 2 with the DPPH assay (10 µmol Trolox eq/g DW), a threefold decrease in lutein (from 2.3 to 0.8 mg/g) but a twofold increase in chlorophyll b (up to 30.0 mg/g). Stauroneis sp. showed a downward response for the antioxidant assays, but its pigment yields did not vary significantly from the control. Diacronema sp. showed reduced antioxidant response and fucoxanthin content (from 4.0 to 0.2 mg/g) when exposed to 0.5 mM H 2 O 2 . R. violacea exposed to H 2 O 2 returned enhanced antioxidant activity and proportions of EPA but was not significantly impacted in terms of pigment content. Results indicate that H 2 O 2 can be used to induce stress and initiate metabolic changes in microalgae. The responses were however speciesspecific, which would require further dosage optimisation to modulate the yields of specific metabolites in individual species. Keywords Microalgae • Hydrogen peroxide • Antioxidants • Pigments • FAMEs Research Highlights• Comparative response to H 2 O 2 treatment assessed for 5 microalgal species from different lineages.• Results showed an enhanced antioxidant response for Rhodella violacea and an overall reduction for the other species. • Pigment yield reduction was observed for most species except for chlorophyll b, which was enhanced in Brachiomonas submarina.• Species-specific variations in the proportions of fatty acids were observed.• Principal component analysis clearly separated the H 2 O 2 -treated Rhodella violacea set from the others, with higher antioxidant response and proportions of EPA.

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