Cellular, inter‐organismal and cross kingdom communication via extracellular vesicles (EVs) is intensively studied in basic science with high expectation for a large variety of bio‐technological applications. EVs intrinsically possess many attributes of a drug delivery vehicle. Beyond the implications for basic cell biology, academic and industrial interests in EVs have increased in the last few years. Microalgae constitute sustainable and renewable sources of bioactive compounds with a range of sectoral applications, including the formulation of health supplements, cosmetic products and food ingredients. Here we describe a newly discovered subtype of EVs derived from microalgae, which we named nanoalgosomes. We isolated these extracellular nano‐objects from cultures of microalgal strains, including the marine photosynthetic chlorophyte Tetraselmis chuii, using differential ultracentrifugation or tangential flow fractionation and focusing on the nanosized small EVs (sEVs). We explore different biochemical and physical properties and we show that nanoalgosomes are efficiently taken up by mammalian cell lines, confirming the cross kingdom communication potential of EVs. This is the first detailed description of such membranous nanovesicles from microalgae. With respect to EVs isolated from other organisms, nanoalgosomes present several advantages in that microalgae are a renewable and sustainable natural source, which could easily be scalable in terms of nanoalgosome production.

Biophysical and biochemical characterisation of microalgae-derived extracellular vesicles.

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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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