Abstract
Eukaryotic microalgae have been classified into several biological divisions and have evolutionarily acquired diverse morphologies, metabolisms, and life cycles. They are naturally exposed to environmental stresses that cause oxidative damage due to reactive oxygen species accumulation. To cope with environmental stresses, microalgae contain various antioxidants, including carotenoids, ascorbate (AsA), and glutathione (GSH). Carotenoids are hydrophobic pigments required for light harvesting, photoprotection, and phototaxis. AsA constitutes the AsA-GSH cycle together with GSH and is responsible for photooxidative stress defense. GSH contributes not only to ROS scavenging, but also to heavy metal detoxification and thiol-based redox regulation. The evolutionary diversity of microalgae influences the composition and biosynthetic pathways of these antioxidants. For example, α-carotene and its derivatives are specific to Chlorophyta, whereas diadinoxanthin and fucoxanthin are found in Heterokontophyta, Haptophyta, and Dinophyta. It has been suggested that AsA is biosynthesized via the plant pathway in Chlorophyta and Rhodophyta and via the Euglena pathway in Euglenophyta, Heterokontophyta, and Haptophyta. The GSH biosynthetic pathway is conserved in all biological kingdoms; however, Euglenophyta are able to synthesize an additional thiol antioxidant, trypanothione, using GSH as the substrate. In the present study, we reviewed and discussed the diversity of microalgal antioxidants, including recent findings.
Highlights
IntroductionEukaryotic microalgae (excluding prokaryotic microalgae in this review) are classified into various phylogenetic divisions, including Chlorophyta (e.g., Chlamydomonas reinhardtii and Chlorella vulgaris), Rhodophyta (e.g., Cyanidioschyzon merolae), Heterokontophyta (e.g., the diatom Phaeodactylum tricornutum), Haptophyta (e.g., Emiliania huxleyi), Dinophyta (e.g., Symbiodinium minutum), and Euglenophyta (e.g., Euglena gracilis) [1]; their evolution, morphology, habitat, and metabolism are extremely diverse
Eukaryotic microalgae are classified into various phylogenetic divisions, including Chlorophyta (e.g., Chlamydomonas reinhardtii and Chlorella vulgaris), Rhodophyta (e.g., Cyanidioschyzon merolae), Heterokontophyta, Haptophyta (e.g., Emiliania huxleyi), Dinophyta (e.g., Symbiodinium minutum), and Euglenophyta (e.g., Euglena gracilis) [1]; their evolution, morphology, habitat, and metabolism are extremely diverse
It has been reported that a wide range of microalgae (Chlorophyta C. reinhardtii and D. tertiolecta, Rhodophyta C. merolae, diatoms P. tricornutum and Thalassiosira weissflogii, and Euglenophyta E. gracilis) markedly induced γEC, GSH, and PC synthesis and resisted heavy metal toxicity when exposed to cadmium [116,117,118,119,120,122,123,124,125]
Summary
Eukaryotic microalgae (excluding prokaryotic microalgae in this review) are classified into various phylogenetic divisions, including Chlorophyta (e.g., Chlamydomonas reinhardtii and Chlorella vulgaris), Rhodophyta (e.g., Cyanidioschyzon merolae), Heterokontophyta (e.g., the diatom Phaeodactylum tricornutum), Haptophyta (e.g., Emiliania huxleyi), Dinophyta (e.g., Symbiodinium minutum), and Euglenophyta (e.g., Euglena gracilis) [1]; their evolution, morphology, habitat, and metabolism are extremely diverse. C. reinhardtii and C. merolae were sequenced prior to the genomes of other algal species [2,3], research using limited algal species is not sufficient to understand the biology of diverse microalgae. Microalgae cannot avoid fluctuating environmental stresses, such as high light, low and high temperatures, and UV irradiation. Exposure to these environmental stresses increases the accumulation of reactive oxygen species (ROS), including H2 O2 , superoxide radical, hydroxyl radical, and singlet oxygen, as they are the byproducts of cellular oxygenic processes. To avoid ROS-induced cytotoxicity, organisms have developed various antioxidants, including carotenoids, ascorbate, and glutathione These antioxidants are the key factors in determining the environmental stress tolerance and outdoor growth efficiency of microalgae [9]. This review describes recent findings regarding the diverse biosynthetic pathways and functions of these antioxidants which act to relieve environmental stress in microalgae
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