Diverse Biosynthetic Pathways and Protective Functions Against Environmental Stress of Antioxidants in Microalgae

plants Review Diverse Biosynthetic Pathways and Protective Functions against Environmental Stress of Antioxidants in Microalgae Shun Tamaki 1,* , Keiichi Mochida 1,2,3,4 and Kengo Suzuki 1,5 1 Microalgae Production Control Technology Laboratory, RIKEN Baton Zone Program, Yokohama 230-0045, Japan; [email protected] (K.M.); [email protected] (K.S.) 2 RIKEN Center for Sustainable Resource Science, Yokohama 230-0045, Japan 3 Kihara Institute for Biological Research, Yokohama City University, Yokohama 230-0045, Japan 4 School of Information and Data Sciences, Nagasaki University, Nagasaki 852-8521, Japan 5 euglena Co., Ltd., Tokyo 108-0014, Japan * Correspondence: [email protected]; Tel.: +81-45-503-9576 Abstract: Eukaryotic microalgae have been classified into several biological divisions and have evo- lutionarily 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 Citation: Tamaki, S.; Mochida, K.; Suzuki, K. Diverse Biosynthetic AsA is biosynthesized via the plant pathway in Chlorophyta and Rhodophyta and via the Euglena Pathways and Protective Functions pathway in Euglenophyta, Heterokontophyta, and Haptophyta. The GSH biosynthetic pathway is against Environmental Stress of conserved in all biological kingdoms; however, Euglenophyta are able to synthesize an additional Antioxidants in Microalgae. Plants thiol antioxidant, trypanothione, using GSH as the substrate. In the present study, we reviewed and 2021, 10, 1250. https://doi.org/ discussed the diversity of microalgal antioxidants, including recent findings. 10.3390/plants10061250 Keywords: microalgae; diversity; antioxidant; carotenoid; ascorbate; glutathione; reactive oxygen Academic Editor: Thomas Nägele species; environmental stress Received: 26 May 2021 Accepted: 15 June 2021 Published: 19 June 2021 1. Introduction Eukaryotic microalgae (excluding prokaryotic microalgae in this review) are classified Publisher’s Note: MDPI stays neutral Chlamydomonas reinhardtii with regard to jurisdictional claims in into various phylogenetic divisions, including Chlorophyta (e.g., published maps and institutional affil- and Chlorella vulgaris), Rhodophyta (e.g., Cyanidioschyzon merolae), Heterokontophyta (e.g., iations. 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. Although the genomes of 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 Copyright: © 2021 by the authors. microalgae. To gain this understanding, a wide range of algal species needs to be studied. Licensee MDPI, Basel, Switzerland. This article is an open access article Notably, as microalgae have photosynthetic ability, fast and autotrophic growth, and vari- distributed under the terms and ous material productivity, they have recently attracted attention because their biomass can conditions of the Creative Commons be used to produce food, fuel, and other valuable materials, and outdoor culture equipment Attribution (CC BY) license (https:// has been developed [4–6]. Under outdoor culture conditions, microalgae cannot avoid creativecommons.org/licenses/by/ fluctuating environmental stresses, such as high light, low and high temperatures, and 4.0/). UV irradiation. Exposure to these environmental stresses increases the accumulation of Plants 2021, 10, 1250. https://doi.org/10.3390/plants10061250 https://www.mdpi.com/journal/plants Plants 2021, 10, 1250 2 of 17 reactive oxygen species (ROS), including H2O2, superoxide radical, hydroxyl radical, and singlet oxygen, as they are the byproducts of cellular oxygenic processes. The superoxide radical is generated by the reduction of molecular oxygen in the photosynthetic electron transport chain and the respiratory chain. It is then converted to H2O2 via the reaction with superoxide dismutase (SOD). The hydroxyl radical is generated from H2O2 in the Fenton reaction with free Cu+ or Fe2+. Singlet oxygen is generated by transferring the energy of the photoexcited pigments to molecular oxygen. At appropriate levels, ROS act as signaling molecules that regulate cellular activities, but when accumulated excessively, they oxidize nucleic acids, proteins, and lipids, leading to oxidative stress damage in cells [7,8]. 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. 2. Carotenoids 2.1. Carotenoid Compounds Carotenoids are isoprenoid compounds with C40 backbones, and their colors range from yellow to red. In nature, more than 750 carotenoid compounds have been structurally defined, and among them, at least 44 are found in eukaryotic microalgae. Most microalgae possess β-carotene and zeaxanthin, whereas other carotenoid compounds are extremely diverse depending on their phylogeny (Figure1)[ 10,11]. Just like land plants, Chloro- phyta species contain an abundance of β-carotene, lutein, neoxanthin, and violaxanthin. Specific carotenoid compounds, such as loroxanthin and siphonaxanthin, were also detected in Chlorophyta [10–13]. In macrophytic-type Rhodophyta (e.g., Porphyra umbilicalis), lutein is a major carotenoid compound, but it is absent in microphytic-type C. merolae, in which β-carotene and zeaxanthin are the predominant carotenoid compounds [14,15]. Diadinoxanthin and fucoxanthin are the major carotenoid compounds in Heterokonto- phyta, Haptophyta, and Dinophyta, whereas only diadinoxanthin is present in Eugleno- phyta [10,11,16–19]. In some microalgae (e.g., Chlorophyta species Haematococcus pluvialis and Chromochloris zofingiensis), astaxanthin synthesis is induced under various stress conditions, such as high light and high salinity [20]. Astaxanthin has also been detected in non-photosynthetic E. gracilis, suggesting that Euglenophyta can potentially synthesize this carotenoid compound [21]. In microalgae, these diverse carotenoid compounds are valuable chemotaxonomic biomarkers [10]. 2.2. Carotenoid Biosynthesis In eukaryotic microalgae, the biosynthetic pathway from phytoene to lycopene is conserved, whereas the downstream pathway from lycopene to each end carotenoid compound is diverse, as reviewed below (Figure1). 2.2.1. Lycopene Synthesis Isopentenyl pyrophosphate (IPP), which is a C5 unit of the carotenoid backbone, is synthesized via the mevalonate (MVA) pathway or via the non-mevalonate (1-deoxy-D- xylulose 5-phosphate/2-C-methylerythritol 4-phosphate, DOXP/MEP) pathway. Most microalgae utilize the DOXP/MEP pathway, whereas the Euglenophyta species are excep- tionally dependent on the MVA pathway [11,22]. IPP is added to farnesyl pyrophosphate (C15), produced from three IPP molecules, by geranylgeranyl pyrophosphate (GGPP, C20) synthase (CrtE, also called GGPS). Two GGPP molecules are condensed by phytoene synthase (CrtB, also called PSY) to produce phytoene (C40). Phytoene is the first carotenoid compound, and its synthesis is known to be the rate-limiting step in the carotenoid biosynthetic pathway [23]. It has been reported that mutations in the PSY gene in C. reinhardtii Plants 2021, 10, 1250 3 of 17 Plants 2021, 10, x FOR PEER REVIEW 3 of 18 cause carotenoid deficiency, colorless cell appearance, and ROS accumulation [24,25], and crtB gene knockdown in E. gracilis showed a similar tendency [19]. Figure 1. Carotenoid structures and biosynthesis in microalgae. Most microalgae contain a series Figure 1. Carotenoid structures and biosynthesis in microalgae. Most microalgae contain a series of carotenoid compounds, from isopentenyl pyrophosphate to violaxanthin; the exception are red of carotenoid compounds, from isopentenyl pyrophosphate to violaxanthin; the exception are red algae (e.g., Cyanidioschyzon merolae), which lack violaxanthin. α-carotene, lutein, loroxanthin, and Cyanidioschyzon merolae α algaesiphonaxanthin (e.g., are found in Chlo),rophyta, which lack diadinoxanthin violaxanthin. and-carotene, diatoxathin lutein, are found loroxanthin, in Hetero- and siphonaxanthinkontophyta, Haptophyta, are found in Dinophyta, Chlorophyta, and diadinoxanthin Euglenophyta, andfucoxanthin diatoxathin is found are found in Heterokonto- in Heterokon-

Load more