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

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.

Suppression of the Lycopene Cyclase Gene Causes Downregulation of Ascorbate Peroxidase Activity and Decreased Glutathione Pool Size, Leading to H2O2 Accumulation in Euglena gracilis

“…Genetic analysis suggests that two endosymbiotic events occurred; in the first, the euglenophyte acquired red lineage genes from Chromalveolata-like prey algae; in the second, green lineage genes from prasinophyte-like green algae were acquired ( Maruyama et al, 2011 ). Since diadinoxanthin, the major carotenoid species of E. gracilis , is present in algae classified in Chromalveolata, such as diatoms and haptophytes, and not in green algae (reviewed in Tamaki et al, 2021 ), E. gracilis is believed to have acquired a series of carotenoid synthetic genes, including LCY , in the first endosymbiotic event. However, our analysis indicates that EgLCY is phylogenetically distinct from both diatom and green algae LCYs ( Supplementary Figure 2 ).…”

“…The unicellular microalga Euglena gracilis contains diadinoxanthin, diatoxanthin, neoxanthin, and β-carotene as the major carotenoid species ( Kato et al, 2017 ). We have genetically and biochemically characterized several carotenoid synthetic genes in E. gracilis to understand the carotenoid synthesis pathway (reviewed in Tamaki et al, 2021 ). In the E. gracilis carotenoid synthetic pathway ( Figure 1 ), the most upstream carotenoid species, phytoene, is synthesized from isopentenyl pyrophosphate by geranylgeranyl pyrophosphate synthase (CrtE) and phytoene synthase (CrtB; Kato et al, 2016 ).…”

“…In many organisms, including animals, plants, and algae, carotenoids act as reactive oxygen species (ROS) scavengers ( Gammone et al, 2015 ; Foyer, 2018 ; Tamaki et al, 2021 ). ROS, such as hydrogen peroxide (H 2 O 2 ), superoxide radicals, hydroxyl radicals, and singlet oxygen are byproducts of aerobic cellular processes, including photosynthesis and respiration, and cause oxidative damage to cells upon excessive accumulation.…”

“…Many organisms have developed a variety of ROS scavenging systems to avoid ROS toxicity ( Apel and Hirt, 2004 ). In addition to carotenoids that mainly scavenge singlet oxygen and do not react with H 2 O 2 and superoxide radicals, other antioxidants, such as ascorbate and glutathione, directly react with various ROS molecules and scavenge them in many organisms including plants and algae ( Foyer, 2018 ; Smirnoff and Arnaud, 2019 ; Tamaki et al, 2021 ). In photosynthetic organisms, including plants and algae, ascorbate peroxidase (APX) detoxifies H 2 O 2 by using reduced ascorbate as an electron donor.…”

“…Ascorbate and glutathione are oxidized during this cycle, but their oxidized forms are regenerated by the ascorbate-glutathione cycle consisting of dehydroascorbate reductase (DHAR), monodehydroascorbate reductase (MDAR), and glutathione reductase (GR). These antioxidants and enzymes are physiologically important as a defense system against ROS-derived oxidative stress ( Mittler et al, 2004 ; Tamaki et al, 2021 ). E. gracilis accumulates both ascorbate and glutathione at high levels and contains ascorbate-glutathione cycle enzymes that have been well characterized biochemically, whereas catalase is absent in this alga ( Shigeoka et al, 1980 , 1987a , b ; Ishikawa et al, 2017 ).…”