Review

Carotenoids and photoprotection in plants: A role for the xanthophyll zeaxanthin

Photochemical reflectance index (PRI) is an optical index that reflects the de-epoxidation state of the xanthophyll cycle. It is strongly correlated with non-photochemical quenching (NPQ) and is expected to be a good indicator of plant stress status in remote sensing. However, PRI is also influenced by other factors, such as leaf pigment composition, and varies widely among plant species, which prevented its practical use in the past. Since the slope of the NPQ-PRI relationship is generally shared among leaves, NPQ can be correctly predicted if the PRI value of NPQ zero (PRI₀) is known. Here we conducted the correlation analysis with PRI₀ and 64,606,179 optical indices across leaves of 22 species from different functional types. Twelve indices were selected that were strongly correlated with PRI₀ and were less sensitive to light intensity. PRI corrected with these 12 optical indices successfully predicted NPQ. Our pigment analysis suggested that PRI₀ was not explained by the content of single pigment but was related to the relative contents of xanthophylls and chlorophylls. The indices proposed in this study are expected to be useful in predicting plant photosynthesis and stress status in remote sensing, contributing to future agriculture technology.

Diatoms, a major group of algae, account for about a quarter of the global primary production on Earth. These photosynthetic organisms face significant challenges due to light intensity variations in their underwater habitat. To avoid photodamage, they have developed very efficient non-photochemical quenching (NPQ) mechanisms. These mechanisms originate in their light-harvesting antenna – the fucoxanthin–chlorophyll protein (FCP) complexes. Spectroscopic studies of NPQ in vivo are often hindered by strongly overlapping signals from the photosystems and their antennae. Fortunately, in vitro FCP aggregates constitute a useful model system to study fluorescence (FL) quenching in diatoms. In this work, we present streak-camera FL measurements on FCPa and FCPb complexes, isolated from a centric diatom Cyclotella meneghiniana, and their aggregates. We find that spectra of non-aggregated FCP are dominated by a single fluorescing species, but the FL spectra of FCP aggregates additionally contain contributions from a redshifted emissive state. We relate this red state to a charge transfer state between chlorophyll c and chlorophyll a molecules. The FL quenching, on the other hand, is due to an additional dark state that involves incoherent energy transfer to the fucoxanthin carotenoids. Overall, the global picture of energy transfer and quenching in FCP aggregates is very similar to that of major light-harvesting complexes in higher plants (LHCII), but microscopic details between FCPs and LHCIIs differ significantly.

This article explores the rising interest in microalgae-derived carotenoids due to their unique properties and wide-ranging industrial potential. These pigments offer higher concentrations and greater diversity compared to other sources, particularly featuring carotenoids like astaxanthin, β-carotene, lutein, and zeaxanthin, known for their potent antioxidant properties that benefit human health. The review provides a comprehensive examination of carotenoids, encompassing their structures, origins, extraction methods, and applications in various sectors such as food, pharmaceuticals, cosmetics, and dietary supplements. It underscores their significance as antioxidants, their role in enhancing human health, and ongoing research in this field. The article also investigates the feasibility of utilizing microalgae as a commercial substrate for carotenoid production, emphasizing their unique qualities and potential future applications. In addition, it discusses the distinctions between natural and synthetic carotenoids, carotenoid synthesis within microalgae, large-scale cultivation, upstream and downstream processing, and commercial applications. The novelty of this review lies in its emphasis on the growing importance and potential of microalgal carotenoids across diverse industries, highlighting their unique attributes, versatility, and considerable value for human health and well-being. It further acknowledges the continuous advancements in cultivation techniques, extraction methods, and biotechnological approaches, indicating the expanding commercialization and utilization of microalgal carotenoids.

Ageing in dry chlorophyllous propagules is leaded by photooxidation through the photosynthetic machinery, but why species differ in longevity and the ageing mechanisms of when light and oxygen are absent are unknown. We hypothesize that the cellular antioxidant capacity is key for the inter- and intra-specific differences in the ageing process. We have tested this hypothesis in chlorophyllous spores of two ferns. They were subjected to four different storage regimes resulting from light/dark and normoxia/hypoxia combinations. Lipophilic and hydrophilic antioxidants, reactive oxygen species (ROS), and photosynthetic pigments were analysed in parallel to germination and the recovery of Fv/Fm over a storage period of up to 22-months. We show that light and oxygen accelerate the ageing process, but their mechanisms (ROS, increase, antioxidant capacity decrease, loss of efficiency of the photosystem II, pigment degradation) appear the same under all conditions tested. The end of the asymptomatic phase of longevity, when a sudden drop of germination occurs, seems to be determined by a threshold in the depletion of antioxidants. Our results support the hypothesis that ageing kinetics in dry plant propagules is determined by the antioxidant system, but also suggests an active role of the photosynthetic machinery during ageing, even in darkness and hypoxia.

For a high production rate of microalgae, a high-cell-density culture is an effective strategy for reducing the light intensity per dry weight in the culture to avoid photoinhibition under high light conditions. However, under high cell density microalgae are exposed to nutrient-deficient conditions, resulting in a reduced biomass production rate. To enhance the production rate of high-value bioactives such as fucoxanthin and eicosatetraenoic acid (EPA) by the diatom Chaetoceros gracilis, a perfusion culture was operated at harvesting rates of 10–40 % under light intensities of 300–2000 μmol m⁻² s⁻¹. At a harvesting rate of 20 % under a high light intensity of 2000 μmol m⁻² s⁻¹, the dry weight increased, and the areal productivity reached 15.6 ± 1.3 g-dry weight (dw) m⁻² d⁻¹, which is comparable to the maximum value previously reported in diatoms. The high biomass production, even at a low harvesting rate of 20 %, could result from sufficient nutrient supply by perfusion culture. At a harvesting rate of 20 %, the EPA areal productivity achieved 183 mg-dw m⁻² d⁻¹ even under 2000 μmol m⁻² s⁻¹ due to the increase in the dry weight using the perfusion culture. However, the fucoxanthin content decreased under 2000 μmol m⁻² s⁻¹. A strong relationship was observed between the specific light intensity per dry weight and fucoxanthin content in C. gracilis. This relationship indicates that the fucoxanthin content does not increase unless the specific light intensity per dry weight is adjusted below 20 μmol g-dw⁻¹ s⁻¹. Thus, perfusion culture proved to be useful to control the biomass-specific light intensity for the effective production of EPA and fucoxanthin in C. gracilis under high light conditions expected for outdoor culture.