The Loroxanthin Cycle: A New Type of Xanthophyll Cycle in Green Algae (Chlorophyta)

Biochemical and phylogenetic analyses of light-harvesting complexes from Tetraselmis striata

Oxygenic photosynthetic organisms employ light-harvesting complexes (LHCs) to capture solar energy and regulate excess excitation. Tetraselmis species belong to Chlorodendrophyceae, one of the earliest-diverging lineages within core Chlorophyta. While these organisms exhibit distinctive pigment compositions, their LHC organization and function remain largely uncharacterized. Here, we examined the biochemical and spectral properties of LHC, PSI-LHCI, and PSII-LHCII complexes from Tetraselmis striata NIES-1019. Pigment analysis identified loroxanthin derivatives, loroxanthin decenoate and loroxanthin dodecenoate, in all three complexes. Notably, these carotenoids are absent in Chlamydomonas reinhardtii and Ostreococcus tauri, implying a lineage-specific adaptation. Fluorescence spectra of PSII-LHCII and PSI-LHCI from T. striata exhibited distinct characteristics compared with their counterparts in C. reinhardtii and land plants, indicating differences in pigment organization. In contrast, LHC fluorescence properties closely resembled those of green-lineage organisms, suggesting conservation of chlorophyll-binding arrangements. Phylogenetic analyses revealed that T. striata possesses LHCBM-based LHCII trimers, consistent with other core Chlorophyta, but its PSI antenna composition diverges from that of these algae. Among LHCIs in the PSI outer belt, only LHCA5a was identified, whereas LHCA4a and LHCA6a were absent, implying structural divergence from C. reinhardtii. These findings provide insights into the evolution of LHCs in Chlorophyta and the distinct pigment-protein interactions underlying Tetraselmis light-harvesting strategies.

Regulation of Microalgal Photosynthetic Electron Transfer

The global ecosystem relies on the metabolism of photosynthetic organisms, featuring the ability to harness light as an energy source. The most successful type of photosynthesis utilizes a virtually inexhaustible electron pool from water, but the driver of this oxidation, sunlight, varies on time and intensity scales of several orders of magnitude. Such rapid and steep changes in energy availability are potentially devastating for biological systems. To enable a safe and efficient light-harnessing process, photosynthetic organisms tune their light capturing, the redox connections between core complexes and auxiliary electron mediators, ion passages across the membrane, and functional coupling of energy transducing organelles. Here, microalgal species are the most diverse group, featuring both unique environmental adjustment strategies and ubiquitous protective mechanisms. In this review, we explore a selection of regulatory processes of the microalgal photosynthetic apparatus supporting smooth electron flow in variable environments.

CRISPR/Cas9-Mediated Knockout of the Lycopene ε-Cyclase for Efficient Astaxanthin Production in the Green Microalga Chlamydomonas reinhardtii

Carotenoids are valuable pigments naturally occurring in all photosynthetic plants and microalgae as well as in selected fungi, bacteria, and archaea. Green microalgae developed a complex carotenoid profile suitable for efficient light harvesting and light protection and harbor great capacity for carotenoid production through the substantial power of the endogenous 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway. Previous works established successful genome editing and induced significant changes in the cellular carotenoid content in Chlamydomonas reinhardtii. This study employs a tailored carotenoid pathway for engineered bioproduction of the valuable ketocarotenoid astaxanthin. Functional knockout of lycopene ε-cyclase (LCYE) and non-homologous end joining (NHEJ)-based integration of donor DNA at the target site inhibit the accumulation of α-carotene and consequently lutein and loroxanthin, abundant carotenoids in C. reinhardtii without changes in cellular fitness. PCR-based screening indicated that 4 of 96 regenerated candidate lines carried (partial) integrations of donor DNA and increased ß-carotene as well as derived carotenoid contents. Iterative overexpression of CrBKT, PacrtB, and CrCHYB resulted in a 2.3-fold increase in astaxanthin accumulation in mutant ΔLCYE#3 (1.8 mg/L) compared to the parental strain UVM4, which demonstrates the potential of genome editing for the design of a green cell factory for astaxanthin bioproduction.

Metabolic engineering and synthetic biology strategies for producing high-value natural pigments in Microalgae

Microalgae are microorganisms capable of producing bioactive compounds using photosynthesis. Microalgae contain a variety of high value-added natural pigments such as carotenoids, phycobilins, and chlorophylls. These pigments play an important role in many areas such as food, pharmaceuticals, and cosmetics. Natural pigments have a health value that is unmatched by synthetic pigments. However, the current commercial production of natural pigments from microalgae is not able to meet the growing market demand. The use of metabolic engineering and synthetic biological strategies to improve the production performance of microalgal cell factories is essential to promote the large-scale production of high-value pigments from microalgae. This paper reviews the health and economic values, the applications, and the synthesis pathways of microalgal pigments. Overall, this review aims to highlight the latest research progress in metabolic engineering and synthetic biology in constructing engineered strains of microalgae with high-value pigments and the application of CRISPR technology and multi-omics in this context. Finally, we conclude with a discussion on the bottlenecks and challenges of microalgal pigment production and their future development prospects.

Accessible versatility underpins the deep evolution of plant specialized metabolism

The evolution of several hallmark traits of land plants is underpinned by phytochemical innovations. The specialized metabolism of plants can appear like a teeming chaos that has yielded an ungraspable array of chemodiversity. Yet, this diversity is the result of evolutionary processes including neutral evolution, drift, and selection that have shaped the metabolomic networks. Deciphering the evolutionary history of the specialized metabolome in the context of plant terrestrialization has only just begun. Studies on phytochemistry of model organisms and crop plants enabled the sketch of a blueprint for the biochemical landscape of land plants and a good idea on the diversity that can be explored. Evolutionary metabolomics has in the past been successfully used to identify traits that were critical for domestication of angiosperms or to unravel key innovations in land plants. Owing to recent advances in the study of non-model land plants and their close streptophyte algal relatives we can now begin to appreciate the variation of metabolic networks across the green lineage-and understand convergent solutions to similar environmental challenges and effects that plant terrestrialization had on these networks. Here, we highlight the significant progress made with regard to identifying metabolomic diversity by adding non-model organisms to the equation. We discuss the role of neutral evolution in the context of metabolomic diversity and the effects that environmental challenges had on the lineage-specific specialized metabolism from an evolutionary point of view.