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The green microalga Dunaliella salina hyperaccumulates β‐carotene in the chloroplast, which turns its cells orange. This does not occur in the sister species Dunaliella tertiolecta. However, the molecular mechanisms of β‐carotene hyperaccumulation were still unclear. Here, we discovered the reasons for β‐carotene hyperaccumulation by comparing the morphology, physiology, genome, and transcriptome between the carotenogenic D. salina and the noncarotenogenic D. tertiolecta after transfer to high light. The differences in photosynthetic capacity, cell growth, and the concentration of stored carbon suggest that these species regulate the supply and utilization of carbon differently. The number of β‐carotene‐containing plastid lipid globules increased in both species, but much faster and to a greater extent in D. salina than in D. tertiolecta. Consistent with the accumulation of plastid lipid globules, the expression of the methyl‐erythritol‐phosphate and carotenoid biosynthetic pathways increased only in D. salina, which explains the de novo synthesis of β‐carotene. In D. salina, the concomitantly upregulated expression of the carotene globule proteins suggests that hyperaccumulation of β‐carotene also requires a simultaneous increase in its sink capacity. Based on genomic analysis, we propose that D. salina has genetic advantages for routing carbon from growth to carotenoid metabolism. Summary statement: Under high light, Dunaliella salina undergoes significant metabolic shifts, while Dunaliella tertiolecta primarily focuses on acclimation in photosynthesis. This metabolic contrast may explain the β‐carotene hyperaccumulation by redirecting carbon from growth to carotenoid metabolism shown in D. salina. [ABSTRACT FROM AUTHOR]

Photosystems must balance between light harvesting to fuel the photosynthetic process for CO2 fixation and mitigating the risk of photodamage due to absorption of light energy in excess. Eukaryotic photosynthetic organisms evolved an array of pigment‐binding proteins called light harvesting complexes constituting the external antenna system in the photosystems, where both light harvesting and activation of photoprotective mechanisms occur. In this work, the balancing role of CP29 and CP26 photosystem II antenna subunits was investigated in Chlamydomonas reinhardtii using CRISPR‐Cas9 technology to obtain single and double mutants depleted of monomeric antennas. Absence of CP26 and CP29 impaired both photosynthetic efficiency and photoprotection: Excitation energy transfer from external antenna to reaction centre was reduced, and state transitions were completely impaired. Moreover, differently from higher plants, photosystem II monomeric antenna proteins resulted to be essential for photoprotective thermal dissipation of excitation energy by nonphotochemical quenching. The functional role of the Photosystem II monomeric antenna complexes CP29 and CP26 was investigated in Chlamydomonas reinhardtii generating single and double mutants by CRISPR‐Cas9 genome editing demonstrating their key role in non‐photochemical quenching of both Photosystem I and II. [ABSTRACT FROM AUTHOR]