Turning red: comparative biology of secondary carotenoid induction in carotenogenic green algae

A limited number of green microalgae develop conspicuous red or orange stress phenotypes through massive accumulation of secondary carotenoids in diverse habitats, including marine lagoons and coastal salterns, hypersaline lakes, freshwater habitats, snowfields, and aeroterrestrial environments. These “turning red” phenotypes occur in phylogenetically distinct algae exposed to extreme or highly variable environmental conditions and represent integrated stress-acclimation responses rather than simple changes in pigment composition. This work compares stress-tolerant, red-forming green algae across contrasting habitats to distinguish shared physiological responses from lineage-specific regulatory, metabolic, and intracellular sequestration strategies. Carotenoid induction is a quantitative response to interacting stresses, particularly irradiance and nutrient limitation, with the relative hierarchy and optimal intensity of these factors differing among taxa. Secondary carotenogenesis is frequently coupled with lipid metabolism, chloroplast remodeling, and redistribution of cellular carbon. Although carotenoid biosynthesis is primarily plastidial, the reviewed taxa differ markedly in intracellular trafficking and sequestration, ranging from plastid-retained β-carotene in Dunaliella to extraplastidial ketocarotenoid storage in cytoplasmic lipid-rich compartments, with some Coelastrella strains additionally producing water-soluble AstaP carotenoproteins. Despite this cellular and regulatory diversity, photoprotection, mitigation of oxidative stress, and enhanced survival under unfavorable conditions emerge as broadly convergent adaptive functions. Several carotenogenic algae are established industrial sources of high-value carotenoids, whereas others offer emerging opportunities for integrated environmental and circular biotechnologies, with production constraints differing across platforms. This cross-habitat comparative framework identifies conserved and lineage-specific features of secondary carotenogenesis and provides a basis for mechanistically informed optimization of carotenoid production.