Light is a paramount parameter driving photosynthesis. However, excessive irradiance leads to the formation of reactive oxygen species that cause cell damage and hamper the growth of photosynthetic organisms. Xanthophylls are key pigments involved in the photoprotective response of plants and algae to excessive light. Of particular relevance is the operation of xanthophyll cycles (XC) leading to the formation of de-epoxidized molecules with energy dissipating capacities. Neoxanthin, found in plants and algae in two different isomeric forms, is involved in the light stress response at different levels. This xanthophyll is not directly involved in XCs and the molecular mechanisms behind its photoprotective activity are yet to be fully resolved. This review comprehensively addresses the photoprotective role of 9′-cis-neoxanthin, the most abundant neoxanthin isomer, and one of the major xanthophyll components in plants’ photosystems. The light-dependent accumulation of all-trans-neoxanthin in photosynthetic cells was identified exclusively in algae of the order Bryopsidales (Chlorophyta), that lack a functional XC. A putative photoprotective model involving all-trans-neoxanthin is discussed.

Bryopsidales (Chlorophyta) are cultured and consumed in several regions of the planet and are known for their high nutritional value and bioprospection potential, due to a high content of relevant polar lipids and polysaccharides. Among other characteristic features, these marine algae are known for possessing unique photosynthetic pigment-protein complexes and for the absence (in nearly all species investigated) of a functional xanthophyll cycle, a ubiquitous photoprotection mechanism present in most algae and plants. With the aim of characterizing the photophysiology of this atypical group of algae, we investigated the changes in pigment content and polar lipidome of two Bryopsidales species (Codium tomentosum and Bryopsis plumosa) exposed for 7 days to low or high irradiance (20 vs. 1,000 μmol photons m–2 s–1). Our results show that high light has a strong effect on the pigment composition, triggering the time-dependent accumulation of all-trans-neoxanthin (t-Neo) and violaxanthin (Viola). High light-acclimated macroalgae also displayed a shift in the characteristic polar lipidome, including a trend of accumulation of lyso-glycolipids, and highly unsaturated phospholipids and betaine lipids. We hypothesize that the observed shifts on the lipid composition could promote the interaction between t-Neo and Viola with the siphonaxanthin–chlorophyll–protein complexes (SCP) of photosystem II (PSII) within the thylakoid membranes of the chloroplasts. Light induced changes in pigment and lipid composition could contribute to the fitness of Bryopsidales algae by reducing damages to the photosynthetic apparatus under increased irradiance.