We have studied carotenoid biosynthesis in the prasinophycean alga Mantoniella squamata, which contains a great variety of carotenoids belonging to both the alpha- and the beta-carotene (Car) biosyntheses pathways. This unusual carotenoid composition allowed us to address the problem of how biosynthesis on the alpha- and the beta-Car pathway is regulated in response to different light regimes. We found that illumination with 4 h of actinic high light (HL, 250 micromol m(-2) s(-1)), and culture growth in permanent light (PL, 60 micromol m(-2) s(-1)). induced the de novo synthesis of violaxanthin (Vx) cycle pigments belonging to the beta-Car pathway. Carotenoid synthesis on the alpha-Car biosynthesis pathway led to a strong accumulation of lutein (L) and dihydrolutein (DhL). Both the newly synthesised Vx cycle pigments and L/DhL can be regarded as intermediate pools of carotenoids that were converted to light-harvesting pigments in low light (LL) periods following the phases of HL illumination. This transition to the light-harvesting pigments included the conversion of Vx to neoxanthin (Nx) on the beta-Car pathway, and the transformation of L/DhL to prasinoxanthin (Px), the main light-harvesting pigment of M. squamata belonging to the alpha-carotenoids. Isolation of light-harvesting complexes from L-enriched M. squamata cells showed that both L and the Vx cycle pigments were loosely bound to the LHC apoprotein. This peripheral binding is in agreement with their proposed role as intermediate pigments in the biosynthesis of light-harvesting pigments, and should allow a smooth detachment from the protein in periods of LL when the synthesis of Nx and Px is stimulated. We conclude that carotenoid synthesis proceeds in general in an economical way: in both, the alpha- and the beta-Car pathway HL induces the formation of photoprotective pigments, which in LL periods following the HL illumination are not degraded, but on the contrary are converted to light-harvesting pigments.