Natural and artificial light-harvesting systems utilizing the functions of carotenoids

Abstract

Carotenoids are essential pigments in natural photosynthesis. They absorb in the blue-green region of the solar spectrum and transfer the absorbed energy to (bacterio-)chlorophylls, and so expand the wavelength range of light that is able to drive photosynthesis. This process is an example of singlet-singlet energy transfer and so carotenoids serve to enhance the overall efficiency of photosynthetic light reactions. Carotenoids also act to protect photosynthetic organisms from the harmful effects of excess exposure to light. In this case, triplet-triplet energy transfer from (bacterio-)chlorophyll to carotenoid plays a key role in this photoprotective reaction. In the light-harvesting pigment-protein complexes from purple photosynthetic bacteria and chlorophytes, carotenoids have an additional role, namely the structural stabilization of those complexes. In this article we review what is currently known about how carotenoids discharge these functions. The molecular architecture of photosynthetic systems will be outlined to provide a basis from which to describe the photochemistry of carotenoids, which underlies most of their important functions in photosynthesis. Then, the possibility to utilize the functions of carotenoids in artificial photosynthetic light-harvesting systems will be discussed. Some examples of the model systems are introduced.

Red Light Control of β-Carotene Isomerisation to 9-cis β-Carotene and Carotenoid Accumulation in Dunaliella salina

“…1 O 2 react readily with fatty acids to form lipid peroxides and will set up a chain of oxygen activation events that may eventually lead to a hyperoxidant state and cell death [29]. Carotenoids protect the photosystems in the following ways: (i) by reacting with lipid peroxidation products and terminating free radical chain reactions as a result of the presence of the polyene chain; (ii) by scavenging 1 O 2 and dissipating the energy as heat; and (iii) by reacting with triplet excited chlorophyll 3 Chl* to prevent formation of 1 O 2 or by dissipation of excess excitation energy through the xanthophyll cycle [3,30,31].…”

“…Carotenoids are synthesized by photosynthetic organisms for light-harvesting and for photo-protection of the pigment-protein light-harvesting complexes and photosynthetic reaction centres in the thylakoid membrane [1,2,3,4]. Dunaliella salina , a halotolerant chlorophyte, is one of the richest sources of natural carotenoids, and accumulates up to 10% of the dry biomass as β-carotene under conditions that are sub-optimal for growth, i.e., high light intensity, sub-optimal temperatures, nutrient limitation and high salt concentrations [5,6,7,8].…”

Carotenoid Production by Dunaliella salina under Red Light

“…Carotenoids are orange, yellow or red pigments which are synthesized by all photosynthetic organisms for light-harvesting and for photo-protection, and for stabilising the pigment–protein light-harvesting complexes and photosynthetic reaction centres in the thylakoid membrane. They may also be accumulated by some non-photosynthetic archaea, bacteria, fungi and animals for pigmentation [1,2,3]. Carotenoids are also the precursors of a range apocarotenoids of biological and commercial importance, such as the phytohormone abscisic acid, the visual and signalling molecules retinal and retinoic acid, and the aromatic or volatile beta-ionone [4].…”

Photoprotective Substances Derived from Marine Algae

“…These photosynthetic pigments consist of two classes of molecules: carotenes and xanthophylls. Carotenoids play an important role in photosynthetic light-harvesting complexes; they absorb the solar spectrum in the blue-green region and transfer the energy to chlorophylls [ 21 ]. Furthermore, carotenoids also act as a photoprotector in photosynthetic organisms.…”