Photosynthetic organisms fuel their metabolism with light energy and have developed for this purpose an efficient apparatus for harvesting sunlight. Chlorophylls serve as light-harvesting antennae by capturing the sunlight and are assisted in their light-harvesting role by carotenoids.
The light-harvesting complex (LHC) is an array of protein and chlorophyll molecules embedded in the thylakoid membrane of plants and cyanobacteria, which transfer light energy to one chlorophyll a molecule at the reaction center of a photosystem (the functional unit in photosynthesis). It consists of a number of chromophores which are complex subunit proteins. LHC is used by plants and photosynthetic bacteria to collect more of the incoming light than would be captured by the photosynthetic reaction center alone. The light which is captured by the chromophores is capable of exciting molecules from their ground state to a higher energy state, known as the excited state. This excited state does not last very long and is known to be short-lived and molecules can return to the ground state or another electronic state of the same molecule. When the excited molecule has a nearby neighbor molecule, the excitation energy may also be transferred, through electromagnetic interactions, from one molecule to another.
Light-harvesting complexes are found in a wide variety among the different photosynthetic species. The main light harvesting complex in Green bacteria is known as the chlorosome. The chlorosome is equipped with rod-like BChl c aggregates with protein embedded lipids surrounding it. Chlorosomes are found outside of the membrane which covers the reaction centre.
In purple bacteria, a light harvesting complex consisting of two pigment protein complexes referred to as LH1 and LH2. The LH1 complex surrounds the reaction centre, while the LH2 complexes are arranged around the LH1 complexes and the reaction centre in a peripheral fashion. Purple bacteria use bacteriochlorophyll and carotenoids to gather light energy. These proteins are arranged in a ring-like fashion creating a cylinder that spans the membrane.
Cyanobacteria that grow in deep-shaded environments can acclimate to harvest far-red light (700–800 nm) by remodeling both their antenna and core photosystems. Under visible light, these organisms produce only one type of Chl, Chl a, and absorb visible photons. Under far red light, however, they become capable of harvesting less energetic quanta by synthesizing the red-shifted Chls f and d. Far red light-induced photoacclimation (FaRLiP) involves replacing the photosystems expressed in (WL-photosystems) with new photosystems containing far red light-specific paralog protein subunits. Far red light-photosystems incorporate a small number of chlorophylls f and d allowing them to absorb up to 750 nm (FRL-PSII), or even 800 nm (FRL-PSI). Under far red light, new APC paralog subunits are produced, which assemble into bicylindrical cores (FRL-BCs). While WL-PBSs absorb up to 670–680 nm, FRL-BCs contain a large number of red-shifted bilins absorbing above 700 nm. The organization of phycobiliproteins in FaRLiP organisms is species-dependent.
In plants, Chlorophyll and carotenoids are important in light-harvesting complexes. Chlorophyll b is almost identical to chlorophyll a, except it has a formyl group in place of a methyl group. Chlorophyll b absorbs light with between 400 and 500 nm more efficiently. Carotenoids are long linear organic molecules that have alternating single and double bonds along their length. Such molecules are called polyenes. Lycopene and beta carotene are examples of carotenoids. These molecules also absorb light most efficiently in the 400 – 500 nm range. Due to their absorption region, carotenoids appear red and yellow and provide most of the red and yellow colours present in fruits and flowers.
The light harvesting complex of cyanobacteria, glaucocystophyta and red algae is known as the phycobilisome which is composed of linear tetrapyrrole pigments. Pigment-protein complexes referred to as R-phycoerythrin are rod-like in shape and make up the rods and core of the phycobilisome. The pigments, such as phycocyanobilin and phycoerythrobilin are the chromophores that bind through a covalent thioether bond to their apoproteins at cysteins residues. The apoprotein with its chromophore is called phycocyanin, phycoerythrin, and allophycocyanin, respectively.
References
Lokstein, Heiko & Renger, Gernot & Götze, Jan Philipp. (2021). Photosynthetic Light-Harvesting (Antenna) Complexes—Structures and Functions. Molecules. 26. 3378. 10.3390/molecules26113378.
Toru Kondo, Shigeru Itoh, Masahiro Matsuoka, Chihiro Azai, and Hirozo Oh-oka. The Journal of Physical Chemistry B 2015 119 (27), 8480-8489 DOI: 10.1021/acs.jpcb.5b03723
Mascoli, V., Bhatti, A.F., Bersanini, L. et al. The antenna of far-red absorbing cyanobacteria increases both absorption and quantum efficiency of Photosystem II. Nat Commun 13, 3562 (2022). https://doi.org/10.1038/s41467-022-31099-5