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Accessory pigment

Accessory pigments are light-absorbing compounds found in photosynthetic organisms, such as plants, , and , that function alongside the primary pigment to broaden the spectrum of light captured for . These pigments, including , (such as and ), and phycobilins (like and ), absorb wavelengths in the and green regions (approximately 400–600 nm) where is less efficient, thereby transferring excitation energy to in the reaction centers of . By enhancing the overall efficiency of light harvesting, accessory pigments enable photosynthetic organisms to utilize a wider range of , including under varying environmental conditions like low-light or high-intensity . Beyond energy transfer, accessory pigments serve critical protective roles in maintaining photosynthetic apparatus integrity. Carotenoids, for instance, act as antioxidants by quenching reactive oxygen species (ROS) generated during excess light absorption, preventing photooxidative damage to chlorophyll and thylakoid membranes in chloroplasts. This photoprotection is mediated through mechanisms like the xanthophyll cycle, where carotenoids such as zeaxanthin and violaxanthin interconvert to dissipate surplus energy as heat, particularly under stress from intense sunlight or during seasonal changes like autumn leaf senescence. Chlorophyll b, comprising about 25% of total chlorophyll in many plants, not only extends light absorption into the blue spectrum (peaking around 455 nm) but also stabilizes the photosynthetic complexes. In aquatic or microbial photosynthesizers, phycobilins attached to phycobiliproteins in phycobilisomes further optimize light capture in deeper water layers by absorbing green to orange light (520–670 nm). The structural and functional diversity of accessory pigments reflects evolutionary adaptations to diverse habitats. , fat-soluble and embedded in membranes, number over 600 types and contribute to the , , and hues visible in leaves during fall, when degrades to facilitate nutrient reabsorption. Overall, these pigments are indispensable for maximizing photosynthetic productivity and safeguarding cellular components against environmental stressors, underscoring their integral role in the global and .

Definition and Overview

Definition

Accessory pigments are light-absorbing compounds found in photosynthetic organisms that function alongside the primary pigment to capture wavelengths of not efficiently absorbed by chlorophyll a alone. These pigments, embedded in the membranes and complexes of chloroplasts or similar structures, transfer absorbed energy to chlorophyll a through resonance energy transfer, thereby enhancing the overall efficiency of light harvesting in . In contrast to primary pigments, chlorophyll a serves as the universal reaction center pigment in all known oxygenic photosynthetic organisms, directly participating in the photochemical charge separation that initiates electron transport. Accessory pigments, however, act in a supportive role by broadening the of usable , particularly in the green-yellow range (500–600 nm), without serving as the primary electron donors in reaction centers. Key categories of accessory pigments include , which absorbs blue-violet and orange-red light; , such as β-carotene and xanthophylls, which capture blue-green wavelengths; and phycobilins, like and , prevalent in and . Evolutionarily, these accessory pigments have enabled photosynthetic organisms to adapt to diverse light environments, from terrestrial canopies filtering to aquatic depths with varying quality, optimizing energy capture across ecological niches.

Historical Context

The concept of accessory pigments in emerged from early investigations into the nature of , the primary green pigment. In 1817, French chemists Pierre Joseph Pelletier and Joseph Bienaimé Caventou first isolated and named from plant leaves, marking the initial step in understanding photosynthetic coloration. By 1864, British physicist George Gabriel Stokes proposed that "" was not a single substance but a mixture of at least two green pigments, based on spectroscopic observations of plant extracts. This laid the groundwork for recognizing multiple pigments beyond the dominant . A major advance came in 1906 when Russian botanist Mikhail Tswett invented and used it to separate leaf pigments into distinct fractions, identifying and as the two green components, along with yellow-orange . Tswett's technique, detailed in his 1911 publications, also formalized the term "" for these non-green pigments, previously isolated in crude form as in 1831 by Heinrich Wilhelm Ferdinand Wackenroder and in 1834 by . In 1882, German physiologist Theodor Wilhelm Engelmann provided the first experimental evidence of pigment-specific light absorption in through his innovative experiment; he exposed filamentous to a spectrum of light and observed aerobic congregating in regions of highest oxygen production, corresponding to and wavelengths absorbed by . During the 1930s and 1940s, refinements in action spectra revealed broader light absorption patterns that exceeded the capabilities of alone, highlighting the contributions of accessory pigments. Researchers such as William H. Hoover (1937) and Robert Emerson with associates (1943) demonstrated through detailed measurements that photosynthetic efficiency dropped sharply in far-red (the "red drop" phenomenon), but combining shorter wavelengths absorbed by accessory pigments like and enhanced overall rates, indicating mechanisms. This period shifted perceptions from viewing accessory pigments merely as colorants or stabilizers to essential participants in light harvesting. By the mid-20th century, comprehensive reviews and experiments solidified their active roles. In their 1965 paper, Eugene Rabinowitch and Govindjee described how pigments, including and , absorb light outside chlorophyll a's range and transfer excitation energy to reaction centers, integrating findings from action spectra into the emerging two-pigment system model of . This understanding, built on decades of spectroscopic and biochemical work, transformed accessory pigments from ancillary to integral components of .

Types of Accessory Pigments

Chlorophyll b and Other Chlorophyll Variants

is a key accessory pigment structurally similar to the primary pigment , differing primarily by the substitution of a formyl group (-CHO) at the C7 position of the B ring in place of a (-CH₃). This modification shifts its to include wavelengths around 450-480 , in addition to a peak near 642 , enabling it to capture light that absorbs less efficiently. Other chlorophyll variants serve as accessory pigments in specific organisms. , prevalent in diatoms and , features a modified ring without the phytyl tail characteristic of and b, with absorption maxima around 447 nm, 580 nm, and 627 nm that broaden light capture in aquatic environments. Chlorophyll d, found in certain , absorbs far-red light near 700 nm due to a formyl group at the C3 position, supporting in low-light, shaded niches. Chlorophyll f, discovered in 2010 in filamentous , represents the most red-shifted variant with absorption in the near-infrared region up to 745 nm, facilitated by a formyl substitution at C21, allowing utilization of deeper-penetrating light in dense microbial mats. These variants, including , are biosynthesized from through enzymatic modifications, primarily via chlorophyllide a oxygenase (CAO), a Rieske-type mononuclear iron oxygenase that oxidizes the C7 to a formyl group using molecular oxygen and reduced . This pathway ensures regulated production tied to environmental light conditions. In higher plants, typically constitutes 20-30% of total content, a proportion that increases under shade conditions to enhance light harvesting by expanding the antenna complexes of and promoting adaptation to low-light environments.

Carotenoids

Carotenoids represent a major class of in photosynthetic organisms, consisting of tetraterpenoid compounds with 40 carbon atoms arranged in a linear polyene chain. These pigments are lipid-soluble and ubiquitous in , , and certain , where they function alongside to enhance light capture during . Carotenoids are broadly classified into two subtypes: carotenes, which are purely molecules lacking oxygen, such as the orange-red β-carotene, and xanthophylls, which are oxygenated derivatives including and . The absorption spectrum of primarily spans the 400-550 nm range, targeting to that absorbs less efficiently, thereby broadening the usable portion of the solar spectrum for . This complementary absorption is crucial in photosynthetic complexes, where transfer captured energy to molecules. In plants, carotenoid biosynthesis occurs exclusively in plastids and initiates from the C5 precursor isopentenyl pyrophosphate (IPP), derived via the methylerythritol 4-phosphate (MEP) pathway, with the committed step catalyzed by phytoene synthase to form phytoene, the first carotenoid intermediate. Subsequent desaturation and cyclization reactions yield diverse carotenes and xanthophylls, regulated by enzymes like phytoene desaturase and lycopene cyclase. More than 1,100 distinct have been identified across , showcasing remarkable structural diversity that influences their specific roles, such as the red ketocarotenoid , which imparts vivid coloration to certain like . This variability allows carotenoids to adapt to environmental demands, with subtypes like accumulating under high light to aid photoprotection.

Phycobilins

Phycobilins are linear pigments that serve as pigments in , covalently attached to proteins to form water-soluble phycobiliproteins, which are primarily found in and . These pigments enable the absorption of light in wavelengths poorly captured by , particularly in aquatic environments where green and yellow light predominates. The main subtypes of phycobiliproteins are , , and allophycocyanin, each distinguished by their color, properties, and role in light harvesting. appears red and absorbs green light in the range of 490–570 , with major peaks at approximately 545 and 565 . is blue and absorbs orange to red light between 550–650 , featuring a primary maximum around 620 . Allophycocyanin, which facilitates to in the photosynthetic reaction centers, absorbs at longer wavelengths near 650 . These phycobiliproteins are organized into large, supramolecular complexes called , which are attached to the stromal surface of membranes in and . consist of a central core of allophycocyanin surrounded by rod-like structures composed of and , optimizing directional energy transfer toward . Biosynthesis of phycobilins begins with , which is cleaved by to produce IXα, followed by enzymatic reductions and isomerizations to yield chromophores like phycocyanobilin and phycoerythrobilin. This pathway is tightly regulated by environmental factors, including and availability such as and iron, which influence and enzyme activity to adapt phycobilisome composition.

Functions in Photosynthesis

Light Harvesting and Absorption

Accessory pigments enhance light harvesting in by absorbing light wavelengths that , the primary pigment, absorbs poorly, thereby broadening the overall spectrum of usable . primarily absorbs in the (around 430 nm) and (around 680 nm) regions, leaving a gap in the green-yellow portion of the (approximately 500-600 nm). Accessory pigments like address this by absorbing light in the 400-550 nm range, particularly the wavelengths, allowing plants to capture a greater portion of incoming that would otherwise be reflected or transmitted unused./08:_Photosynthesis/8.05:The_Light-Dependent_Reactions_of_Photosynthesis-_Absorption_of_Light) These pigments are embedded within complexes that surround the reaction centers of I () and II (PSII), forming organized light-harvesting complexes (LHCs) that efficiently collect photons. In , the LHCI complex incorporates and alongside , while in PSII, the major LHCII trimer binds multiple molecules and up to four molecules per , creating a structured array that funnels energy toward the core. This integration into LHCs not only expands absorption capacity but also positions the pigments in close proximity to facilitate rapid energy capture across diverse light environments. The presence of accessory pigments significantly boosts the of , particularly in non-optimal light conditions, by increasing the proportion of photons effectively utilized for photochemical reactions. This enhancement allows for greater , with studies indicating improvements in light-limited scenarios through expanded spectral coverage. Plants adapt to varying light availability by adjusting accessory pigment ratios, with shade leaves showing higher proportions of these pigments relative to chlorophyll a compared to sun leaves. For example, shade-adapted leaves exhibit lower chlorophyll a/b ratios (around 2.6-2.7) and increased light-harvesting chlorophyll-carotenoid proteins, enabling better capture of diffuse light in low-intensity environments. This acclimation optimizes energy acquisition where direct sunlight is scarce, contrasting with sun leaves that prioritize higher chlorophyll a content for rapid processing of intense light.

Energy Transfer Mechanisms

Accessory pigments in photosynthetic light-harvesting complexes (LHCs) primarily transfer absorbed energy to the reaction center through non-radiative mechanisms, ensuring high in . The dominant process is (FRET), a dipole-dipole interaction that enables energy to pass from excited donor pigments, such as or , to acceptor molecules without emission. This mechanism operates effectively over short distances of 1-10 nm, typical of pigment arrangements in LHCs, where the transfer rate is highly sensitive to intermolecular spacing. The rate k is quantified by the equation k = \frac{1}{\tau_D} \left( \frac{R_0}{r} \right)^6, where \tau_D is the donor's lifetime (often around 1 for ), R_0 is the Förster radius (approximately 5-6 nm for pairs, marking 50% transfer efficiency), and r is the donor-acceptor distance. This sixth-power dependence ensures rapid transfer within the densely packed pigment-protein environment of LHCs, with rates on the timescale. In LHCs, occurs via direct and indirect pathways, where excitation migrates stepwise from outer accessory pigments to inner . For instance, in the major LHCII of , energy from or funnels inward through intermediate molecules, achieving near-unity efficiency by optimizing spectral overlap and pigment orientation. Exciton migration complements by treating energy as delocalized packets that hop coherently between closely coupled in arrays, rather than strictly point-to-point transfers. This delocalization, facilitated by strong electronic coupling in pigment clusters, allows energy to explore multiple pathways efficiently, reducing losses in systems like the bacterial LH2 complex. Recent studies have revealed quantum coherence in these processes, where energy exhibits wavelike behavior, enhancing transfer efficiency beyond classical predictions. Evidence from two-dimensional electronic on the Fenna-Matthews-Olson (FMO) complex in green sulfur bacteria demonstrates long-lived coherences at physiological temperatures, enabling optimal pathway selection and near-100% efficiency in directing energy from accessory bacteriochlorophylls to the reaction center. These findings, emerging post-2007, underscore how quantum effects in natural light-harvesting systems outperform incoherent models.

Photoprotection

Accessory pigments play a crucial role in photoprotection by mitigating damage from excess light energy in photosynthetic organisms. Under high light conditions, these pigments prevent , a process where excessive excitation leads to the degradation of photosynthetic reaction centers, primarily . By dissipating surplus energy and neutralizing harmful reactive species, accessory pigments maintain the integrity of the photosynthetic apparatus. One primary mechanism of photoprotection involves (NPQ), where accessory pigments, particularly , dissipate excess absorbed energy as heat rather than allowing it to drive potentially damaging photochemical reactions. In the xanthophyll cycle, violaxanthin is converted to under high light intensity through the action of violaxanthin de-epoxidase, a process that enhances NPQ and reduces the risk of oxidative damage. This cycle operates in the membranes, enabling rapid adjustment to fluctuating light environments. Accessory pigments also serve an antioxidant function by scavenging (ROS), such as , which are generated during excess light exposure. effectively quench through physical energy transfer or chemical reactions, preventing in thylakoid membranes and subsequent membrane disruption. This protective action is vital in high-light stress scenarios where ROS production overwhelms the photosynthetic . Another key protective role is the quenching of triplet states formed in chlorophyll molecules. When chlorophyll absorbs excess light, it can enter a triplet excited state, which readily generates singlet oxygen. Accessory pigments, especially carotenoids, accept this triplet energy via triplet-triplet energy transfer, safely dissipating it and averting ROS formation. This mechanism operates within light-harvesting complexes, ensuring efficient photoprotection at the molecular level. The activation of these photoprotective processes is tightly regulated, often in a pH-dependent manner within the . During high light, proton accumulation lowers the luminal pH, triggering NPQ and the cycle to dissipate excess energy and minimize . This pH sensing allows for dynamic responses that balance photoprotection with .

Chemical Properties

Molecular Structures

Accessory pigments exhibit diverse molecular architectures tailored to their roles in light absorption and energy transfer in photosynthetic organisms. , a key accessory pigment in higher and , features a ring system coordinated with a central magnesium ion (Mg²⁺). This macrocyclic structure consists of four rings linked by methine bridges, with a distinctive formyl group (-CHO) attached at the C7 position on ring B, distinguishing it from , which has a (-CH₃) there. The overall molecular formula of chlorophyll b is C₅₅H₇₀O₆N₄Mg. Carotenoids, another major class of accessory pigments, are lipophilic tetraterpenoids composed of a linear polyene chain typically containing 40 carbon atoms and featuring a series of conjugated double bonds that enable extended π-electron delocalization. These molecules often terminate in cyclic end groups, such as the β-ionone rings in , which has the molecular formula C₄₀H₅₆ and exemplifies the hydrocarbon nature of carotenes. Structural variations within include oxygenated derivatives known as xanthophylls, which incorporate functional groups like hydroxyl or moieties (e.g., in or ), contrasting with the purely carotenes. Phycobilins, prevalent in algae and cyanobacteria, are linear tetrapyrrole chromophores lacking a central metal ion, unlike chlorophylls, and are covalently bound via thioether linkages to cysteine residues within phycobiliproteins. Their open-chain structure comprises four pyrrole rings connected by single-carbon bridges, with propionic acid side chains and vinyl or ethylidene substituents influencing chromophore conformation. For instance, phycoerythrobilin, the primary chromophore in phycoerythrin, has the molecular formula C₃₃H₃₈N₄O₆ and adopts an extended conformation when attached to the protein scaffold.

Absorption Spectra

Accessory pigments exhibit distinct absorption spectra that complement the primary pigment , enabling broader capture of (PAR) in the 400-700 nm range. , a key chlorophyll variant, displays major absorption peaks at approximately 453 nm in the blue region and 645 nm in the red region when measured in organic solvents. , including and xanthophylls, primarily absorb in the blue-green spectrum between 400 and 500 nm, with notable peaks around 450 nm and 475 nm, corresponding to their conjugated polyene structures. Phycobilins, such as and , absorb in the green to orange region from 550 to 650 nm, with peaking near 565 nm and around 620 nm in aqueous environments. The spectrum of a pigment illustrates the wavelengths it absorbs, whereas the measures the relative efficiency of across wavelengths, often determined by or bacterial distribution in classic experiments. In Engelmann's 1882 experiment using filamentous and aerobic , the peaked in blue and red regions but showed broader activity in green wavelengths due to pigments, highlighting their role in enhancing beyond a's absorption alone. This difference underscores how pigments transfer absorbed energy to reaction centers, contributing to a more uniform that aligns closely with cumulative pigment absorption but extends into underutilized wavelengths. Collectively, the spectra of accessory pigments provide complementary coverage across the PAR spectrum, minimizing gaps in harvesting. extends red slightly beyond , fill the 450-550 nm blue-green gap, and phycobilins capture 550-650 nm green-orange , resulting in near-complete overlap from 400 to 700 nm when combined. Conceptual graphs of these spectra reveal overlapping peaks with minimal valleys; for instance, bridges the low-efficiency green region of chlorophylls, while phycobilin bands overlap tails to ensure efficient PAR utilization in diverse organisms. Absorption characteristics of accessory pigments are influenced by environmental factors such as solvent polarity and protein binding, which can induce spectral shifts. For chlorophyll b, polar solvents like methanol cause a red-shift of up to 5-10 nm in the red peak compared to non-polar solvents like hexane, due to altered molecular interactions. In phycobilins, binding within phycobilisome protein complexes red-shifts absorption by 10-20 nm—for example, allophycocyanin peaks shift from ~650 nm in solution to 653-712 nm in vivo—optimizing energy transfer through excitonic coupling. These shifts demonstrate how microenvironmental contexts fine-tune pigment optics for specific light conditions.

Occurrence and Distribution

In Higher Plants

In higher plants, the primary accessory pigments are chlorophyll b and , which complement in capturing light for . Chlorophyll b typically constitutes 20-25% of the total content in leaves, with a standard chlorophyll a to b of approximately 3:1 under optimal conditions. , including , , , and violaxanthin, make up about 20-25% of the total photosynthetic pigments by molecular count in complexes, with being the most abundant carotenoid, often comprising 40-50% of the total carotenoid pool. These pigments are localized within the chloroplasts, specifically embedded in the thylakoid membranes and associated with light-harvesting complexes (LHCs), where they form pigment-protein assemblies that optimize light across a broader . is predominantly found in the peripheral LHCs of , enhancing the capture of green wavelengths, while are integrated into both core and peripheral antenna complexes, with bound to specific sites in LHCII trimers. Higher plants exhibit adaptations in accessory pigment composition to environmental cues. In shade-adapted plants, chlorophyll b levels increase relative to , lowering the a:b ratio to as low as 2.5:1, which improves utilization of green light penetrating canopies. During autumn , chlorophyll degradation unmasks persistent , leading to the characteristic yellow and orange foliage colors as and β-carotene become visible. Under abiotic stresses like , plants often show a relative increase in content—up to 20-30% higher than in well-watered conditions—to mitigate oxidative damage, while may decline more rapidly than , elevating the a:b ratio.

In Algae and Cyanobacteria

Green algae (Chlorophyta) possess accessory pigments similar to those in higher plants, primarily chlorophyll b and such as and , which aid in light harvesting in freshwater and marine environments. In , accessory pigments are primarily organized into phycobilisomes, large supramolecular complexes that serve as the main light-harvesting antennae, enabling efficient capture of green and orange wavelengths (500–650 nm) that penetrate aquatic environments. These structures consist of , which absorbs orange light around 620 nm, and , which targets green light at approximately 565 nm, allowing to thrive in water columns where dominates but complementary wavelengths are available. play a secondary role in these organisms, mainly providing photoprotection rather than primary light harvesting. Red algae, such as those in the Rhodophyta phylum, rely heavily on as their dominant accessory pigment within phycobilisomes, which is particularly adapted for absorbing green light (around 550–570 nm) that penetrates deeper into compared to other wavelengths. This specialization enables to occupy niches in the mesophotic zone, where light intensity drops below 1% of surface levels and green wavelengths prevail due to selective absorption of shorter and longer spectra by water molecules. In chromalveolate algae, including (Phaeophyceae) and certain green-pigmented groups like diatoms, acts as the principal carotenoid accessory pigment, absorbing green light (450–550 nm) to complement and the accessory , which extends absorption into the blue-green range. , present as variants c1, c2, or c3, enhances light capture in these translucent aquatic habitats, supporting in low-light coastal and oceanic waters. Cyanobacteria and algae exhibit adaptations in phycobilin composition to varying light quality in stratified water columns, where surface layers favor blue-green light and deeper strata enrich for green-orange penetration, leading to evolutionary shifts such as increased in low-light-adapted marine strains. In low-light conditions, mutants of like can lose or downregulate phycobilisomes, reducing reliance on these pigments to optimize under blue-dominated illumination.