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Chlorophyll
Chlorophyll
Chlorophyll
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Chlorophyll at various scales
Lemon balm leaves
Chlorophyll is responsible for the green color of many plants and algae.
A microscope image of plant cells, with chloroplasts visible as small green balls
Seen through a microscope, chlorophyll is concentrated within organisms in structures called chloroplasts – shown here grouped inside plant cells.
A leaf absorbing blue and red light, but reflecting green light
Plants are perceived as green because chlorophyll absorbs mainly the blue and red wavelengths but green light, reflected by plant structures like cell walls, is less absorbed.[1]
The structure of chlorophyll d
There are several types of chlorophyll, but all share the chlorin magnesium ligand which forms the right side of this diagram.

Chlorophyll is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants.[2] Its name is derived from the Greek words χλωρός (khloros, "pale green") and φύλλον (phyllon, "leaf").[3] Chlorophyll allows plants to absorb energy from light. Those pigments are involved in oxygenic photosynthesis, as opposed to bacteriochlorophylls, related molecules found only in bacteria and involved in anoxygenic photosynthesis.[4]

Chlorophylls absorb light most strongly in the blue portion of the electromagnetic spectrum as well as the red portion.[5] Conversely, it is a poor absorber of green and near-green portions of the spectrum. Hence chlorophyll-containing tissues appear green because green light, diffusively reflected by structures like cell walls, is less absorbed.[1] Two types of chlorophyll exist in the photosystems of green plants: chlorophyll a and b.[6]

History

[edit]

Chlorophyll was first isolated and named by Joseph Bienaimé Caventou and Pierre Joseph Pelletier in 1817.[7] The presence of magnesium in chlorophyll was discovered in 1906,[8] and was the first detection of that element in living tissue.[9]

After initial work done by German chemist Richard Willstätter spanning from 1905 to 1915, the general structure of chlorophyll a was elucidated by Hans Fischer in 1940. By 1960, when most of the stereochemistry of chlorophyll a was known, Robert Burns Woodward published a total synthesis of the molecule.[9][10] In 1967, the last remaining stereochemical elucidation was completed by Ian Fleming,[11] and in 1990 Woodward and co-authors published an updated synthesis.[12] Chlorophyll f was announced to be present in cyanobacteria and other oxygenic microorganisms that form stromatolites in 2010;[13][14] a molecular formula of C55H70O6N4Mg and a structure of (2-formyl)-chlorophyll a were deduced based on NMR, optical and mass spectra.[15]

Photosynthesis

[edit]
Absorbance spectra of free chlorophyll a (blue) and b (red) in a solvent. The spectra of chlorophyll molecules are slightly modified in vivo depending on specific pigment-protein interactions.
  Chlorophyll a
  Chlorophyll b

Chlorophyll is vital for photosynthesis, which allows plants to absorb energy from light.[16]

Chlorophyll molecules are arranged in and around photosystems that are embedded in the thylakoid membranes of chloroplasts.[17] In these complexes, chlorophyll serves three functions:

  1. The function of the vast majority of chlorophyll (up to several hundred molecules per photosystem) is to absorb light.
  2. Having done so, these same centers execute their second function: The transfer of that energy by resonance energy transfer to a specific chlorophyll pair in the reaction center of the photosystems.
  3. This specific pair performs the final function of chlorophylls: Charge separation, which produces the unbound protons (H+) and electrons (e−) that separately propel biosynthesis.

The two currently accepted photosystem units are photosystem I and photosystem II, which have their own distinct reaction centres, named P700 and P680, respectively. These centres are named after the wavelength (in nanometers) of their red-peak absorption maximum. The identity, function and spectral properties of the types of chlorophyll in each photosystem are distinct and determined by each other and the protein structure surrounding them.

The function of the reaction center of chlorophyll is to absorb light energy and transfer it to other parts of the photosystem. The absorbed energy of the photon is transferred to an electron in a process called charge separation. The removal of the electron from the chlorophyll is an oxidation reaction. The chlorophyll donates the high energy electron to a series of molecular intermediates called an electron transport chain. The charged reaction center of chlorophyll (P680+) is then reduced back to its ground state by accepting an electron stripped from water. The electron that reduces P680+ ultimately comes from the oxidation of water into O2 and H+ through several intermediates. This reaction is how photosynthetic organisms such as plants produce O2 gas, and is the source for practically all the O2 in Earth's atmosphere. Photosystem I typically works in series with Photosystem II; thus the P700+ of Photosystem I is usually reduced as it accepts the electron, via many intermediates in the thylakoid membrane, by electrons coming, ultimately, from Photosystem II. Electron transfer reactions in the thylakoid membranes are complex, however, and the source of electrons used to reduce P700+ can vary.

The electron flow produced by the reaction center chlorophyll pigments is used to pump H+ ions across the thylakoid membrane, setting up a proton-motive force a chemiosmotic potential used mainly in the production of ATP (stored chemical energy) or to reduce NADP+ to NADPH. NADPH is a universal agent used to reduce CO2 into sugars as well as other biosynthetic reactions.

Reaction center chlorophyll–protein complexes are capable of directly absorbing light and performing charge separation events without the assistance of other chlorophyll pigments, but the probability of a single chlorophyll molecule doing so under a given light intensity is small. Thus, the other chlorophylls in the photosystem and antenna pigment proteins all cooperatively absorb and funnel light energy to the reaction center. Besides chlorophyll a, there are other pigments, called accessory pigments, which occur in these pigment–protein antenna complexes. These pigments complement chlorophyll by absorbing photons at wavelengths outside of chlorophyll's narrow absorption spectrum and deliver additional electrons to the photosystem.

Chemical structure

[edit]
Space-filling model of the chlorophyll a molecule

Several chlorophylls are known. All are defined as derivatives of the parent chlorin by the presence of a fifth, ketone-containing ring beyond the four pyrrole-like rings. Most chlorophylls are classified as chlorins, which are reduced relatives of porphyrins (found in hemoglobin). They share a common biosynthetic pathway with porphyrins, including the precursor uroporphyrinogen III. Unlike hemes, which contain iron bound to the N4 center, most chlorophylls bind magnesium. The axial ligands attached to the Mg2+ center are often omitted for clarity. Appended to the chlorin ring are various side chains, usually including a long phytyl chain (C20H39O). The most widely distributed form in terrestrial plants is chlorophyll a. Chlorophyll a has methyl group in place of a formyl group in chlorophyll b. This difference affects the absorption spectrum, allowing plants to absorb a greater portion of visible light.

The structures of chlorophylls are summarized below:[18][19]

Chlorophyll a Chlorophyll b Chlorophyll c1 Chlorophyll c2 Chlorophyll d Chlorophyll f[15]
Molecular formula C55H72O5N4Mg C55H70O6N4Mg C35H30O5N4Mg C35H28O5N4Mg C54H70O6N4Mg C55H70O6N4Mg
C2 group −CH3 −CH3 −CH3 −CH3 −CH3 −CHO
C3 group −CH=CH2 −CH=CH2 −CH=CH2 −CH=CH2 −CHO −CH=CH2
C7 group −CH3 −CHO −CH3 −CH3 −CH3 −CH3
C8 group −CH2CH3 −CH2CH3 −CH2CH3 −CH=CH2 −CH2CH3 −CH2CH3
C17 group −CH2CH2COO−Phytyl −CH2CH2COO−Phytyl −CH=CHCOOH −CH=CHCOOH −CH2CH2COO−Phytyl −CH2CH2COO−Phytyl
C17−C18 bond Single
(chlorin) 		Single
(chlorin) 		Double
(porphyrin) 		Double
(porphyrin) 		Single
(chlorin) 		Single
(chlorin)
Occurrence Universal Mostly plants Various algae Various algae Cyanobacteria Cyanobacteria
  • Structures of chlorophylls
  • chlorophyll a
    chlorophyll a
  • chlorophyll b
    chlorophyll b
  • chlorophyll c1
    chlorophyll c1
  • chlorophyll c2
    chlorophyll c2
  • chlorophyll d
    chlorophyll d
  • chlorophyll f
    chlorophyll f

Chlorophyll e is reserved for a pigment that has been extracted from algae in 1966 but not chemically described. Besides the lettered chlorophylls, a wide variety of sidechain modifications to the chlorophyll structures are known in the wild. For example, Prochlorococcus, a cyanobacterium, uses 8-vinyl Chl a and b.[20]

Measurement of chlorophyll content

[edit]
Chlorophyll forms deep green solutions in organic solvents.

Chlorophylls can be extracted from the protein into organic solvents.[21][22][23] In this way, the concentration of chlorophyll within a leaf can be estimated.[24] Methods also exist to separate chlorophyll a and chlorophyll b.

In diethyl ether, chlorophyll a has approximate absorbance maxima of 430 nm and 662 nm, while chlorophyll b has approximate maxima of 453 nm and 642 nm.[25] The absorption peaks of chlorophyll a are at 465 nm and 665 nm. Chlorophyll a fluoresces at 673 nm (maximum) and 726 nm. The peak molar absorption coefficient of chlorophyll a exceeds 105 M−1 cm−1, which is among the highest for small-molecule organic compounds.[26] In 90% acetone-water, the peak absorption wavelengths of chlorophyll a are 430 nm and 664 nm; peaks for chlorophyll b are 460 nm and 647 nm; peaks for chlorophyll c1 are 442 nm and 630 nm; peaks for chlorophyll c2 are 444 nm and 630 nm; peaks for chlorophyll d are 401 nm, 455 nm and 696 nm.[27]

Ratio fluorescence emission can be used to measure chlorophyll content. By exciting chlorophyll a fluorescence at a lower wavelength, the ratio of chlorophyll fluorescence emission at 705±10 nm and 735±10 nm can provide a linear relationship of chlorophyll content when compared with chemical testing. The ratio F735/F700 provided a correlation value of r2 0.96 compared with chemical testing in the range from 41 mg m−2 up to 675 mg m−2. Gitelson also developed a formula for direct readout of chlorophyll content in mg m−2. The formula provided a reliable method of measuring chlorophyll content from 41 mg m−2 up to 675 mg m−2 with a correlation r2 value of 0.95.[28]

Also, the chlorophyll concentration can be estimated by measuring the light transmittance through the plant leaves.[29] The assessment of leaf chlorophyll content using optical sensors such as Dualex and SPAD allows researchers to perform real-time and non-destructive measurements.[30][31] Research shows that these methods have a positive correlation with laboratory measurements of chlorophyll.[32]

Biosynthesis

[edit]
Main article: Chlorophyllide

In some plants, chlorophyll is derived from glutamate and is synthesised along a branched biosynthetic pathway that is shared with heme and siroheme.[33][34][35] Chlorophyll synthase[36] is the enzyme that completes the biosynthesis of chlorophyll a:[37][38]

chlorophyllide a + phytyl diphosphate chlorophyll a + diphosphate

This conversion forms an ester of the carboxylic acid group in chlorophyllide a with the 20-carbon diterpene alcohol phytol. Chlorophyll b is made by the same enzyme acting on chlorophyllide b. The same is known for chlorophyll d and f, both made from corresponding chlorophyllides ultimately made from chlorophyllide a.[39]

In Angiosperm plants, the later steps in the biosynthetic pathway are light-dependent. Such plants are pale (etiolated) if grown in darkness. Non-vascular plants and green algae have an additional light-independent enzyme and grow green even in darkness.[40]

Chlorophyll is bound to proteins. Protochlorophyllide, one of the biosynthetic intermediates, occurs mostly in the free form and, under light conditions, acts as a photosensitizer, forming free radicals, which can be toxic to the plant. Hence, plants regulate the amount of this chlorophyll precursor. In angiosperms, this regulation is achieved at the step of aminolevulinic acid (ALA), one of the intermediate compounds in the biosynthesis pathway. Plants that are fed by ALA accumulate high and toxic levels of protochlorophyllide; so do the mutants with a damaged regulatory system.[41]

Senescence and the chlorophyll cycle

[edit]

The process of plant senescence involves the degradation of chlorophyll: for example the enzyme chlorophyllase (EC 3.1.1.14) hydrolyses the phytyl sidechain to reverse the reaction in which chlorophylls are biosynthesised from chlorophyllide a or b. Since chlorophyllide a can be converted to chlorophyllide b and the latter can be re-esterified to chlorophyll b, these processes allow cycling between chlorophylls a and b. Moreover, chlorophyll b can be directly reduced (via 71-hydroxychlorophyll a) back to chlorophyll a, completing the cycle.[42][43] In later stages of senescence, chlorophyllides are converted to a group of colourless tetrapyrroles known as nonfluorescent chlorophyll catabolites (NCC's) with the general structure:

Nonfluorescent chlorophyll catabolites

These compounds have also been identified in ripening fruits and they give characteristic autumn colours to deciduous plants.[43][44]

Distribution

[edit]

Chlorophyll maps from 2002 to 2024, provided by NASA, show milligrams of chlorophyll per cubic meter of seawater each month.[45] Places where chlorophyll amounts are very low, indicating very low numbers of phytoplankton, are blue. Places where chlorophyll concentrations are high, meaning many phytoplankton were growing, are yellow. The observations come from the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Aqua satellite. Land is dark gray, and places where MODIS could not collect data because of sea ice, polar darkness, or clouds are light gray. The highest chlorophyll concentrations, where tiny surface-dwelling ocean plants are, are in cold polar waters or in places where ocean currents bring cold water to the surface, such as around the equator and along the shores of continents. It is not the cold water itself that stimulates the phytoplankton. Instead, the cool temperatures are often a sign that the water has welled up to the surface from deeper in the ocean, carrying nutrients that have built up over time. In polar waters, nutrients accumulate in surface waters during the dark winter months when plants cannot grow. When sunlight returns in the spring and summer, the plants flourish in high concentrations.[45]

Uses

[edit]

Culinary

[edit]

Synthetic chlorophyll is registered as a food additive colorant, and its E number is E140. Chefs use chlorophyll to color a variety of foods and beverages green, such as pasta and spirits. Absinthe gains its green color naturally from the chlorophyll introduced through the large variety of herbs used in its production.[46] Chlorophyll is not soluble in water, and it is first mixed with a small quantity of vegetable oil to obtain the desired solution.[citation needed]

In marketing

[edit]

In years 1950–1953 in particular, chlorophyll was used as a marketing tool to promote toothpaste, sanitary towels, soap and other products. This was based on claims that it was an odor blocker — a finding from research by F. Howard Westcott in the 1940s — and the commercial value of this attribute in advertising led to many companies creating brands containing the compound. However, it was soon determined that the hype surrounding chlorophyll was not warranted and the underlying research may even have been a hoax. As a result, brands rapidly discontinued its use. In the 2020s, chlorophyll again became the subject of unsubstantiated medical claims, as social media influencers promoted its use in the form of "chlorophyll water", for example.[47]

See also

[edit]
Wikimedia Commons has media related to Chlorophyll.

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Chlorophyll

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from Grokipedia
Chlorophyll is a class of green pigments essential for , the process by which , , and convert light energy into chemical energy to sustain most life on . These pigments, first isolated and named in , primarily include —the universal form present in all oxygenic photosynthesizers—and , an found in higher and that broadens the range of light absorption. Other variants, such as in certain , chlorophylls d and f in , and bacteriochlorophylls in photosynthetic bacteria, enable diverse light-harvesting strategies across ecosystems. Located in chloroplasts within cells, chlorophyll molecules are organized into where they capture photons, initiating and driving the production of oxygen and glucose. Structurally, chlorophyll consists of a ring system with a central and a hydrophobic tail that anchors it in membranes. This configuration allows chlorophyll a to absorb light maximally at wavelengths around 430 nm (blue-violet) and 662 nm (red), while shifts absorption slightly to 453 nm and 642 nm, optimizing energy capture from sunlight. The pigments' conjugated double bonds facilitate excitation and energy transfer, with over 100 known variants differing in side-chain modifications like formyl groups. begins with and involves 15 enzymes encoded by 27 genes, tightly regulated to match environmental light conditions. Beyond photosynthesis, chlorophyll plays roles in photoprotection, sensing light quality for , and emerging applications in and technology, such as antioxidants with potential anticancer properties and biosensors for . Its degradation products, like pheophytin, participate in electron transport chains, while synthetic derivatives are used as natural colorants (E140) in . As of , more than 43,000 scientific references since 2016 underscore chlorophyll's foundational impact on global oxygen production and carbon fixation.

Fundamentals

Definition and Importance

Chlorophyll is a green pigment present in the chloroplasts of , algae, and cyanobacteria, where it functions as the primary photoreceptor for capturing sunlight during . This pigment enables these organisms to absorb light energy, particularly in the blue and red wavelengths, initiating the process that sustains much of life on . The importance of chlorophyll lies in its central role in transforming into through , which forms the foundation of global food chains by producing organic compounds that support herbivores, carnivores, and decomposers. Additionally, this process generates oxygen as a byproduct, with oceanic —microscopic containing chlorophyll—responsible for approximately 50% of Earth's atmospheric oxygen production, underscoring chlorophyll's critical contribution to the planet's breathable atmosphere. Chlorophyll's evolutionary origins date back to ancient photosynthetic organisms, likely , that emerged between 3.5 and 2.7 billion years ago, revolutionizing Earth's environment by enabling and paving the way for aerobic life.

Types of Chlorophyll

is the most ubiquitous form of chlorophyll, present in all oxygenic photosynthetic organisms, including , , and . It serves as the primary pigment in the reaction centers of both (PSI) and (PSII), where it directly participates in the initial charge separation during . Structurally, features a chlorin ring with a (-CH₃) at the C7 position and a (-CH=CH₂) at C3, esterified with a tail at C17. Chlorophyll b functions as an primarily in higher plants and , enhancing the absorption of light in the blue and red regions to broaden the spectrum available for . It differs from by having a formyl group (-CHO) instead of a at the C7 position, which shifts its absorption maximum to slightly longer wavelengths compared to . This structural variation allows chlorophyll b to transfer excitation energy to in light-harvesting complexes, optimizing energy capture in terrestrial environments. In various algae and cyanobacteria, additional chlorophyll types specialize in light harvesting under specific environmental conditions. Chlorophyll c, found in chromophyte algae such as diatoms and dinoflagellates, lacks the isocyclic ring E and phytol ester of chlorophyll a, featuring instead acrylic or propionic acid side chains and variations like ethyl or vinyl groups at C8 in its subtypes (c1, c2, c3). It acts as an accessory pigment in peripheral light-harvesting antennas, absorbing blue-green light to support photosynthesis in aquatic habitats. Chlorophylls d and f occur in certain cyanobacteria, enabling the use of far-red light; chlorophyll d has a formyl group at C3 (replacing the vinyl of chlorophyll a), while chlorophyll f bears a formyl at C2. These modifications red-shift their absorption (chlorophyll d to ~700 nm, chlorophyll f to ~720 nm), allowing energy transfer to reaction centers in shaded or far-red-dominated niches, with chlorophyll d also functioning in PSI reaction centers as P740. Chlorophyll e is a rare and unconfirmed variant reported in extracts from yellow-green algae such as Vaucheria, with undetermined structure and debated natural occurrence, possibly an artifact of chlorophyll a isolation; its photosynthetic role, if any, is minor. These side-chain differences across types influence their spectral properties, with formyl substitutions generally extending absorption into longer wavelengths.

Chemical Structure and Properties

Molecular Composition

Chlorophyll molecules are composed of a , a derivative featuring four rings linked by methine bridges, with one of the pyrrole rings partially reduced to form a chlorin rather than a full . At the center of this planar ring system is a divalent magnesium ion (Mg²⁺) coordinated to the four nitrogen atoms of the pyrrole rings, which stabilizes the structure and imparts unique electronic properties. This tetrapyrrole structure is analogous to that found in heme, the prosthetic group of hemoglobin, but with magnesium instead of iron as the central ion. A distinctive fifth ring, a (ring E), is fused to one of the pyrroles at positions C13–C15, bearing a carbomethoxy substituent at C13². Various peripheral groups adorn the , including a at C3, a methyl at C7, an ethyl at C8, methyls at C12 and C18, and a side chain at C17 that is esterified with , a long-chain diterpenoid alcohol (C₂₀H₃₉OH) consisting of four units, which serves to embed the molecule in membranes. The molecular formula of chlorophyll a, the primary form in most photosynthetic organisms, is C55H72MgN4O5C_{55}H_{72}MgN_4O_5, with a molecular weight of approximately 893.5 g/mol. This formula accounts for the chlorin core (C₂₀H₁₄MgN₄O₂, roughly), the ring E and substituents (adding C₄H₆O₃), and the (C₂₀H₃₉O + C₂H₃O for the linkage). The structure can be described as a nearly flat, spanning about 10 Å in diameter for the ring, with the tail extending linearly up to 25 Å, enhancing in nonpolar environments. Key derivatives include chlorophyllide, which lacks the phytol tail and features a free at C17³, resulting in the formula C35H34MgN4O5C_{35}H_{34}MgN_4O_5 for chlorophyllide a and greater . Pheophytin, another derivative, arises from demetallation where the central Mg²⁺ is replaced by two protons, yielding C55H74N4O5C_{55}H_{74}N_4O_5 for pheophytin a while retaining the full chain and altering the coordination chemistry of the ring nitrogens. These modifications highlight the modular nature of the chlorophyll scaffold, where the core remains conserved across variants.

Spectral Characteristics

Chlorophyll a, the primary pigment in , exhibits strong absorption in the with peaks at approximately 430 nm in the blue-violet region and 662 nm in the red region, enabling efficient capture of these wavelengths for energy transfer. Chlorophyll b, an , complements this by absorbing at slightly shifted peaks of 453 nm (blue) and 642 nm (red), which broadens the overall absorption range and enhances light harvesting in diverse environmental conditions. These absorption characteristics stem from the electronic transitions in the conjugated macrocyclic of the chlorophyll molecules, as explored in the molecular composition section. The action spectrum of photosynthesis, representing the relative effectiveness of different wavelengths in promoting or carbon fixation, closely parallels the absorption spectrum of chlorophyll, showing peak efficiencies in the (400–500 nm) and (600–700 nm) regions where absorption is maximal. However, a sharp decline in photosynthetic occurs beyond approximately 680 nm in green plants, a phenomenon termed the "red drop," which reflects the inability of to efficiently utilize far-red light without supplementary shorter wavelengths. This drop highlights the wavelength-specific partitioning of energy between I and II. When absorbed light energy exceeds the capacity for photochemical use, chlorophyll releases excess energy as fluorescence, emitting light primarily between 650 and 800 nm with broadband peaks centered at about 685 nm (red) and 740 nm (far-red). The intensity and spectral shape of this fluorescence provide insights into photosynthetic efficiency and stress responses. In remote sensing applications, solar-induced chlorophyll fluorescence in the 680–760 nm range serves as a non-invasive proxy for gross primary productivity, allowing global monitoring of vegetation health via satellite observations.

Biological Functions

Role in Photosynthesis

Chlorophyll plays a central role in by absorbing light energy and facilitating its conversion into through electron transport. In the , chlorophyll molecules within membranes capture photons, exciting electrons that drive the reduction of NADP⁺ and the generation of ATP. This process occurs primarily in two , where chlorophyll acts as the key for light harvesting and electron donation. Light harvesting begins in antenna complexes, which are pigment-protein assemblies surrounding the reaction centers of photosystems . These complexes contain multiple chlorophyll molecules, along with , that collectively increase the effective absorption cross-section of the by approximately 100-fold compared to isolated reaction centers. Upon absorption, excited chlorophyll molecules transfer energy to neighboring pigments through a series of rapid steps, ultimately funneling it to the core reaction center chlorophylls. This energy migration occurs on timescales from femtoseconds to picoseconds, ensuring efficient delivery to initiate charge separation. In photosystem II (PSII), the primary electron donor is P680, a specialized chlorophyll a species with a redox potential of at least +1.17 V in its oxidized form, enabling the challenging oxidation of water. P680, upon excitation by light absorbed directly or via the antenna, donates an electron to pheophytin, the initial electron acceptor, leaving behind the oxidized P680⁺ that is subsequently reduced by the oxygen-evolving complex. Similarly, in photosystem I (PSI), P700 serves as the primary electron donor, a heterodimer of chlorophyll a and chlorophyll a' with an excited-state potential of approximately -1.2 V. Excited P700 transfers an electron to a chlorophyll a acceptor (A₀), and P700⁺ is reduced via the electron transport chain from PSII. These reaction center chlorophylls are the heart of the photosystems, where light energy is first converted to electrochemical potential. Energy transfer between chlorophyll molecules in the antenna and to the reaction centers primarily occurs via Förster resonance energy transfer (FRET), a non-radiative dipole-dipole mechanism that depends on the overlap of donor emission and acceptor absorption spectra, as well as their proximity (typically 1-10 nm). In photosynthetic systems, FRET enables incoherent hopping of excitations among weakly coupled chlorophylls, while stronger excitonic coupling in closely spaced pairs allows delocalized energy states for faster transfer. This process underpins the high quantum efficiency of light harvesting, with minimal energy loss. The overall flow in is depicted by the Z-scheme, a conceptual diagram illustrating the potentials and sequential transfers from to NADP⁺. In this scheme, light-driven excitation of in PSII boosts an to a high-energy state, enabling its transfer through , the cytochrome b₆f complex, and to P700 in . A second excites P700, propelling the to and ultimately NADP⁺, while the protons generated contribute to ATP synthesis via . Chlorophyll's role as the in both is essential to spanning the thermodynamic gap required for non-cyclic transport.

Biosynthesis

The biosynthesis of chlorophyll in , , and follows a conserved pathway that branches from synthesis, starting with the glutamate as the initial precursor. Glutamate is first reduced to glutamate-1-semialdehyde by glutamyl-tRNA reductase (GluTR), the rate-limiting in , and then transaminated to δ-aminolevulinic acid (ALA) by glutamate-1-semialdehyde-2,1-aminomutase; in some and , ALA is directly synthesized from and by ALA synthase. ALA condenses to form porphobilinogen, which polymerizes through uroporphyrinogen III and coproporphyrinogen III to (PPIX) via a series of enzymatic steps involving porphobilinogen deaminase, uroporphyrinogen III synthase, and other decarboxylases and oxidases. At the , magnesium chelatase—a heterotrimeric complex consisting of CHLI, CHLD, and CHLH subunits, often activated by the regulatory protein GUN4—inserts Mg²⁺ into PPIX to form Mg-protoporphyrin IX, committing the pathway to chlorophyll production rather than . Subsequent steps include by S-adenosyl-L-methionine:magnesium-protoporphyrin IX methyltransferase (CHLM) to yield Mg-protoporphyrin IX monomethyl ester, oxidative cyclization by Mg-protoporphyrin IX monomethyl ester cyclase (CHL27/CRD1/CTH1) to protochlorophyllide, and phytylation and reduction to form . A critical step in the pathway is the reduction of protochlorophyllide to chlorophyllide, catalyzed by protochlorophyllide oxidoreductase (POR). In angiosperms and most , light-dependent POR (LPOR, encoded by POR genes A, B, or C) predominates, requiring photoexcitation for the stereospecific hydride transfer and utilizing as a cofactor; this light dependence ensures chlorophyll accumulation aligns with development under illumination. In contrast, gymnosperms, some , and anaerobic employ a dark-operative POR (DPOR), a nitrogenase-like complex (ChlL, ChlN, ChlB subunits) that functions independently of , enabling chlorophyll synthesis in etiolated or non-phototrophic conditions. These enzymes highlight the pathway's adaptability across oxygen levels and environments, with LPOR and DPOR often coexisting in certain organisms for flexible . Recent genetic studies have elucidated transcriptional regulation of the pathway, involving factors that integrate light, hormone, and stress signals to fine-tune chlorophyll levels. For instance, the bHLH transcription factors GLK1 and GLK2 activate genes for chloroplast development and tetrapyrrole biosynthesis, while GATA-type factors GNC and GNL respond to gibberellins and light via GATA motifs to promote POR expression. The bZIP factor HY5, activated by phytochrome signaling, directly upregulates LPOR and other biosynthetic genes in light, whereas PIF repressors inhibit the pathway in darkness until light-induced degradation relieves repression; REVEILLE1 cooperates with HY5 to enhance POR transcription. These mechanisms prevent photooxidative damage by synchronizing synthesis with photosynthetic capacity. Recent studies have also identified the BCM1-EGY1 module, which maintains chlorophyll homeostasis through post-translational regulation of biosynthesis and degradation enzymes. Advances in synthetic biology have engineered novel variants of the pathway in microbes for applications like biofuel production; for example, heterologous expression of chlorophyllide a oxygenase in the chlorophyll b-lacking microalga Nannochloropsis salina enabled chlorophyll b synthesis, boosting photosynthetic efficiency by up to 17%, growth by up to 31%, and lipid accumulation by up to 43%, thereby increasing biomass yield for biodiesel feedstock. Similar optimizations in cyanobacteria and E. coli have produced chlorophyll derivatives, supporting scalable photosynthetic platforms for renewable fuels.

Degradation and Senescence

Chlorophyll degradation is a key process during leaf senescence, enabling the recycling of valuable nutrients such as nitrogen, magnesium, and carbon from the photosynthetic apparatus to support reproductive growth or storage organs. This breakdown begins with the action of the enzyme chlorophyllase (CLH), which catalyzes the hydrolytic removal of the phytol tail from chlorophyll a, yielding chlorophyllide a and free phytol. Subsequently, magnesium dechelation occurs, primarily mediated by NON-YELLOW COLORING1/STAY-GREEN (NYE1/SGR) proteins, converting chlorophyllide a to pheophorbide a. Pheophorbide a then serves as the substrate for pheophorbide a oxygenase (PAO), which initiates the irreversible opening of the porphyrin macrocycle, leading to the formation of fluorescent and non-fluorescent catabolites that are transported to the vacuole for safe storage. An alternative route in the degradation pathway involves the initial removal of magnesium from by SGR, producing pheophytin a, followed by of the phytol chain by pheophytinase (PPH) to generate pheophorbide a. This pheophytin-mediated path contributes to the chlorophyll catabolic cycle, where intermediates facilitate the disassembly of chlorophyll-protein complexes in membranes, promoting efficient nutrient remobilization. The overall process ensures that typically 40–90% of nitrogen can be retranslocated during , depending on the . Environmental stresses such as drought and nutrient deficiency accelerate chlorophyll degradation by inducing (ABA) signaling, which upregulates catabolic enzymes like PAO and SGR. In autumn, shorter days and cooler temperatures trigger in trees, where chlorophyll breakdown unmasks underlying and anthocyanins, producing the characteristic yellow, orange, and red foliage colors. This seasonal degradation is evolutionarily conserved across higher plants, optimizing before .

Occurrence and Distribution

In Plants and Algae

In green , chlorophyll is most abundant in leaves, where it typically constitutes 0.5 to 2% of the dry weight, with concentrations ranging from 5 to 20 mg per gram of dry tissue depending on and environmental conditions. This pigment is primarily localized in chloroplasts, enabling efficient light capture for , and its levels are highest during periods of active growth. Variations occur across plant ; for instance, maintain chlorophyll in their foliage for extended periods—often 2 to 4 years—due to prolonged lifespan and reduced seasonal degradation compared to . In , chlorophyll concentrations peak in summer and decline in autumn as part of processes. Algae, as eukaryotic phototrophs, exhibit diverse chlorophyll profiles adapted to aquatic environments. serves as the primary pigment across all algal groups, but accessory chlorophylls enhance light absorption in specific lineages; for example, is prevalent in diatoms and other chromalveolates, aiding in the utilization of wavelengths in marine waters. Chlorophyll f, though less common, occurs in certain often associated with algal assemblages, extending absorption into far-red light for low-light niches. Oceanic , dominated by diatoms and other algae, represent a major component of global phototrophic and account for approximately 50% of Earth's through their high turnover rates. Plants and algae display adaptations in chlorophyll content and composition to optimize light harvesting under varying conditions. Sun-exposed leaves in plants accumulate higher chlorophyll concentrations per unit leaf area to maximize photon capture in intense light, whereas shade leaves prioritize chlorophyll per gram of dry weight for efficient use of diffuse light. The chlorophyll a:b ratio adjusts accordingly, typically higher (around 3:1) in sun leaves to favor photosystem I activity under high irradiance, and lower (around 2.5:1) in shade leaves to broaden the absorption spectrum for poorer light quality. Similar adjustments occur in algal populations, where pigment ratios shift in response to water column depth and light availability.

In Microorganisms and Other Organisms

Green sulfur , such as those in the genus Chlorobium, employ chlorophyll a alongside s c, d, or e in their chlorosomes for efficient light harvesting during . These pigments enable the to absorb light in the near-infrared spectrum, supporting their anaerobic lifestyle in sulfur-rich environments. In contrast, , including species like Rhodobacter sphaeroides, primarily utilize a and its variant b, which are embedded in intracytoplasmic membranes to facilitate energy transfer in photosynthetic reaction centers. These variants allow to varying light conditions, with b shifting absorption peaks to optimize under low-light or far-red illumination. Chlorophyll occurs rarely in animals through , where certain s sequester functional chloroplasts from algal prey. For instance, the Elysia chlorotica incorporates chloroplasts from the alga Vaucheria litorea, enabling it to perform and sustain itself without food for months via chlorophyll a-mediated light reactions. Similarly, Elysia timida induces modifications in stolen chloroplasts from Acetabularia acetabulum to enhance photoprotection, demonstrating how these symbiotic associations extend chlorophyll's role beyond microbial and domains. In , chlorophyll's spectral signatures serve as potential biosignatures for detecting photosynthetic on exoplanets. The red-edge absorption feature of around 680–700 nm, combined with oxygenic byproducts, could indicate surface on habitable worlds, as modeled for direct imaging missions like the Habitable Worlds Observatory. These signatures, detectable via high-resolution , distinguish biological pigments from abiotic reflectances on Earth-like exoplanets within 15 parsecs. Ocean acidification, driven by climate change, has induced shifts in microbial chlorophyll distribution in marine environments up to 2025. Studies show that lowered levels (e.g., from 8.1 to 7.8) alter communities, increasing chlorophyll a concentrations in some microbial food webs while reducing formation and primary productivity in others, particularly affecting coastal bacterial and assemblages. Combined with warming, acidification promotes dominance over diatoms, reshaping chlorophyll-based productivity in nutrient-enriched waters.

Measurement and Analysis

Quantification Techniques

Laboratory quantification of chlorophyll typically begins with extraction from biological samples using organic solvents such as 80-90% acetone or 95-96% , which effectively solubilize the pigments while minimizing degradation. Acetone is widely preferred for its high extraction efficiency and compatibility with downstream spectrophotometric , often involving grinding or homogenization of plant material followed by to obtain a clear supernatant. offers a greener alternative with comparable yields, particularly for tissues, though it may require adjustments for solvent-specific coefficients. These extracts are then analyzed spectrophotometrically at wavelengths around 645 nm and 663 nm, corresponding to chlorophyll absorption peaks, using equations derived from empirical calibrations with pure standards. A seminal approach for spectrophotometric quantification is the set of equations proposed by Arnon in 1949, which allow calculation of chlorophyll a, b, and total concentrations from absorbance readings. For example, in 80% acetone, chlorophyll a is computed as Chl a=12.7×A663−2.69×A645\text{Chl a} = 12.7 \times A_{663} - 2.69 \times A_{645} where A663A_{663} and A645A_{645} are the absorbance values at 663 nm and 645 nm, respectively, with concentrations expressed in mg/L; similar formulas apply for chlorophyll b (Chl b=22.9×A645−4.68×A663\text{Chl b} = 22.9 \times A_{645} - 4.68 \times A_{663}) and total chlorophyll. These equations, validated against pure pigment standards, provide rapid estimates but assume negligible interference from carotenoids or degradation products, with adaptations for other solvents like ethanol to account for shifted absorption spectra. Updated variants, such as those for 90% acetone, refine coefficients to improve accuracy across diverse sample types. For higher resolution separation of chlorophyll types (a, b, c, d) and isomers, (HPLC) is employed, often using reversed-phase C18 columns with methanol-acetone or gradients as mobile phases. A widely adopted method, developed by et al. in 1991, enables quantification of over 20 pigments in marine extracts by monitoring at 440-665 nm, with detection limits around 0.1-1 ng per injection and linearity up to 100 µg/mL. This technique distinguishes chlorophyll derivatives like pheophytin, which may form during extraction, ensuring purity assessment. Coupled with (LC-MS), HPLC provides structural confirmation via molecular ions (e.g., m/z 893 for chlorophyll a [M+H]+ in positive ESI mode), particularly useful for identifying metallo-chlorophyll complexes or degradation artifacts in complex matrices. Accurate quantification relies on calibration with pure chlorophyll standards, typically sourced from commercial suppliers like , which are verified by their absorbance ratios (e.g., A663/A645 ≈ 3.6-4.0 for chlorophyll a in acetone). Standard curves are constructed across 0.1-50 µg/mL ranges, with correlation coefficients (R²) exceeding 0.99 for both spectrophotometric and HPLC methods. However, errors can arise from degradation during extraction, such as acid-catalyzed conversion to pheophytin (reducing apparent chlorophyll a by up to 20-30% in acidic conditions) or light/heat-induced allomerization, which alters structure and . To mitigate these, extractions are performed in dim light at 4°C, with antioxidants like added, and recoveries validated against spiked samples yielding 90-110% efficiency.

Environmental Assessment

Environmental assessment of chlorophyll in ecosystems relies on non-destructive, indirect methods that enable large-scale monitoring of concentrations and their ecological implications without disrupting natural habitats. These approaches, primarily through and techniques, provide proxies for chlorophyll levels, facilitating the study of , productivity, and environmental changes. Such methods are essential for tracking spatiotemporal variations in chlorophyll distribution across , forests, and other biomes. In marine environments, satellite-based remote sensing, particularly from the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard NASA's Aqua satellite, estimates near-surface chlorophyll-a concentrations using empirical algorithms derived from in situ data. These algorithms employ blue-to-green reflectance ratios, exploiting chlorophyll's strong absorption in blue wavelengths (around 443 nm) and lower absorption in green (around 547 nm), to derive concentrations in mg m⁻³ with applicability across global ocean color sensors. Validation efforts confirm the reliability of these estimates for open ocean waters, though regional adjustments may be needed for coastal areas. Recent advancements as of 2025 include the integration of Sentinel-2 data for higher-resolution chlorophyll-a mapping in coastal and inland waters. On land, the Normalized Difference Vegetation Index (NDVI), computed from red and near-infrared reflectance bands as
NDVI=NIR−RedNIR+Red\text{NDVI} = \frac{\text{NIR} - \text{Red}}{\text{NIR} + \text{Red}}
serves as an indicator of vegetation health and chlorophyll content, with values ranging from -1 to 1; higher positive values (typically >0.3) denote dense, chlorophyll-rich canopies reflecting healthy photosynthetic activity. NDVI data from MODIS products have been instrumental in assessing global vegetation vigor since 2000. In situ fluorometry offers a complementary, field-deployable method for rapid chlorophyll assessment, measuring the fluorescence emitted by chlorophyll-a when excited by blue light (around 460 nm), typically yielding real-time concentrations calibrated against laboratory standards. Portable fluorometers, such as those used in aquatic and terrestrial surveys, enable non-invasive sampling in rivers, lakes, or soils, with studies showing strong linear correlations (r > 0.8) between fluorescence signals and actual chlorophyll levels under varying environmental conditions. These measurements often correlate positively with primary productivity; in oceans, elevated chlorophyll-a concentrations signal higher phytoplankton biomass and net primary production rates, driven by nutrient availability and light, as evidenced by satellite validations in productive zones like coastal upwelling regions. Similarly, in forests, NDVI-derived chlorophyll proxies link to gross primary production, where declines indicate reduced carbon fixation. These techniques underpin critical monitoring applications, such as detecting harmful algal blooms (HABs) through spikes in satellite-observed chlorophyll-a, which have intensified in frequency and extent from 2020 to 2025 due to warming and runoff. models, such as networks, have improved one-day-ahead HAB predictions using chlorophyll-a as a key indicator as of 2025. For example, MODIS and similar sensors have mapped HAB events in the and coastal waters, enabling early warnings for and . In terrestrial settings, NDVI tracks impacts by revealing sharp drops in vegetation greenness—often exceeding 20% in affected areas—corresponding to chlorophyll loss and heightened vulnerability, as demonstrated in Amazonian and African biomes through 2025 analyses showing post-clearing productivity reductions of up to 15%. Lab confirmations occasionally validate these field estimates for accuracy.

Historical Development

Early Discovery

The discovery of chlorophyll emerged from early investigations into the green coloration of plants and its role in light-dependent processes. In 1779, conducted experiments demonstrating that only the green parts of release oxygen in , linking this phenomenon to the formation of from and , thus establishing the foundational context for chlorophyll's involvement in . A significant milestone occurred in 1817 when French chemists Joseph Pelletier and Joseph Bienaimé Caventou isolated the green pigment from plant leaves using alcohol extraction. They treated pressed and washed plant pulp with cold distilled alcohol, evaporated the solution to yield a dark green resinous substance, and further purified it by dissolving in hot water to remove impurities, resulting in a substance they named "chlorophylle" (from Greek chloros for green and phyllon for leaf). Pelletier and Caventou described it as a distinct, hydrogen-rich plant principle essential for physiological functions, separate from resins or starches, and considered their isolate pure as a "principe immédiat." Throughout the 19th century, scientists debated the homogeneity of chlorophyll, questioning whether it constituted a single uniform substance or a mixture of pigments varying across plant species. George Gabriel Stokes in 1864 proposed that chlorophyll might comprise at least two distinct green components based on spectroscopic observations, a view later supported by researchers like Mikhail Tsvett and Léon Marchlewski, challenging the notion of its chemical uniformity. These discussions highlighted uncertainties about its composition despite its consistent green appearance in diverse plants. Advances in in the late clarified chlorophyll's localization within plant cells. In 1883, German botanist Andreas Franz Wilhelm Schimper examined the development of pigment bodies and coined the term "Chloroplastiden" (later simplified to chloroplasts) for the chlorophyll-containing granules in plant cells, distinguishing them from other plastids like colorless leukoplasts and colored chromoplasts. Schimper's work emphasized that these organelles arise from the division of preexisting ones and are sites of chlorophyll accumulation, providing early evidence of their structured role in green tissues.

Modern Characterization

In the early , Richard Willstätter advanced the characterization of chlorophyll through meticulous isolation techniques, culminating in the purification of in crystalline form by 1915, a achievement that earned him the for his work on pigments. His research demonstrated that constitutes the primary in , with a molecular structure centered around a magnesium coordinated within a ring, marking the first identification of magnesium's essential role in a and paralleling iron's function in . Willstätter's group also distinguished as a secondary during experiments from 1906 to 1914, confirming its distinct absorption properties and through fractional and spectroscopic , which revealed its role in broadening the light-harvesting in . Mid-20th-century progress shifted toward structural elucidation of chlorophyll within photosynthetic complexes using . The landmark 1985 determination of the bacterial photosynthetic reaction center structure by Deisenhofer, Epp, Miki, Huber, and Michel at 3 Å resolution revealed the arrangement of molecules—close analogs to chlorophyll—in chains, providing the first atomic model of a involved in light energy conversion and Nobel recognition in 1988. This breakthrough informed subsequent studies on oxygenic photosystems; by 2001, high-resolution structures of plant (PSI) at 2.5 Å and (PSII) at 3.8 Å, solved by Jordan et al. and Zouni et al. respectively, illustrated how and b molecules are embedded in protein scaffolds to facilitate charge separation and oxidation. These models highlighted chlorophyll's precise positioning for efficient energy transfer, with over 100 chlorophylls per coordinating excitons across the complexes. Recent decades have leveraged advanced techniques like nuclear magnetic resonance (NMR) spectroscopy and cryogenic electron microscopy (cryo-EM) to achieve atomic-level insights into chlorophyll's dynamics and interactions. NMR studies, building on earlier assignments of chlorophyll's proton and carbon environments, have probed solution-state conformations and aggregation behaviors in synthetic analogs, revealing subtle shifts in ring puckering that influence photostability. Cryo-EM has revolutionized in situ structural biology, yielding near-atomic models of photosystems; for instance, a 2024 cryo-EM structure of a PSI variant at 2.03 Å resolution detailed the binding of plastoquinone derivatives near chlorophyll sites, elucidating adaptations in electron transfer pathways. Similarly, a 2025 cryo-EM analysis of mangrove PSI–LHCI supercomplex at 2.1 Å exposed environmental adaptations in chlorophyll coordination under saline conditions. Genetic studies up to 2025 have linked chlorophyll's function to genes through analyses and genome-wide association. CRISPR-Cas9 of genes like those encoding magnesium chelatase in protoplasts has produced targeted chlorophyll-deficient phenotypes, confirming their regulatory roles in accumulation. In , the OsNF-YB7 was shown in 2024 to repress chlorophyll by inactivating OsGLK1, reducing expression of protochlorophyllide reductase and leading to variegated leaves in knockouts. mapping in identified candidate genes such as those in the pathway influencing leaf chlorophyll content, with variants explaining up to 20% of phenotypic variation across diverse populations. These findings underscore how genetic perturbations directly impact chlorophyll integration into , informing models of .

Applications and Uses

Natural and Culinary Uses

In nature, chlorophyll contributes to the green coloration of certain animals, enhancing their against predators by mimicking surrounding vegetation. For instance, the sacoglossan Elysia chlorotica sequesters functional chlorophyll from ingested , allowing it to maintain a bright green appearance that blends seamlessly with algal habitats. This incorporation not only supports temporary but also provides visual in marine environments. Chlorophyll derivatives have a history in , particularly for in remedies. Early clinical studies in the showed that water-soluble chlorophyll preparations accelerated tissue repair and reduced in suppurative wounds when applied topically, promoting and epithelialization. These applications, often derived from plant extracts like , were used in ointments and dressings to deodorize and stimulate , though modern evidence confirms benefits primarily for topical use rather than ingestion. In culinary contexts, chlorophyll functions as a natural green pigment for coloring foods and beverages, classified as the additive E140 (chlorophylls) in the . Extracted from sources such as or nettles, it imparts an olive-to-dark green hue to products like , ice creams, sauces, and processed , offering a plant-based alternative to synthetic dyes. However, its instability under heat, light, and acidic conditions limits widespread use, often requiring stabilization through brief extraction processes or formulation with natural carriers. Regulatory bodies approve E140 for specific food categories at levels in the EU, while in the , related copper chlorophyllin (not pure chlorophyll) is permitted at up to 0.2% in certain dry mixes under FDA guidelines. Health claims associated with culinary chlorophyll intake include potential detoxification effects, such as binding to carcinogens like aflatoxin-B1 to reduce their absorption and urinary DNA adducts. A small human trial demonstrated a 55% reduction in aflatoxin biomarkers with 100 mg daily chlorophyllin supplementation over four months in high-exposure populations. Similar in vitro studies show chlorophyll chelating endocrine disruptors like p-nonylphenol, supporting liver detoxification pathways. Nonetheless, evidence remains limited, relying on preliminary animal, in vitro, and small-scale human data, with no large randomized trials confirming broad detox benefits from dietary sources. Marketing trends have popularized as a wellness beverage, with continued growth in U.S. sales driven by promotion for , clearer , and reduced , including a 114% year-over-year increase in views as of 2025. Products like Chlorophyll Water® emphasize natural sourcing and fortification, capitalizing on clean-label demands. Experts caution that while generally safe as a food colorant, these claims lack robust scientific backing, and regulatory oversight treats it primarily as a non-nutritive additive rather than a therapeutic agent.

Industrial and Emerging Applications

Chlorin e6, a derivative of , serves as a key in (PDT) for cancer treatment, where it absorbs light to generate that selectively destroy tumor cells. This FDA-approved compound exhibits high ROS production efficiency and deep tissue penetration due to its absorption at around 660 nm, enabling applications in various cancers including lung, , and skin tumors. Nanosystems such as liposomes and nanoparticles enhance its solubility and tumor targeting, improving therapeutic outcomes when combined with or . Synthetic chlorophyll analogs, including pheophorbide and chlorin derivatives, are incorporated into thin-film photovoltaic cells as photoactive electron donors, mimicking natural light-harvesting processes to convert into . These materials benefit from their broad absorption spectra, though challenges in film morphology and charge mobility limit scalability, with self-assembled J-aggregates showing promise for bio-inspired . Emerging bioengineering efforts introduce synthesis into like salina via heterologous , enhancing and accumulation for production. This modification increases growth by 18–31% and productivity by up to 43% under medium light conditions, supporting sustainable from algal sources without compromising cellular health. Similarly, mimicking chlorophyll structures, such as Mg-N₄-decorated , facilitate by integrating light absorption and catalysis for CO₂ reduction. These systems achieve reaction rates of 9.67 mmol·g⁻¹·h⁻¹ in CO₂ under mild conditions, offering stable, metal-free alternatives for generation. Chlorophyll fluorescence detection via in-situ sensors monitors algal blooms as indicators of in aquatic environments, enabling real-time assessment of from sources like agricultural runoff. These optical fluorometers quantify concentrations to predict water quality degradation. In , the chlorophyll spectral at 670 nm persists in and spring analogs under Mars-like conditions, detectable by rover instruments like Pancam for identifying potential past on planetary surfaces. This feature withstands UV radiation and low pressure for weeks to months when shielded by minerals, aiding missions to explore habitable zones on Mars and beyond.

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