IT'S NOT love that makes the world go round, it'sphotosynthesis . Virtually all plants and animals dependon it . Photosynthesis is the process by which plants trap

sunlight andturn it into chemical energy. Today, most plantstake in carbon dioxide from the air andcombine it with waterusing light energy, to produce sugars and oxygen. It is acomplicated chemical process that took a long time to evolve .Even nowthe process has its shortcomings . The most obviousone is that it does not make the most of the light available.Plants look green because they contain a pigment, chloro-

phyll, which absorbs light at the red and blue ends of thespectrum and uses it for photosynthesis . The plant reflectsthe wavelengths in between, principally those of green light;this is what we see. Ideally, a photosynthetic pigment shouldbe black, that is, it should absorb light at all visiblewavelengths so that it wastes none of its energy . So why didnature choose green?To find an answer,we first have to go to a salt lake andthen

back in time, almost to the beginning of life . The salt lakeprobably contains an unusual photosynthetic bacterium,called Ilalobaclerium holohium . This species bears all thehallmarks of a living fossil . Halobaclerium lives in salt lakeswhere the concentration of salt may be as high as 20 per cent(seawater is only 4 per cent). Almost all other organismswould die in salt at this concentration, Halobacterium diesif the salt concentration falls to levels at which otherorganisms can grow . The bacterium is therefore isolated inan environment free of competition from other organisms.Because of this, much of evolution has passed it by; it still

Why trees are greenThe countryside is green. In an ideal world it would be black . A new theory explains why

Andrew Goldsworthy

has many of the attributes of early forms of life .One of Halohacterium's ancient characteristics is that it

has unusual membranes. These contain branched terpenoidsin place of the unbranched fatty acid molecules that modernorganisms have in their membranes. Perhaps terpenoidspredate fatty acids as components of the cell's membranes.The way lIalobaclerium photosynthesises is also extremely

primitive. The bacterium has no chlorophyll. Instead, itcontains a purple pigment, bacteriorhodopsin, so calledbecause it is similar to rhodopsin, the light-sensing pigment inthe eyes of animals. lIalobacterium's photosynthetic pigmentis based on a terpenoid molecule of the same size as those inits membrane and might have evolved from them. It isdifficult to say exactly how old Halobacterium is becauseorganisms as simple and as small as this leave no fossils-atleast not that we would recognise easily. However, MelvinCalvin, who discovered a vital biochemical pathway in photo-synthesis (the Calvin cycle) found terpenoids resemblingthose in Halobacterium's membranes in rocks about 3000million years old. If these compounds are relics of organismssimilar to Ilalobacterium, they must have been widespreadby this time . They, might have originated much earlier-perhaps close to the dawn oflife, which wasprobably between3000 and 4000 million years ago.We know agood deal about the photosynthesis and metab-

olism of Ilalobacterium, mainly as a result ofresearch in the1970sby Walther Stoeckenius of the University ofCaliforniaat San Francisco. Unlike organisms that contain chlorophyll,Halobacterium cannot take carbon dioxide and water and

convert them to sugars by photosynthesis . It needs a supply oforganic substances to grow . Instead, the bacterium uses lightenergy to swim, to absorb nutrients and for making adenosinetriphosphate (ATP), which, in turn, makes energy for othermetabolic reactions (see Box 1) . Such an organism would bewell suited to the conditions in the so-called "primeval broth"of complex organic materials when life was just beginning .The chemicals in the "broth" had gradually accumulated inthe seas over millions of vears as a result of the action onrelatively simple chemicals of ultraviolet light and electricaldischarges in the atmosphere .The purple colour of bacteriorhodopsin probably resulted

from natural selection for a photosynthetic pigment thatabsorbed a broad band of wavelengths in the middle of thevisible spectrum, Only wavelengths in or near the visibleregion of the spectrum have the right amount of energy tobe useful in photosynthesis . Furthermore, water stronglyabsorbs light outside this region, so the light would not havebeen available to the aquatic organisms in which photo-synthesis first evolved .

Bacteriorhodopsin lies in the bacterium's externalmembrane ; it needs light energy to pump positively chargedhydrogen ions (protons) out of the cell . As a result, a concen-

l : From purple pigment to modern methods of making energy~,JORGANISMS similar to Halobacterium

were probably among the first to uselight energy for photosynthesis . With it,they could absorb food more efficiently andmake virtually unlimited amounts of ATPto provide energy for metabolism . Theseorganisms probably pioneered the chemi-osmotic production of ATP which is nowalmost universal .

As in the rest of evolution, Halo-bacterium's type of photosynthesis and itscoupling to transport and the synthesis ofATP must have arisen in small manageablestages, with each step having a selectiveadvantage over the one before, We willprobably never know exactly how ithappened, but we can make some educatedguesses .

The first step was probably the evolutionof selective systems of passive transport,followed closely by the development ofactive transport and the production ofATf,The earliest organisms had very simple

metabolism and obtained most of theirmetabolites ready-made from the primevalbroth . Their cell membranes . like those oforganisms today . had to control the passageof materials to allow the entrv of nutrientsbut prevent the loss of metabolites . To dothis, most of the membrane is made of oilymaterials. Although the membrane is onlytwo molecules thick, the water-solublecontents of the cell cannot dissolve in it andremain trapped within the cell . To allowthe entry of nutrients that are solublein water, there are "carrier" proteins"floating" in the membrane . Theirjob is torecognise the nutrient molecules and letthem in . They do this by combining rever-sibly with specific nutrients. The resultingcompound is free to change its shape sothat the part attached to the nutrient canpass freely back and forth through themembrane . These simple systems are selec-tive . but allow passage only down adiffusion gradient .

Sometimes, organisms need to transportmaterials actively against a diffiision

New Scientist 10 December 1987

gradient from a dilute solution into aconcentrated one . This needs a pumpingmechanism driven by energy . Probably oneof the first systems of active transport toevolve was a pump for the excretion ofhydrogen ions (protons) . The cell generatesprotons in the course of metabolism . Itmust remove the protons, usually against adiffusion gradient, to keep the pH of thecell at the right value for its enzymes tofunction properly . This may require aconsiderable input of energy because theconcentration of hydrogen ions outside acell may be more than 100 times greaterthan that within .Modern organisms contain three main

types of proton pump depending on theirsource of energy . Some are driven by theoxidation-reduction reactions of respira-tion, some by photosynthesis and others byATP. Those driven by ATP probablyevolved first because the primitive atmo-sphere did not contain oxygen and photo-synthesis has to stop at night . But ATPgenerated by anaerobic fermentationscould provide the steady source of energyneeded to keep the cell's pH constant dayand night .

Unfortunately, anaerobic fermentationsare very inefficient at producing ATP, andwith the depletion of the primeval broth,the necessary substrate was runningout . The first proton pump driven bylight, perhaps based on bacteriorhodopsin,must have given its owner a considerableadvantage .Bacteriorhodopsin

consists of a single pro-tein molecule attachedto an unsaturated ter-penoid (retinal) whichgives it its colour . It liesin the external mem-brane and . when itabsorbs light, it changesits shape and thrusts aproton out of the cell .The evolution of a

system that actively

tration gradient builds up across the membrane and thebacterial cell becomes negatively charged relative to itssurroundings . The cell then releases the energy stored in thisgradient when it allows protons back into the cell . Thisprocess drives the active uptake of nutrients, providespower for the whiplike flagella that propel the bacteria andgenerates ATP.Organisms like Halobacterium would have had the advan-

tage over earlier forms of life that depended for their energyon the anaerobic fermentation of nonrenewable resources inthe primeval broth . The advent of these simple photo-synthetic bacteria provided an almost unlimited supply ofmetabolic energy . The bacteria could even recycle the spentproducts of earlier fermentations, using solar energy . Thebiosphere gained a new lease of life . These simple photo-synthetic organisms must have been extremely successfulcompared with their nonphotosynthetic ancestors and theywould probably have become the dominant form of life . Inthose days, the seas might well have been purple .

Successful as they might have been, purple bacteria suchas Halobacterium had a serious failing that ultimately ledto their downfall . they could not fix carbon dioxide .The fermentation reactions of all the nonphotosynthetic

Halobacterium halobium, theJirst photosynthesiser?

expelled protons yielded an added bonus ; itprovided the basis for the so-called co-transport mechanisms that Halobacteriumnand virtually all modern plants and bacte-ria use to take up nutrients . To do this, allthat was needed was a slight modificationof the carrier molecules to give them abinding site for protons next to that for thenutrient . Movement was permitted onlywhen both sites were full or both wereempty, so that protons and nutrientsalways crossed the membrane together . Bylinking the passage of the two, the steepdiffusion gradient for protons trying to re-enter the cell could overcome a weakerdiffusion gradient for nutrients trying toleave . Thus, cells could absorb nutrientsfrom solutions that were more dilutethan its own contents . Mechanisms such asthis enabled organisms to continue toabsorb nutrients as the primeval broth ranout of carbon .

Bacteriorhodopsin probably brought aneven more important bonus . The very steepproton gradient created by its photo-chemical reactions could drive the ATP-linked proton pumps backwards . Protonspumped out of the cell by the light couldnow flow back in, generating ATP as theydid so . Because the force that drives thisreaction is similar to osmosis, the process iscalled chemiosmosis. This may have beenhow the chemiosmotic production of ATPin virtually all modern organisms firstevolved .

11

49

so

The mast advanced plantshave the least <fcient systemfor absorbing light . :Mosses,ferns and higher plants are

"backtivard"intheir photosynthesis . Brown

algae (right) have extrapigments to make the most

oJ'the light that filtersthrough the murky ivater

organisms in the broth generated a great deal of carbondioxide as a waste product . In the absence of any effectivemeans to refix the carbon dioxide into organic chemicals, thismust have led to a steady loss of carbon from the biosphere .If the broth ran out of carbon, life would almost certainlyhave become extinct .

Fortunately, an organism with the potential to fix carbondioxide was waiting in the wings . This organism was probablya bacterium that lived deep underwater on the surface ofsediments . The organism had a different photosyntheticpigment, probably similar to chlorophyll a, the greenpigment that modern plants contain . Like bacteriorhodopsin,chlorophyll lies in the cell membrane . But unlike bac-teriorhodopsin, which functions on its own, chlorophyllworks in conjunction with a number ofother molecules in themembrane, not only to pump protons out of the cell but alsoto pump electrons in . This capacity to pump electronsenabled it to reduce carbon dioxide to sugars {see Box 2},

In simple chemical terms, chlorophyll uses light energy todrive a system of redox reactions (reduction and oxidation),to reduce carbon dioxide using what, at first sight, would

New Scientist 10 December 1981

Ir:~Pá6~~fr~~ ,__

Accessory pigments absorb more light.1he primitive cyanobacteria (above andcentre) absorb red and orange light. Redalgae (top) are almost the ideal colourand absorb even green light

seem an impossibly weak reducing agent . Bacteria withchlorophyll probably evolved as a result of natural selectionas the supply of organic materials in the primeval brothdwindled and carbon dioxide was the only major source ofcarbon left.

Life still would not have been easy for these bacteria. Theyneeded a supply ofsuitable compounds in their environmentto provide them with electrons for their type of photo-synthesis . As with modern photosynthetic bacteria, theseorganisms probably used compounds from decaying organicmatter in the sediments on which they lived . They mighthave made use of a variety of electron donors, but themost important were probably sulphur compounds such ashydrogen sulphide . The primeval broth had an ample supplyof sulphur compounds because the nonphotosyntheticorganisms in the sediment produced them as waste productsduring anaerobic fermentation .As the sediments on which the new type of photo-

synthesisers lived were deep underwater, the bacteria musthave been short of light . What little light they had was at thered and blue ends of the spectrum-all that remained after

the purple bacteria swimming above them had taken theirshare. It is not surprising, then, that they evolved a photo-synthetic pigment that absorbed specifically at these wave-lengths. The red and blue absorption peaks of chlorophyll acomplement almost exactly the green absorption peak ofbacteriorhodopsin (see Diagram below) . Modern photo-synthetic bacteria that live in sediments are similar in thisrespect . However, because modern bacteria must cope not withpurple bacteria swimming above them, but with organismscontaining chlorophyll, they contain special chlorophylls,such as bacteriochlorophyli, in which the main absorptionpeaks are pushed even further outwards into the infrared .

P HOTOSYNTIIESIS with chlorophyllreduces carbon dioxide to sugars

using electrons taken From a donor mole-cule . The transfer of' electrons betweenmolecules is called a redox reaction: onesubstance is oxidised when it donateselectrons to another, and the secondcompound is reduced in the process. Forelectrons to flow between molecules in thisway, the receiving molecule must have agreater attraction for the electrons than thedonor. This difference can be measured asa voltage, or redox potential . Most coin-monly available compounds in the envi-ronment do not release their electrons withenough potential to reduce carbon dioxide.

Photosynthesis based on chlorophyllsolved this problem. Unlike bacterio-rhodopsin, which uses light energy to trans-fer protons across the cell membrane . chlo-rophyll uses light to transfer electrons .Chlorophyll takes these electrons from rela-tively weak donor molecules outside thecell . When light excites the chlorophyll, thepigment molecule ejects these electronsinto the interior with enough energy toreduce carbon dioxide. The chlorophylldoes not reduce the carbon dioxidedirectly . Instead . the electrons it ejectsfeed into a chain of electron "carrier" mole-cules which pass them on from one toanother until they finally enter a biochem-ical pathway called the Calvin cycle.

'I he reactions in the Calvin cycle reducecarbon dioxide to sugar . The cycle needsATP as well as electrons. Photosynthesiswith chlorophyll provides the ATP. Someof the electron carrier molecules arearranged in the membrane so that as theytransport the "high-energy" electronscoming from chlorophyll, they combinewith protons on the inside of the cell andrelease them on the outside. In this way.some nf' the energy of the ciectrom helpsto generate a proton gradient across

Z : Photosynthesis with green pigments

New Scientist 70 Qecerriber 7987

the membrane for themanufacture of ATP. Ifthe cell needs extra ATPfor any reason . some ofthe electrons, instead ofbeing passed to carbondioxide, are returned tothe starting point via achain ofelectron carriersand go through thephotochemical systemagain . Each time the velectrons pass throughthe circuit they add tothe proton gradient andgenerate ATP.

dIn modern photo-

synthetic bacteria,chlorophyll is concen-trated in folds in theouter membrane called chromalophores .This enhances the effectiveness of' theproton transport system . After crossing themembrane, the protons remain confinedwithin the chromatophore [which, tech-nically, is outside the cell] and build up asteeper gradient . The steeper the gradient,the more efficient the production of A 1'Pwhen the protons return to the cell .The system of electron carriers and

enzymes that services the chlorophyllmolecule is complex and bulky. To avoidthe need for such a system for each chloro-phyll molecule, photosynthetic organismsevolved a phenomenon called resonancetransfer, which allows many pigmentmolecules to share the same enzymes.Resonance transfer enables one pigmentmolecule to transfer to another the energyit absorbs from light . Chlorophyll mole-cules are arranged in groups of betweenabout 5(1 and 3U(1, called photosyntheticunits . At the hub of each unit . at the so-called reaction centre, are specialisedchlorophyll molecules that collect the

The absorptionspectrum rfchlorophyll asuperimposed on theabsorption spectrumof, membranescontaininghacteriorhndop5in(shadedj .Chlorophyll'sabsorption peaks ftneat[ °oneither sideofbacteriorhudnpsin's

Careen plants keep their chloruphy11 in chloroplasts

energy from the other chlorophyll mole-cules and use it to transfer electrons acrossthe membrane .

In bacteria, the photosynthetic mem-branes are simply infoldings of the cell'souter membrane . In higher plants, thephotosnthetic membranes lie withinspecial' organelles called chloropiasls .Chloroplasts bear a remarkable resem-blance to photosynthetic bacteria, but theyare surrounded by an extra membrane .Biologists think that chloroplasts originatedwhen a primitive, nonphotosyntheticorganism engulfed one of the types ofphotosynthetic bacteria that produceoxygen . Once inside a vacuole (now theouter membrane of the chloroplast), thebacterium lived in a svitthiotic relationshipwith the organism that had engulfed it . Theinner membrane of the chloroplast . whichcorresponds to the outer membrane of theoriginal bacterium, is thrown into acomplex pattern of folds, the thylakoids,which contain chlorophyll and correspondto the bacterial chromatophores .

0

As the biosphere ran out of organic compounds, the abilityto fix carbon dioxide became more important. The greenbacteria began to have the advantage as the purple bacteriastarved to death . Life had won a reprieve when organismsbegan to fix carbon dioxide using electrons from sulphurcompounds. But the relative scarcity of electron donorslimited the amount of carbon dioxide these organismscould recapture from the atmosphere . Salvation came inthe form of photosynthesis that produced oxygen . Theprinciple of this process is simple . Instead of taking electronsfrom relatively scarce sulphur compounds, organisms tookthem from water. of which there is an almost unlimitedsit pplv . The removal ofelectrons from water to reduce carbondioxide results in the decomposition of water molecules withthe production of oxygen .

This apparently simple act, however, involved a hugechange in biochemistry . The voltage that a single light-energised chlorophyll molecule can generate by its electron-pumping action is not enough to strip electrons from waterand give them to carbon dioxide. To accomplish this, natureused the simple expedient of taking two such electron pumpsand arranging them in series, rather like putting two cellsinto a flashlight to obtain a higher voltage. A second photo-chemical electron pump evolved alongside the first . A chainof electron carriers connected the two electrically_ so that elec-trons had to pass through each pump in turn on their wayfrom water to carbon dioxide and so received a double dose

57

Why leaves are green the cells within a leaj'are packed withchlorophyll, which absorbs red light

ofenergy . The old electron pump (now called photosystem 1)delivered electrons to the systems that fixed carbon dioxide,and the new one (photosystem 2) removed the electrons fromwater and delivered them to photosystem 1 .

The evolution of photosystem 2 was perhaps one of themost important steps in evolution . For the first time, fixationofcarbon dioxide was independent of the amount of decayingorganic material and sulphur compounds in the environ-ment. As a result, life began to flourish . The amount ofcarbon in the biosphere grew steadily until the carbon dioxidehad all but disappeared from the atmosphere and the massiveamounts of oxygen produced allowed the evolution of theanimal kingdom .

By the time this new form of photosynthesis had evolved,most of the purple bacteria had probably disappeared . Thismeant that much more light in the centre of the spectrumbecame available and natural selection sought ways to use it .Yet black plants-which would absorb all this light--did notevolve . Because photosynthesis is such a complex process,it would have been difficult to exchange chlorophyll foranother, fundamentally different, pigment. Instead, a processevolved in which a number of"accessory pigments" absorbedlight in the hitherto unused centre ofthe spectrum and passedtheir energy on to chlorophyll by a simple physical processcalled resonance transfer .

Different plants have evolved with accessory pigmentsabsorbing at different wavelengths . Often they have severalsuch pigments with overlapping absorption bands so thatthey make the most of a large part of the spectrum (seediagram above) . Perhaps the simplest organisms to makeextensive use of accessory pigments were the cyanobacteria .Their pigments, phycocyanin and allophycocyanin, absorb inthe orange region, but even here they dó not cover the wholespectrum .An improvement came with the evolution ofred algae . Red

algae have not only the orange-absorbing pigments of thecyanobacteria but also contain large amounts of a redpigment, phycoerythrin, which absorbs in the green . Betweenthem, these pigments cover a much larger part of the spec-trum . Despite their name the red algae tend to be very dark,approaching black-the ideal colour. Similarly, the darkcolour of brown algae, the familiar seaweeds, is . due to acombination of chlorophyll and the accessory pigmentfucoxanthol . Both red and brown algae usually grow in rela-tively deep water where the light is dim . The pressure on

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350 400 450 500 550 600 650 700Wavelength (nm)

The absorptionspectra ofthe mainaccessory pigmentsfrom the red algae.Light energy passesfrom left to rightalong this chain ofoverlapping pigmentsand then to the redabsorption peak ofchlorophyll a

plants to absorb light more efficiently has produced accessorypigments that cover a good range of the spectrum .Where selection pressure has been weaker, plants have not

developed such a range of pigments. Green algae, which livenear the surface of water, and the land plants that evolvedfrom them, are not so short of light and depend almostentirely on chlorophyll . Green plants have two kinds ofchlo-rophyll . Chlorophyll a is the main photosynthetic pigment ;chlorophyll b, which has two peaks of absorption near thecentre of the spectrum, functions only as an accessorypigment . Although chlorophyll b absorbs some of the lightmissed by chlorophyll a there is still a large gap between itstwin absorption peaks where light escapes unabsorbed .Carotenoids partly filled this gap, absorbing light towards theblue end of the spectrum . But these orange and yellowpigments do not pass their energy efficiently to chlorophylland no pigment absorbs significantly ~in the green,Consequently, the plant wastes a considerable amount ofenergy in the middle of the spectrum and reflects green light .

Paradoxically, the most advanced plants have the leastadvanced system for absorbing light-because they grow inadequate light and do not have to extract as much energy aspossible from light . This "backwardness" does have a bonus .Few of us would like to see our green countryside turn blackin the interests of photosynthetic efficiency . Instead, we mayowe our thanks for pleasant green surroundings to the sea ofpurple bacteria countless millions of years ago . These tinypurple organisms probably provided the selection pressurethat made modern vegetation green . Not only that, theirphotosynthetic pigment gave rise to our visual pigment andso gave us the eyes to see it .

117 _r

Andrew Goldsworthy lectures in plant physiology and biochemistryat Imperial College, London .