Botanists have found that plants process a much wider range of the sun’s rays than believed possible, enabling them to survive in almost any environment. Photo: ChinaFotoPress Science
By Jamie Brown
Imagine a super-efficient transistor capable of switching electronic components on or off at lightning speed. Coated with nature’s own light filter, chlorophyll, such a transistor would make tomorrow’s smartphones and iPads faster and more powerful and alluring than ever.
And picture plants that grow more readily and prolifically than usual, even under the toughest conditions. Genetically modified, in order to absorb more light, these plants may one day help offset some of the effects of global warming.
Finally, envisage solar panels that are lighter and yet more efficient at converting sunlight into much-needed green electricity.
Such potential advances are not pie in the sky but the result of pure research by botanists keen to understand how plants, both terrestrial and marine, process a much wider range of the sun’s rays than believed possible, enabling them to survive in almost any environment.
The discovery was made after a team of Australian scientists came upon a new form of chlorophyll, the green plant pigment that absorbs the solar energy needed to synthesise carbohydrates from carbon dioxide and water.
The team leader, Sydney University biologist Min Chen, says scientists previously knew of only four chemically distinct types of chlorophyll. The fifth, labelled chlorophyll f, uses long-wavelength light – which has a lower energy content per photon of light than any other known type.
“We are already finding potential applications for the first new form of chlorophyll to be discovered in more than six decades,” explains Dr Chen, whose findings, published in the American journal Science, challenges botanists’ understanding of the physical limits of photosynthesis.
In contrast with animals, plants, algae and some bacteria obtain their sustenance not directly from food but by absorbing energy from the sun’s rays, a process called photosynthesis. Using a complicated metabolic pathway, the sunlight is used to convert carbon dioxide and water into glucose, an energy-rich sugar that powers plants and helps them grow and reproduce.
Photosynthesis occurs in the leaves of plants, inside minuscule structures called chloroplasts, which contain the crucial molecule chlorophyll that absorbs sunlight shining on the leaves. In addition to producing glucose, photosynthesis also gives off oxygen, which is released as a form of waste product through the plants’ pores.
Until recently, photosynthesis was believed to occur at wavelengths of between 400 and 700 nanometres, mainly because chlorophyll was thought to be limited to absorbing light in this range.
This long-held belief, described in earlier textbooks, was overturned in 1996 when scientists found a cyanobacterium, a form of photosynthetic bacteria, better known as blue-green algae, found in fresh and salt water, containing a modified chlorophyll molecule, named chlorophyll d. It could photosynthesise using light just outside the 400-700-nanometre range – at a wavelength of 710 nanometres, the infrared region of the electromagnetic spectrum.
The finding perplexed scientists who struggled to understand how chlorophyll d was able to muster sufficient energy from infrared light for photosynthesis to take place.
Now the rules of photosynthesis need to be rewritten again, says Dr Chen, with the newly discovered chlorophyll f that uses light with lower photon energy at a wavelength of 730 nanometres – less than any other known type of green pigment.
“It seems that minor changes to the molecular structure of chlorophyll allow photosynthetic organisms to survive in almost any environment,” she explains.
Chlorophyll f was discovered by accident within stromatolites collected from Western Australia’s Shark Bay. These rock-like structures are built by photosynthetic microbes in the form of single-celled cyanobacteria.
The new chlorophyll allows cyanobacteria to photosynthesise using low-energy infrared sunlight.
“Finding the new chlorophyll was totally unexpected – it was one of those serendipitous moments of scientific discovery,” recounts Dr Chen, who was searching for chlorophyll d, which she knew could be found in cyanobacteria living under low-light conditions. “I thought stromatolites would be a good place to look, since the bacteria in the middle of the structures don’t get as much light as those on the edge.”
After obtaining a sample of stromatolite, Dr Chen searched for chlorophyll d by culturing the sample in infrared light. This ensured that only cyanobacteria with chlorophylls able to absorb and use infrared light could survive.
High-tech analyses of the cultured sample performed six months later revealed trace amounts of not just chlorophyll d, but also the new pigment – chlorophyll f.
An interdisciplinary team – including Sydney University’s Dr Martin Schliep and Dr Zhengli Cai, Macquarie University’s Associate Professor Robert Willows, University of New South Wales’ Professor Brett Neilan and Munich University’s Professor Hugo Scheer – characterised the absorption properties and chemical structure of chlorophyll f.
Testing its absorption spectrum revealed that chlorophyll-f could absorb much longer wavelengths of light than any other known type – 10 nanometres longer than chlorophyll-d and more than 40 nanometres longer than chlorophyll-a.
The sophisticated technique of nuclear magnetic resonance spectroscopy was used to determine the new pigment’s chemical structure, with details published in the journal Organic Letters.
Results indicated that most chlorophyll varieties exhibit remarkably similar chemical structures. Yet the minuscule nuances enable them to function in starkly different sorts of light environments.
The discovery has completely overturned the traditional notion that photosynthesis needs high-energy light, Dr Chen says. “It’s amazing that this new molecule, with a simple change to its chemical structure, can absorb extremely low-energy light. This means photosynthetic organisms can use a much larger portion of the solar spectrum than previously thought and that the efficiency of photosynthesis is much greater than we imagined.”
Chlorophyll f’s ability to absorb infrared light is expected to have numerous applications in industries such as plant biotechnology and bio-energy. It could also be used to help make solar panels more efficient by allowing them to convert a higher proportion of light into electricity.
Chlorophyll is very efficient at absorbing light, says Wei-Hua Wang of Taiwan’s Institute of Atomic and Molecular Sciences, Academia Sinica. He is working on a transistor coated with chlorophyll that increases the efficiency of switches in electronic components.
“Chlorophyll is remarkably stable and abundant,” Dr Wang explains.
“This is the first time that a biomaterial such as chlorophyll has been used as a photosensitiser for a graphene-based phototransistor. The fact that the performance of the device is comparable to other systems is impressive and shows a promising future.”
For Dr Chen and her team, the next challenge is to work out how the new chlorophyll works in the process of photosynthesis.
“Is its job to capture additional red light and pass it on to another form of chlorophyll, such as chlorophyll a?” she asks. “Or is it the only chlorophyll responsible for photosynthesis in cyanobacteria? And if it is, we will need to understand how this molecule can get enough energy from infrared light to make oxygen from water.”
The discovery, she believes, will transform scientists’ understanding of plant life: “It opens our mind to the many ways that organisms adapt to survive.”
Learn more about the world of photosynthesis at: http://photosynthesisforkids杭州夜生活m/
Get to grips with the intricacies of chlorophyll at: http://science.howstuffworks杭州夜生活m/dictionary/plant-terms/chlorophyll-info.htm
Indulge in some light work at: http://abyss.uoregon.edu/~js/ast122/lectures/lec04.html
Read the papers: “A Red-Shifted Chlorophyll” published in the US journal Science at: sciencemag杭州夜生活/content/329/5997/1318.short
“Spectra Expansion and Antenna Reduction Can Enhance Photosynthesis for Energy Product,” published in Current Opinion in Chemical Biology at sciencedirect杭州夜生活m/science/article/pii/S1367593113000562
“Expanding the Solar Spectrum Used by Photosynthesis” published in Trends in Plant Science at: sciencedirect杭州夜生活m/science/article/pii/S1360138511000598
Tomorrow (Tuesday) at 1pm, watch the IMAX Melbourne Museum movie The Earth Wins, exploring the delicate balance between humanity and the Earth. Details: imaxmelbourne杭州夜生活m.au/movie/the-earth-wins/education
AusVELS Science: Biological sciences: http://ausvels.vcaa.vic.edu.au/Science/Curriculum/F-10
Science: http://ausvels.vcaa.vic.edu.au/Science/Curriculum/F-10 (in particular, “Science as a Human Endeavour” strand, levels 5-10)
Please send bright ideas for new topics to [email protected]杭州夜生活m.au
The original release of this article first appeared on the website of Hangzhou Night Net.