Enjoying the sun in a shady nook
Photosynthesis is the basis of virtually all life on Earth. The process uses solar radiation to reduce carbon dioxide to energy-rich molecules, which not only nourish their phototrophic producers, but also feed heterotrophic organisms including humans. In oxygenic photosynthesis, which is quantatively the most important variant, water acts as the reducing agent. Green plants, algae and cyanobacteria (blue-green algae) have tapped this practically unlimited source of reducing agent; they all carry out oxygenic photosynthesis, producing oxygen as a waste product. Water is difficult to activate chemically, however, oxygenic photosynthesis therefore requires a relatively large amount of energy.
The molecular machinery responsible for photosynthesis has remained virtually unchanged for more than 2 billion years. Chlorophylls, the pigments to which leaves owe their green colour, play crucial roles at many stages of the process. They absorb sunlight, and transfer energy and electrons to various acceptors. It was long thought that chlorophyll a, the type most abundant in green plants, was the only chlorophyll capable of converting light energy into chemical energy in the so-called reaction centers. Chlorophyll a can absorb light of wavelengths of less than 700 nanometers, and this was thought to be the minimal energy level capable of driving oxygenic photosynthesis.
More recently it was shown that chlorophyll d, found in certain cyanobacteria, can absorb light of lower energy, with wavelengths just above 700 nm. Targeted searches then revealed that chlorophyll d is also present in many other phototrophs. “These searches were motivated by the idea that the widespread use of chlorophyll a should pose a dilemma for oxygenic organisms“, says Hugo Scheer, retired Professor at LMU Munich. “They must all compete for the same light. Particularly in very densely populated biotopes, the organisms on the top storey would absorb all the visible light, and those in the shade below would be left with nothing. This should exert a strong selection pressure on the latter to exploit the tiny amounts of visible light that still reach them, or to use near-infrared light, which chlorophyll a cannot absorb.” Indeed, it was in such ecological niches that further chlorophyll d-containing organisms were discovered.
The analysis of so-called stromatolites by Chen’s team in Australia led to a further and far greater surprise -- chlorophyll f. Stromatolites are dense mats composed of masses of cyanobacteria, and they are among the oldest surviving biological communities on Earth. “By illuminating the cells with infrared light, it was possible to obtain cultures enriched in organisms that contained the novel chlorophyll f”, explains Scheer. “We were then able to show that it is structurally very similar to chlorophyll a, but can absorb infrared light at lower wavelengths than any of the four known types of chlorophyll.“ A so-called formyl group located at a specific position in the molecule is responsible for the ”red shift“ in the absorption spectrum. Chlorophyll f is the first new chlorophyll to be found in oxygenic phototrophs in over 60 years.
The structure of the new molecule was determined largely by Scheer. The findings may make it possible to modify the absorption spectra of other chlorophylls in useful ways. “This might allow technical installations that use light as a source of energy to exploit it more efficiently“, says Scheer. “In addition, there are possible medical applications. In photodynamic therapy of cancers, light-sensitive drugs including chlorophylls accumulate at the tumor site and are then activated by irradiation with an external source of light. Chlorophylls that absorb in the near infrared are interesting in this regard, because light in this region of the spectrum can penetrate deeply into tissues.“ (suwe)
“A red-shifted chlorophyll”,
Min Chen, Martin Schliep, Robert Willows, Zheng-Li Cai, Brett A. Neilan, Hugo Scheer
Science online, 19 August 2010
Professor Hugo Scheer
Department of Biology I - Division of Botany