Landmarks in photosynthesis research

Modern photosynthesis research dates from 177 1 when Joseph Priestley discovered that plants can grow in air ‘injured by the burning of candles’ and restore the air so that a candle can burn again. He thus showed that oxygen gas is a product of the photosynthesis reaction. In 1779, the Dutch physician Jan Ingenhousz confirmed Priestley’s findings and also discovered that plants can improve air only when illuminated, thus showing the importance of sunlight in the reaction. He also showed that only the green parts of the plant could carry out this process. In 1788, the Swiss pastor Jean Senebier noted that ‘fixed air’ or carbon dioxide was a necessary ingredient for the growth of plants. In 1804, another Swiss scholar, Nicolas Theodore de Saussure, demonstrated that water was consumed in the reaction. In 1817, two French chemists, Pelletier and Caventou, isolated the green pigment of leaves and called it chlorophyll.

It was not until 1845 that the German physician, Julius Robert Mayer, first realised the importance of photosynthesis as an energy-storage reaction storing sunlight as chemical energy. The stoichiometry of photosynthesis was established in 1864 by the work of the French plant physiologist T. B. Boussingault, who showed that the mole ratio of oxygen evolved to carbon dioxide consumed is close to unity. Finally, also in 1864, the German botanist, Julius Sachs, demonstrated that carbohydrates are produced in photosynthesis via the now famous starch-iodide experiment on a half-darkened leaf. In 1880, Engelmann determined that chlorophylls are the key pigments of green algae, by using oxygen-seeking motile bacteria to demonstrate that oxygen was evolved only by the chloroplast in the algae, and that the red and blue portions of the spectrum (absorbed strongly by chlorophyll) were the most active in generating oxygen.
 

The concepts of the antenna pigment system and the photosynthetic unit were developed by Emerson and Arnold (1932), who measured 0 2 evolution from algae subjected to short flashes of light, and concluded that one O2 molecule could be evolved per -2400 chlorophyll molecules. Later, Gaffron and Wohl (1936) suggested that absorbed quanta are transferred from one chlorophyll molecule to another until they can be trapped at a reaction centre.
 

The Dutch microbiologist C. B. van Niel (1935) hypothesised that the oxygen evolved in green-plant photosynthesis comes from H20 (rather than from C02. as had been previously thought). Ruben et al. (1941) later confirmed this hypothesis by the use of ‘*O-labelled water. Another important development was the isolation by Hill (1937) of chloroplasts from leaves. He showed that these isolated chloroplasts were able to evolve oxygen when supplied with an artificial electron acceptor such as ferricyanide, but were not able to reduce C02, thus proving that the oxygen evolution and C02 reduction reactions are physically separable. In the same year, Pirson (1937) established the requirement for manganese in oxygenic photosynthesis. Hill and Bendall (1960) first proposed the coupling of the two photosystems in the 2-scheme. The detailed mechanism of C02 fixation was elucidated by Melvin Calvin and his co-workers between 1947 and 1950 (Bassham and Calvin, 1957), using the I4C radioactive tracer technique to detect the chronological history of the pathway of carbon in C3 plants from its initial fixation into phosphoglyceric acid to the ‘ultimate’ product D-glucose. For this work Calvin received the Nobel Prize for Chemistry in 1961. Several years later, Hatch and Slack (1966) discovered the C4 pathway for carbon fixation, in which C02 is initially converted to C4 dicarboxylic acids (malic, aspartic and oxaloacetic) before entering the C3 pathway. Photophosphorylation (the production of ATP in the light) was discovered in the chloroplasts of algae and green plants by Arnon et al. (1954), and in the chromatophores of photosynthetic bacteria by Frenkel (1954). Vishniac and Ochoa (195 l), Arnon (195 1) and Tolmach (1951) independently discovered the photochemical reduction of NADP’ by chloroplasts. A number of useful chloroplast-driven reduction reactions other than COZ fixation are thermodynamically possible. By far the most interest has centred on the photobiological production of hydrogen from water. Gaffron and Rubin ( 1942) were the first to observe that certain algae, containing the enzyme hydrogenase, adapted in certain conditions to evolve hydrogen. Arnon et al. (1961) observed hydrogen evolution from isolated chloroplasts coupled to hydrogenase in the presence of artificial electron donors. Benemann et al. ( 1973) demonstrated hydrogen evolution from isolated chloroplasts plus hydrogenase in a system where water was clearly the electron donor.
 

Enormous advances in our knowledge of the mechanisms of photosynthesis have been made since these pioneering studies by workers such as Witt, Duysens, Kok, Jagendorf, Clayton, Feher, Sauer, Joliot, Babcock and many others (see Ke, 2001). However, prior to the 1980s, the structures of reaction centres and other important photosynthetic components could only be inferred indirectly from spectroscopic and kinetic studies. Then in 1982, Hartmut Michel succeeded in crystallising the reaction centre protein of the purple photosynthetic bacterium Rhodopseudomonas viridis (Michel, 1982). X-ray crystallographic determination of the structure followed (Deisenhofer et al., 1984, 1985). This seminal achievement, for which Johann Deisenhofer, Robert Huber and Hartmut Michel shared the 1988 Nobel Prize in Chemistry, revealed for the first time the exact locations of the redox active cofactors involved in the earliest steps of photosynthesis and the arrangement of the bacteriochlorophyll special pair. Since then, several reaction centre proteins and lightharvesting complexes have been crystallised and their structures determined.