PHOTOSYNTHESIS is first and foremost a process for converting the radiant energy of sunlight into physiological chemical energy, that is, reducing power and high energy phosphate bonds1. Since, for historical and quantitative reasons, however, photosynthesis in a broad sense is commonly identified with the reduction of carbon dioxide to carbohydrate, the fact is often overlooked that photosynthetic organisms manufacture their most representative biomolecules—proteins and nucleic acids—not only from carbon dioxide, but also from other oxidised inorganic nutrients; especially nitrate, that, more or less directly, has to be reduced to ammonia by water before being incorporated into amino acids2,3. It was first thought4 that the reduction of nitrate in green cells was a process far removed from the light reactions of photosynthesis and that carbohydrates were the in vivo source of reducing power for nitrate reduction, but it now seems that, at least in blue-green algae5–7, the photosynthetic reduction of nitrate is even more direct than that of carbon dioxide, since it does not depend on adenosine triphosphate (ATP) and reduced nicotinamide adenine dinucleotide phosphate (NADPH), but only on reduced ferredoxin, the first electron acceptor from photosystem I. Also the enzymes for the reduction of nitrate to ammonia are retained tightly bound by the pigment-containing particles from these prokaryotes.
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