Evolution of photosystems of photosynthetic organisms.

It is generally accepted that two photosystems function successively in photosynthetic electron transport chain of plants and algae. The interaction of these photosystems results in the enhancement of photosynthesis. It was suggested that only one photosystem is present in purple bacteria, the most primitive photosynthetic organisms. The functioning of this photosystem is accompanied by absorption changes at 890 nm. Recently new spectral changes were found in Chramatium chromatophores under reductive conditions, more favorable for bacterial growth. Some of that spectral changes take place even at liquid nitrogen temperature. It is proposed these absorption changes could be related to other photosystem functioning in low potential region. Such a photosystem is necessary for reduction of NAD in Chromatium, for which the reverse electron transport to NAD was not shown. In contrast to photosystems of plants, the bacterial photosystems appear to function independently because the enhancement of bacterial photosynthesis is not found. Apparently the evolution of photosystems involved interaction between independent photosystmes, one of them functioning under more oxidative conditions.