[Antenna replacement in the evolutionary origin of chloroplasts]

Endosymbiotic origin of chloroplasts from unicellular cyanobacteria is presently beyond doubt. Oxygenic photosynthesis is based on coordinated action of two photosystems (PS), PS I and PS II, cooperating with several variants of the pigment antenna. In cyanobacteria, red algae, and glaucophytes, phycobilisomes (PBS) act as antennae, while in terrestrial plants, as well as most macro- and microalgae antennae are formed by chlorophyll a/b- and chlorophyll a/c-containing proteins. Advantages and disadvantages of the PBS antenna compared to other light-gathering complexes form the basis for adaptive variations of the antenna in the course of development of eukaryotic photosynthesis. During the evolution of the "green" and "chromophyte" lineages of the chloroplasts, PBS, in spite of their optimal features of light absorption,were replaced by chlorophyll a/b- and chlorophyll a/c-containing light-gathering complexes. Development of the cell wall associated with limited motility and with tissue formation in photosynthetic eukaryotes were the factors responsible for the antenna shift. The subsequent redistribution of cell resources in favor of cellulose biosynthesis required increased for CO2 consumption, higher PS II levels, and greater number and density of the thylakoids in the chloroplasts, got incompatible with the energy-consuming and overly large PBS antenna.