THE PHOTOSYNTHETIC MACHINERY IN PROCHLOROPHYTES - STRUCTURAL-PROPERTIES AND ECOLOGICAL SIGNIFICANCE

The Prochlorophytes are a diverse group of photosynthetic prokaryotes which falls within the cyanobacterial lineage, yet lack phycobilisomes as light harvesting structures. Instead, the Prochlorophytes have a l ight-harvesting apparatus composed of the higher plant pigments chloro phylls a and b. This review discusses antenna structures, photosynthet ic properties and evolutionary relationships among these bacteria, wit h focus on the role of photosynthesis in their natural habitat. Most o f the available information is obtained from studies on Prochlorothrix , the model organism of this group in laboratory studies. Our analysis yields a consensus from studies on two Prochlorophytes, Prochloron an d Prochlorothrix, as to how the thylakoid membrane is organized. Lack of laboratory studies on an abundant third Prochlorophyte, Prochloroco ccus, does not (yet) allow to include this specie's in the consensus. Overall, we propose that the structure of the light-harvesting complex es from Prochlorophytes is very different from those of chloroplast sy stems, and is evolutionarily very ancient. The light-harvesting appara tus is considered to maintain a strong structural and functional assoc iation with Photosystem I in both Prochlorothrix and Prochloron. Photo system II in Prochlorothrix differs from other photosynthetic systems in structural and functional properties of both donor and acceptor sid es of its reaction center. A demonstrated capacity for Photosystem I-d ependent anoxygenic photosynthesis in Prochlorothrix may indicate that there is an increased dependence on cyclic photophosphorylation in th ese organisms. A description of the natural habitats of the Prochlorop hytes has been employed as a jumping board for speculation on the role of the photosynthetic apparatus in occupying, proliferating and survi ving in their ecological niches. Prochlorophytes seem to thrive in sta ble environments of low light, sufficient nitrogen supply and possibly the presence of essential organic solutes.