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 light-harvesting apparatus composed of the higher plant pigments chlorophylls a and b. This review discusses antenna structures, photosynthetic properties and evolutionary relationships among these bacteria, with focus on the role of photosynthesis in their natural habitat. Most of 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 and Prochlorothrix, as to how the thylakoid membrane is organized. Lack of laboratory studies on an abundant third Prochlorophyte, Prochlorococcus, does not (yet) allow to include this species in the consensus. Overall, we propose that the structure of the light-harvesting complexes from Prochlorophytes is very different from those of chloroplast systems, and is evolutionarily very ancient. The light-harvesting apparatus is considered to maintain a strong structural and functional association with Photosystem I in both Prochlorothrix and Prochloron. Photosystem II in Prochlorothrix differs from other photosynthetic systems in structural and functional properties of both donor and acceptor sides of its reaction center. A demonstrated capacity for Photosystem I-dependent anoxygenic photosynthesis in Prochlorothrix may indicate that there is an increased dependence on cyclic photophosphorylation in these organisms. A description of the natural habitats of the Prochlorophytes has been employed as a jumping board for speculation on the role of the photosynthetic apparatus in occupying, proliferating and surviving in their ecological niches. Prochlorophytes seem to thrive in stable environments of low light, sufficient nitrogen supply and possibly the presence of essential organic solutes.