The Photosynthetic Apparatus of Chlorophyll b- and d-Containing Oxyphotobacteria

Abstract

Although the vast majority of oxyphotobacteria harvest light with complex supramolecular structures, the phycobilisomes, three atypical prokaryotic genera share the property of using a mere intrinsic chlorophyll (Chl) a/b-protein complex: Prochloron, Prochlorothrix and Prochlorococcus. Some strains of the latter genus do contain some phycoerythrin, but at too low concentration for having a significant role in light harvesting. In the context of the endosymbiotic theory of the origin of chloroplasts, it was proposed that these three prokaryotic genera might belong to a separate clade among Cyanobacteria, the ‘prochlorophytes’, which would have had a recent common ancestor with green algae and higher plants. Phylogenetic analyses using diverse genes however strongly suggest that these genera are polyphyletic within the cyanobacterial radiation and on a different branch than the one bearing green plastids. With this idea in mind, it was interesting to re-examine to what extent the photosynthetic apparatus of these Chl b-containing oxyphotobacteria are truly similar to one another and whether they share some relationships with those of green plastids. This chapter shows that the main trait linking these prokaryotes is the similar nature of their antenna proteins, called Pcbs for ‘prochlorophy te chlorophyll b-binding’ proteins. These are different from the light-harvesting complex (Lhc) proteins found in eukaryotes, but closely resemble iron-stress induced (IsiA) proteins, used for light-harvesting by Photosystem I in some cyanobacteria when iron-starved. More than the mere occurrence of Chl b, this similar antenna system likely conditions most of the other apparent similarities between these three atypical organisms, including those at the ultrastructural level. Besides reviewing the recent literature about these ‘green oxyphotobacteria,’ we discuss a newcomer among atypical oxygenic prokaryotes, Acaryochloris marina, which contains Chl d as the main light-harvesting pigment and a Pcb-like antenna system. The insights that the study of these prokaryotes have brought into the more general fields of algal photosynthesis and evolution are underlined.