Prochlorophytes: The ‘Other’ Cyanobacteria?

Abstract

The interest in Prochlorophytes (Oxychlorobacteria) was originally boosted by the endosymbiont theory on chloroplast evolution. The first organism of this type: Prochloron didemni, was described as a procaryote performing oxygenic photosynthesis and containing both Chls a andb. The combination of those two pigments was until then only characteristic of Chlorophyte chloroplasts. The prochlorophytes accordingly were portrayed to be the potential endosymbionts from which (green) chloroplasts originated. Before Prochloron sp., the cyanobacteria were the only procaryotes known to perform oxygenic photosynthesis. Because of this qualifying property, the origin of chloroplasts was linked with endosymbiontic cyanobacteria. Following the discovery of more prochlorophyte species (e.g., Prochlorothrix hollandica, Prochlorococcus marinus) research efforts in various fields demonstrated that the predicted relationship between the prochlorophytes and green chloroplasts if any at all existed, was not very direct. Moreover, sequence analysis of conserved genes in general supported a closer relationship between prochlorophytes and cyanobacteria rather than chloroplasts. This has even led some to consider the prochlorophytes as another type of cyanobacteria. Given the identification of a number of distinctive differences in the organization of the photosynthetic apparatus in these two types of procaryotes performing oxygenic photosynthesis it would be appropriate to support and extend the suggestion of Gibbons and Murray (1978) and Florenzano et al. (1986) to institute two orders—i.e., Cyanobacteria and Oxychlorobacteria—within the class of the Oxyphotobacteria.