Cyanobacteria: Acaryochloris marina

Acaryochloris marina was isolated as a minor symbiont from a colonial ascidian in Palau (Miyashita et al. 1996) together with Prochloron, a major symbiont. A. marina is the only well-studied species that contains Chl d as the major photosynthetic pigment. This species is therefore unique among all oxygenic photosynthetic organisms so far studied. Chl d (3-desvinyl-3-formyl Chl a) has a formyl group at the 3 position in ring A of the structure, that induces a red-shift of the absorption maximum up to approximately 700 nm in organic solvents and 715 nm in cells (Miyashita et al. 1997) with the tail of the cell absorption reaching 740 nm. The extension of Chl d absorption into the near infrared, beyond the range of any other oxygenic photosynthetic organisms, could have immense agricultural consequences. If Chl d could be incorporated into higher plants, it has a potential capacity of increasing the energy conversion of sunlight by 5% compared to that of the Chl a-containing organisms.

In addition to Chl d,A. marina also contains small amounts of Chl a (Miyashita et al. 1997). The content of Chl a varies from 1 to 5% depending on light intensity for the growth, however it never disappears (Mimuro et al. unpublished). The major carotenoid is a -carotene, instead of b -carotene usually found, and the phycobiliprotein content is very low. These modifications presumably induce important changes in the antenna and electron transfer.

          Photomicrograph of Acaryochloris marina

The constitution of pigments in the reaction center complexes is also unique in A. marina. This species contains two kinds of reaction center complexes, PSI and PSII, as in other cyanobacteria and does O 2 evolution. In PSI, the primary electron donor is clearly Chl d (Hu et al. 1999; Kumazaki et al. 2002; Mi et al. 2003), which is the only known exception to the rule of Chl a being the primary donor of PSI in cyanobacteria. The primary electron acceptor is also identified as Chl d (Akiyama et al. 2001, 2002). This is unique among all photosynthetic organisms including anaerobic photosynthetic bacteria that use a Chl a-type pigment as the primary acceptor (Kobayashi et al. 2000). The identity of the primary donor in PSII is not yet certain (Mimuro et al. 1999, 2004)

Chl d has been proposed as a transitional pigment that is both structurally and energetically intermediate between the bacteriochlorophylls found at longer wavelengths in anoxygenic phototrophs and the Chl a that is found in all other known oxygenic photosynthetic organisms (Blankenship and Hartman 1998). Alternatively, it may represent a recent adaptation to a particular environment. Currently, it is not possible to choose between these two alternatives.

Significance of the Acaryochloris marina Genome Sequence and Choice of Organism

The genome sequence of A. marina would be of great value for the following reasons:

1) Changes in photosynthetic apparatus behind a unique pigment composition. According to the 16S rRNA phylogenetic tree, A. marina is relatively close to Prochlorococcus sp., Prochlorothrix hollandica and Thermosynechococcus sp. It may be that the common ancestor of all these species, including A. marina, possessed Chl a. In this view, Chl d was acquired during the evolutionary divergence to Acaryochloris. Changes in photosynthetic pigment may have induced modification of protein sequences in many of the pigment-protein complexes and associating proteins to accommodate a new pigment, Chl d. The genome analysis of A. marina will reveal those changes, and will give information on the structure-function relationship of the primary processes of photosynthesis. By comparing with those reaction systems of other cyanobacteria, the relationship will be reinforced. For example, the function of Chl a in A. marina might be indicated by the conserved structure of the reaction center complex. Furthermore, mechanisms of sensing of light conditions, expression of genes for primary reactions, and modifications of reaction centers will be revealed by whole genome analysis including the genus-specific genes of A. marina.

2) Diversity of pigments and their biosynthesis, and evolutionary implications. Acquisition of the gene(s) for biosynthesis of a new pigment is postulated to occur during the evolution of pigment system (Tomitani et al. 1999). This acquisition process is thought to be common to Chl a/b and Chl a/c organisms. When the gene(s) for Chl d-biosynthesis and relating to this reaction are identified, we will be able to experimentally reproduce the evolutionary processes of pigment acquisition by introducing those genes in other organisms. This in vivo reconstitution method will be a new and powerful tool for the analysis of the core component of reaction centers. For the development of this method, a complete genome sequence is indispensable.

Chl d has also been reported for several eukaryotic species of macrophytic red algae, however its presence has not been reproducible, thus it has been enigmatic to date. The EST library of one red alga, Porphyra yezoensis, has been published (Nikaido et al. 2000). By sequence comparison, the continuity or discontinuity of pigment biosynthesis pathways can be resolved, and this can lead to a general understanding of the continuity of chlorophyll evolution from prokaryotic cyanobacteria to eukaryotic red algae .

A. marina was isolated as a symbiont with no known information on the properties leading to a symbiosis with another organism. In general, a symbiont is affected by a host, and thus this species could function as a model for cross talk between a host and symbiont. Since it is generally accepted that the chloroplast evolved from a symbiotic relationship, the study of a current photosynthetic symbiont may give some insights into complex relationships that exist between nuclear genes and chloroplast genes in plant cells.

In the Mimuro lab, the type specimen of A. marina has been maintained axenically for the last seven years, and Dr. Miyashita, who isolated the organisms, is in charge of this culture. The growth rate is fast, so it is easy to obtain enough cells for sequence analysis. The genome size has been estimated to be 4.2 +/- 0.5 Mb by pulsed field gel electrophoresis (unpublished).