Marine cyanobacteria belonging to the Prochlorococcus genus (Chisholm et al, 1992) are the most abundant photosynthetic organisms on Earth (Partensky et al, 1999). They are virtually ubiquitous in the latitudinal band 40°N/40°S. Typical concentrations are 1-3 10⁵ cell per mL over the first top 100 m and Prochlorococcus cells can be found as deep as 150-200 m at depths reached by only 0.1% of the irradiances in surface. This single genus contributes to 40-50 % of the phytoplanctonic biomass in oligotrophic areas. With a cell size between 0.5-0.7 µm, these cyanobacteria are also the smallest photosynthetic organisms known to date. This tiny size optimizes uptake of nutrients such as N or P, the concentration of which is often below the detection limit in central parts of the oceans, dominated by Prochlorococcus.

Besides its ecological relevance, the Prochlorococcus genus possess numerous peculiarities with regard to other cyanobacteria. Thus, it has a specific pigment complement, including unique divinyl derivatives of chlorophyll a and b (Chl a2 and b2 ) (Goericke et Repeta, 1992). Furthermore, in sharp contrast to typical cyanobacteria which harvest light with large extrinsic multi-subunit complexes (the phycobilisomes), its major light-harvesting complex is constituted by a Chl a2/b2-binding protein which is intrinsic to the thylakoid membrane (La Roche et al, 1996). This antenna complex is structurally closer from that of higher plants, although the nature of the proteins is different (6-helix Pcb protein in Prochlorococcus vs. 3-helix Lhc in plants) .

A number of studies made on Prochlorococcus cultures and populations from the field have shown that at least two ecological forms are occupying different (although overlapping) light niches in the ocean (Campbell et Vaulot, 1993; Moore et al, 1998). The top 100 m layer characterized by high photon fluxes densities and low nutrient concentrations is colonized by the high-light-adapted form. The second form (low-light-adapted form) preferentially thrive at depth (80 - 200 m) reached by low irradiances but not nutrient-limited. These two forms have different Chl b2 to Chl a2 ratios and optimal irradiances for growth and photosynthesis. They are also genetically distinct (e.g. when comparing their 16S rRNA sequences), and probably belong to different species.

Recent studies have revealed some of the molecular bases of ecophysiological differences existing between the 2 Prochlorococcus forms, including a large number of genes encoding the light-harvesting complex of the low-light-adapted strain SS120 compared to only one in the high-light-adapted strain MED4 (Garczarek et al, 2000). Multiplication of this specific gene family, which is accompanied by a diversification in the functions of these genes, seems to result from an adaptation to the low-light niche. Other examples include multiplication of high-light-induced proteins in the high-light-adapted form.

In order to better understand evolution processes in Prochlorococcus and to identify novel genes involved in the niche adaptation of this organism, the genome of the low-light-adapted type strain P. marinus SS120 (CCMP1375) has been completely sequenced by the Genoscope (Evry, France). This strain is one of the best ecophysiologically characterized. Furthermore, the Joint Genome Institute (Walnut Creek, California) has completed the sequences of the surface strain P. marinus MED4 and another deep strain (P. marinus MIT9313), genetically fairly distant from SS120. Furthermore, this institute has also sequenced the genome of the phylogenetically close genus Synechococcus sp. WH8102.

Characteristics of these four genomes are reported here.