Prochlorococcus marinus Chisholm et al. 1992 subsp pastoris subsp nov strain PCC 9511, the first axenic chlorophyll a(2)/b(2)-containing cyanobacterium (Oxyphotobacteria)

The formal description of Prochlorococcus marinus Chisholm et al. 1992, 299 was based on the non-axenic nomenclatural type, strain CCMP 1375(T). The p urification and properties of the axenic strain PCC 9511, derived from the same primary culture (SARG) as the type species, are reported here. Prochlo rococcus PCC 9511 differs from the latter in possessing horseshoe-shaped th ylakoids, exhibiting a low chlorophyll b(2) content and lacking phycoerythr in, but shares these phenotypic properties with Prochlorococcus strain CCMP 1378. This relationship was confirmed by 16S rRNA sequence analyses, which clearly demonstrated that the axenic isolate is not co-identic with the no menclatural type. Strain PCC 9511 has a low mean DNA base composition (32 m ol% G+C) and harbours the smallest genome of all known oxyphotobacteria (ge nome complexity 1.3 GDa = 2 Mbp). Urea and ammonia are the preferred source s of nitrogen for growth, whereas nitrate is not utilized. Several differen t organic phosphorus compounds efficiently replace phosphate in the culture medium, indicative of ecto-phosphohydrolase activity. In order to distingu ish strain PCC 9511 from the nomenclatural type, a new subspecies is propos ed, Prochlorococcus marinus Chisholm et al. 1992 subsp.pastoris subsp. nov.