INTERNATIONALJOURNAL OF SYSTEMATICBACTERIOLOGY, Apr. 1986, p. 351-353 Vol. 36, No. 2 OO20-7713/86/020351-03$02.OO/O

Nomenclature of Prochloron didernni (Lewin 1977) sp. nov., norn. rev. ? Prochloron (Lewin 1976) gen. nov. , norn. rev.? Prochloraceae fam. nov., Prochlorales ord. nov., norn. rev. in the class Photobacteria Gibbons and Murray 1978 GIN0 FLORENZANO,” WALDEMARO BALLONI, AND RICCARDO MATERASSI Centro di Studio dei Microrganismi Autoiroji del C.N.R. e Istituto di Microbiologia Agraria dell’ Universita di Firenze, 50144 Florence, Italy

We propose that the photosynthetic procaryotes containing chlorophylls a and b in the species Prochloron dideemni sp. nov., genus Prochloron gen. nov., be placed under the International Code of Nomenclature of Bacteria by including the genus Prochloron in the family Prochloraceae fam. nov., order Prochlorales ord. nov. in the class Photobacteria Gibbons and Murray 1978, listed on the approved lists of bacterial names.

The discovery by Lewin (12, 13) of the procaryotic pho- while a (possibly more ancient) chlorophyll a-b protein tosynthetic microorganisms that have two forms of chloro- complex served this function in Prochloron (28). In any case, phyll (a and b) and lack phycobilins led him to create a new it is quite evident that the surviving lineages of oxygenic division of algae termed Prochlorophyta, based on the genus procaryotes, cyanobacteria and Prochloron, are more Prochloron. This interpretation of the taxonomic position of closely related to each other than they are to other groups of these organisms does not appear reasonable, because they photosynthetic organisms (9, 30). Since the relationships have a procaryotic structure which constitutes the definitive among these two groups are not firmly established, it is not property of bacteria. advisable to suggest the inclusion of the phylum The discovery of these new procaryotes has stimulated Prochlorophyta in the cyanobacterial lineage. The only speculation about their phylogeny (1, 2,4, 5, 10, 16, 30), the indication about the existence of a close relationship be- possible endosymbiont origins of the plastids of eucaryotic tween Prochloron and the cyanobacteria comes from the cells (14, 18), and the evolution of procaryotic photosynthe- studies of Seewaldt and Stackebrandt (25) and Stackebrandt sis. The most significant questions concerning the evolution et al. (26) on the 16s rRNA sequence homology. However, of these microrganisms can be summarized as follows. First, Van Valen (28) has challenged their conclusion by showing since the Didemnidae (colonial ascidians) are relatively that their data allow the construction of an alternative young in evolutionary terms, the symbiotic procaryotes of phylogeny in which Prochloron falls outside the cyano- the genus Prochloron must presumably either have existed bacteria. Therefore, the best provisional solution seems to previously as free-living organisms or have evolved since the be their separation from the cyanobacteria by the creation of Didemnidae evolved. The first possibility is suggested by the an order in the class Photobacteria Gibbons and Murray demonstration that Prochloron sp. can be grown, although 1978. for only a few generations, in a laboratory culture in medium Lewin’s statement, “Prochloron is clearly a unique MN of Rippka et al. (23) supplemented with tryptophan (20). procaryote in that it makes chlorophylls a and b” (15), is Second, hypotheses about these procaryotes postulate evo- consistent with present knowledge of the ultrastructure, lution from a typical cyanobacterium in which the ability to biochemistry, and molecular biology of this group (2, 17, 25, synthesize chlorophyll b was acquired either by molecular 28, 31). Stanier and van Niel (27) concluded that “A defini- evolution or by transfer of the genetic information from tion of a bacterium is only possible if one includes the some green alga. As a consequence of the possession of blue-green algae”. As pointed out by Rippka and Cohen- chlorophyll b, further evolutionary steps, such as the loss of Bazire (22), “They undoubtedly would have extended their phycobilins and the modified thylakoid arrangement, might argument to include Prochloron, if this organism had not have occurred rapidly. If there is a strong selective advan- evaded discovery for so long”. The procaryotic nature of tage for these procaryotes to possess chlorophyll b rather Prochloron implies that its systematic position must be than phycobilin pigments, then it would seem that further found within the procaryotic domain and in accordance with examples of chlorophyll b-containing procaryotes may be the International Code of Nomenclature of Bacteria (8). found in shallow marine waters (30). Another possibility is Consequently, we propose that the genus Prochloron Lewin that the cyanobacteria and members of the genus Prochloron (with Prochloron didemni as type species), the family evolved from a common photosynthetic ancestor which Prochloraceae, and the order Prochlorales in the class contained chlorophyll a but not phycobiliproteins (7). The Photobacteria Gibbons and Murray 1978, should be taken phycobiliproteins would have developed as accessory pig- into consideration by the Subcommittee on Phototrophic I1 ments for photosystem in the cyanobacterial lineage, Bacteria for inclusion in the Validation List. Description of the order Prochlorales ord. nov., nom. rev. Prochlorales (Pro .chlo.ra’les. M .L. fem. n. Prochloraceae * Corresponding author. type family of order; ales ending to denote an order; M.L.

351 352 NOTES INT. J. SYST.BACTERIOL. fem. pl. n. Prochlorales the Prochloraceae order) cells are await further work with cell chemistry and physiology of unicellular or multicellular gram-negative photosynthetic future isolates. procaryotes, containing chlorophylls a and b and carotenoids. Motile or nonmotile, Free living or symbionts LITERATURE CITED on invertebrate animals (didemnid ascidians, sponges, or tectibranch molluscs). Aerobic or facultatively anaerobic. 1. Bjorn, G., and L. 0. Bjorn. 1982. Prochloron sidospor eller felande Lank. Sven. Bot. Tidskr. 76:43-45. Perform oxygenic photosynthesis. Photoautotrophic or 2. Bonen, L., W. F. Doolittle, and G. E. Fox. 1979. Cyanobacterial myxotrophic. evolution: results of 16s ribosomal ribonucleic acid sequence The photopigments are located on paired (not single) analysis. Can. J. Biochem. 57:879-888. thylakoids without phycobilisomes. The arrangement of 3. Buchanan, R. E., and N. E. Gibbons (ed.). 1974. Bergey's DNA, scattered in the periphery of the cell (therefore the manual of determinative bacteriology, 8th Ed. Williams and center appears hollow), is different from that of all the Wilkins Co., Baltimore. cyanobacteria (6). The cells do not contain gas vacuoles or 4. Cavalier-Smith, T. 1982. The origins of plastids. Biol. J. Linn. cyanophicin. This broad definition of the ordinal rank is SOC.17 :289-306. made not to restrict Prochlorales solely to the photosyn- 5. Chadefaud, M. 1978. Sur la notion de Prochlorophytes. Rev. thetic procaryotes having a proven symbiotic capacity in a Algol. 13:203-206. 6. Coleman, A. W., and R. A. Lewin. 1983. The disposition of given host, but to make possible the inclusion of free-living DNA in Prochloron (Prochlorophyta). Phycologia 22~209-212. members, which can be cultivated away from the host. 7. Doolittle, W. F. 1982. Molecular evolution. In N. G. Carr and Presently it is suggested that Prochlorales includes one B. A. Whitton (ed.), The biology of Cyanobacteria. Blackwell family with the following diagnosis. Scientific Publications, Ltd., Oxford. Prochloraceae fam. nov. Members of the family 8. Gibbons, N. E., and R. G. E. Murray. 1978. Proposal concerning Prochloraceae (Pro. chlo .ra' ceae. M. L. n. Prochloron type the higher taxa of bacteria. Int. J. Syst. Bacteriol. 28:1-6. genus of the family; -aceae ending to denote a family; M.L. 9. Herdman, M. 1981. Deoxynbonucleic acid base composition fem. n. the Prochloron family) are unicellular photosynthetic and genome size of Prochforon. Arch. Microbiol. 129:314- procaryotes, containing chlorophylls a and 6, but not 316. 10. Kott, P. 1977. Algal supporting didemnid ascidians of the Great phycobilins. At present the family comprises a single genus Barrier Reef, p. 615-622. Proceedings of the 3rd International Prochloron. Coral Reef Symposium, University of Miami, Fla. Prochloron Lewin 1977 gen. nov., nom. rev. Prochloron 11. Kremer, B. P. 1980. 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Prochloron. A status report. Phycologia connected by fine strands. Aerobic or microaerophilic. 23~203-208. Ascidian symbionts or free living. Nonsymbiotic growth is 17. Mackay, R. M., D. Saldago, L. Bonen, G. Stackebrandt, and poor and dependent on aminoacids. Photoautotrophic or W. F. Doolittle. 1982. The 5s ribosomal RNAs of Paracoccus photoorganotrophic metabolism. The mol% G+C of DNA denitrijicans and Prochloron. Nucleic Acids Res. 10:2963-2970. ranges from 39 to 41. Type species: Prochloron didernni. 18. Margulis, L. 1981. Symbiosis in cell evolution. W. H. Freeman, Prochloron didemni sp. nov., nom. rev. Prochloron didemni San Francisco. M.L. Didernnum 19. McCourt, R. M., A. F. Michaeles, and R. W. Hoshaw. 1984. (di.de' mni. of Lewin 1976 [synomym Seasonality of symbiotic Prochloron (Prochlorophyta) and its Synechocystis didemni Schulz-Baldes and Lewin 19761 [24] didemnid host in the northern Gulf of California. 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bacterium. Arch. Mikrobiol. 42:17-35. biology of Cyanobacteria. Blackwell Scientific Publications, 28. Van Valen, L. M. 1982. Phylogenies in molecular evolu- Ltd., Oxford. tion:Prochforon. Nature (London) 298:493494. 31. Withers, N., W. Vidaver, and R. A. Lewin. 1978. Pigment 29. Wallace, D. C., and H. J. Morowitz. 1973. Genome size and composition, photosynthesis and fine structure of a non-blue- evolution. Chromosoma 40:121-126. green prokaryotic algal symbiont (Prochioron sp.) in a 30. Whitton, B. A., and N. G. Carr. 1982. Cyanobacteria: current didemnind ascidian from Hawaiian waters. Phycologia perspectives. In N. G. Carr and B. A. Whitton (ed.), The 17: 167-17 1.