LNTERNATIONAL JOURNAL OF SYSTEMATIC BACTERIOLOGY, July 1989, p. 250-257 Copyright 0 1989, International Union of Microbiological Societies 0020-77 13/89/030250-08$02 .OO/O Vol. 39. No. 3 Prochlorothrix hollandica gen. nov., sp. nov., a Filamentous Oxygenic Photoautotrophic Procaryote Containing Chlorophylls a and b: Assignment to Prochlovotrichaceae fam. nov. and Order Prochlorales Florenzano, Balloni, and Materassi 1986, with Emendation of the Ordinal Description TINEKE BURGER-WIERSMA," LUCAS J. STAL, AND LUUC R. MUR Laboratory for Microbiology, University of Amsterdam, Nieuwe Achtergracht 127, NL-1018 WS Amsterdam, The Netherlands The genus Prochlorothrix gen. nov., with one species, Prochlorothrix hollandica sp. nov., is proposed for filamentous oxygenic photoautotrophic procaryotes containing chlorophylls a and b. The cylindrical cells are arranged in trichomes which lack sheaths, are not motile, and do not possess differentiated cells. P . hollandica grows in liquid media or on solid mineral media and is incapable of dinitrogen fixation. Optimum growth occurs between 20 and 30°C and at pH 8.4. The deoxyribonucleic acid base composition of this organism is 53 mol% guanine plus cytosine. Emendation of the description of the order Prochlorales Florenzano, Balloni, and Materassi 1986 is proposed since the original definition of this taxon would exclude the genus Prochlorothrix. To accommodate the filamentous genera in the order Prochlorales, we propose the family Prochlorotrichaceae fam. nov. The organisms belonging to the Prochlorophyta (16) are oxygenic, photoautotrophic procaryotes which contain chlo- rophylls a and b. Usually, this combination of pigments is associated with green algae and higher plants. However, the main criterion discriminating prochlorophytes from green algae is their typical procaryotic cellular ultrastructure. Therefore, Florenzano et al. (6) proposed that the photosyn- thetic procaryotes containing chlorophylls a and b should be under the jurisdiction of the International Code of Nomen- clature of Bacteria (15). The property of oxygenic photosyn- thesis suggests a close relationship among this new group of bacteria, the oxychlorobacteria, and the cyanobacteria. Al- though studies on the homologies of the 16s ribosomal ribonucleic acid (rRNA) sequences (23, 24) confirmed this relationship, the conclusions of these studies were debated by Van Valen (29). A less close relationship can be hypoth- esized on the basis of the thylakoid structure. The lack of phycobilin pigments and the presence of chlorophyll b may have consequences for the structure of the thylakoid mem- branes. The accessory phycobilin pigments of cyanobacteria are located in complexes extrinsic to the thylakoid mem- branes, whereas the light-harvesting chlorophyll alb com- plexes in plants and green algae are intrinsic to the photo- synthetic membranes. To separate the oxychlorobacteria from the cyanobacteria, Florenzano et al. (6) suggested that an order should be created in the class Photobacteria Gibbons and Murray 1978 (7). In connection with this, these authors proposed descriptions for the order Prochlorales, the family Prochloraceae, and the genus Prochloron, as well as the type species Prochloron didemni (17). Hitherto, Prochloron didemni (6, 15) was the only de- scribed species in this order. This organism is a unicellular, extracellular symbiont of certain marine invertebrates. Re- cently, Burger-Wiersma et al. (2) reported the isolation of a nonsymbiotic chlorophyll alb-containing procaryote from the freshwater Loosdrecht Lakes in The Netherlands, where * Corresponding author. 250 it has been abundant in the summer phytoplankton over the last 5 years. In contrast to Prochloron didemni, the newly isolated strain is filamentous with long cylinder-shaped cells. All of the currently available results on ultrastructure (2), cell wall organization (13), and carotenoid (2) and lipid (30) compositions have established the procaryotic nature of the new strain and its resemblance to the cyanobacteria. Ac- cording to a 16s rRNA-based phylogenetic analysis, this organism is a deeply branching member of the cyanobacte- rial line of descent (26a). However, there is a distinct difference between the cyanobacteria and the oxychlorobac- teria. For both the organism described in this paper (20a) and Prochloron spp. (9), lateral heterogeneity of the thylakoid membranes has been reported. This Ceature is common in green algae and plants, but has never been observed in cyanobacteria. This supports the proposal that the oxychlo- robacteria and the cyanobacteria should be separated at the ordinal level (6). A close relationship between the photosynthetic apparatus of the new strain and that of Prochloron sp. has not been established yet, but is suggested by the fact that both organisms contain a chlorophyll alb antenna apoprotein of comparable molecular weights, which is distinct from the protein that is usually observed in green algae and plants (1). In this paper, we formally describe the new isolate and propose the name Prochlorothrix hollandica. Since the or- dinal definition of Florenzano et al. (6) would exclude Prochlorothrix hollandica, we propose emendation of the original description of the order Proc hlorales. MATERIALS AND METHODS Strain. The new strain was isolated from a sample from a mixed water column from eutrophic Lake Loosdrecht, The Netherlands (52"20' North latitude, 5"5' East longitude). The temperature and pH of the lake water on the sampling date (9 July 1984) were 18°C and 8.8, respectively. Growth media. The liquid medium contained NaNO, (500 mglliter), MgSO, . 7H,O) (50 mg/liter), CaCl, . 2H,O (13