Abstract Two new phototrophic consortia, “Chlorochro- matium lunatum” and “Pelochromatium selenoides”, were observed and collected in the hypolimnion of several dim- ictic lakes in Wisconsin and Michigan (USA). The two consortia had the same morphology but different pigment composition. The cells of the photosynthetic components of the consortia were half-moon-shaped. This morphology was used to differentiate them from the previously de- scribed motile phototrophic consortia “Chlorochromatium aggregatum” and “Pelochromatium roseum”. These pho- totrophic cells did not resemble any described unicellu- lar green sulfur bacteria. The predominant pigments de- tected were bacteriochlorophyll d and chlorobactene for the green-colored “Clc. lunatum”, and bacteriochlorophyll e and isorenieratene for the brown-colored “Plc. selenoi- des”. Their pigment compositions and the presence of chlorosomes attached to the inner face of the cytoplasmic membrane in both kinds of photosynthetic cells confirmed this new half-moon-shaped morphotype as a green sulfur bacterium. Both consortia were found thriving in lakes with low concentrations of sulfide (< 60 μM), below the layers of “Clc. aggregatum” and “Plc. roseum”. The green consortia were observed in lakes where the oxic-anoxic interface was located at shallow depths (2–7 m), while the brown consortia were found at greater depths (8–16 m). The two newly described consortia were never detected together at the same depth in any lake. Key words Phototrophic consortia · “Chlorochromatium lunatum” · “Pelochromatium selenoides” · “Chlorochromatium aggregatum” · “Pelochromatium roseum” · Green sulfur bacteria · Syntrophic associations Abbreviations BChl Bacteriochlorophyll · Chl Chlorophyll · Clc Chlorochromatium · Plc Pelochromatium Introduction Syntrophic associations between phototrophic and non- phototrophic bacteria have been repeatedly observed and described to thrive on the surface of anoxic mud or in anoxic hypolimnia (Pfennig 1989). These associations have been described as consortia by Buder (1913), who detailed the motile “Chloronium mirabile” (“Chlorochromatium aggregatum”). Although he and other investigators (Uter- möhl 1924; Skuja 1956; Mechsner 1957) have used species names for the consortia, Trüper and Pfennig (1971) have pointed out that the names have no taxonomic value; therefore, they have been accepted only as trivial names. The most commonly observed consortia consist of a motile central bacterium surrounded by one or several lay- ers of green or brown cells of green sulfur bacteria (Pfen- nig 1980; Croome and Tyler 1984). Four consortia of this characteristic type have been described: “Chlorochromatium aggregatum” (Lauterborn 1906), “Pelochromatium roseum” (Lauterborn 1913), “Chlorochromatium glebulum” (Skuja 1956), and “Pelochromatium roseo-viride” (Gorlenko and Kuznetsov 1971). Two other nonmotile consortia have been described. These are the platelet-forming “Chloro- plana vacuolata” (Dubinina and Kuznetsov 1976), which consists of several alternate rows of vacuolated pho- totrophic and nonphototrophic bacteria, and the filamen- tous “Cylindrogloea bacterifera” (Perfiliev 1914), which has been mentioned only in two other publications (Skuja 1956; Gorlenko et al. 1983). The physiology of these consortia still remains unclear. Pfennig (1980) has proposed a model based on a sulfate- reducing central bacterium that might provide the sur- rounding phototrophic cells with sulfide. Another model based on an exchange of organic matter between the part- ners of the consortia is still under discussion. Hirsch (1984) has postulated that the cell divisions of both part- C. A. Abella · X. P. Cristina · A. Martinez · I. Pibernat · X. Vila Two new motile phototrophic consortia: “Chlorochromatium lunatum” and “Pelochromatium selenoides” Arch Microbiol (1998) 169 : 452–459 © Springer-Verlag 1998 Received: 30 April 1997 / Accepted: 17 January 1998 ORIGINAL PAPER C. A. Abella () · X. P. Cristina · A. Martinez · I. Pibernat · X. Vila Laboratory of Microbiology, Institute of Aquatic Ecology, University of Girona, Campus Montilivi, E-17071 Girona, Spain e-mail: charles_a_abella@morgat.udg.es Fax +34-72-418748