The Epibiont

Early electron microscopic studies of phototrophic consortia from the natural samples revealed the presence of chlorosomes in the epibiont cells (Truper and Pfennig 1971; Caldwell and Tiedje 1974), suggesting that they belonged to the group of green sulfur bacteria. This conclusion was supported by the observation that maximum concentrations of green sulfur bacterial pigments (bacterio-chlorophylls c, d and e) in freshwater lakes coincided with population maxima of phototrophic consortia. The phylogenetic affiliation of the epibionts with the Chlorobiaceae could be verified by fluorescence in situ hybridization after a highly specific oligonucleotide probe for green sulfur bacteria had become available and an improved hybridization protocol had been developed (Tuschak et al. 1999).

Recently, repeated cultivation attempts led to the isolation in pure culture of the epibiont from a "Chlorochromatium aggregatum" (Vogl et al. 2005). Exhaustive physiological testing of this bacterium did not reveal any unusual physiological capabilities as compared to the known strains of Chlorobiaceae, suggesting that epibionts of phototrophic consortia may grow photolithoauto-trophically like their free-living counterparts. Indeed, in situ measurements of light-dependent H14CO3- fixation in a natural population of phototrophic consortia and determination of the stable carbon isotope ratios (813C) of their bacteriochlorophyll molecules revealed that photoautotrophic growth of the epibionts occurs under natural conditions (Glaeser and Overmann 2003a). It is to be concluded that the epibionts are obligate photolithoautotrophs as all other known Chlorobiaceae (Overmann 2001a). The only unusual feature of green sulfur bacteria associated with phototrophic consortia detected so far is the low cellular concentration of carotenoids in the epibiont of "Chlorochromatium aggregatum," (Vogl et al. 2005) and in the brown epibiont from "Pelochromatium roseum" (Glaeser et al. 2002; Glaeser and Overmann 2003a). More subtle differences were observed with respect to the light-dependence of growth. In cultures of phototrophic consortium "Chlorochromatium aggregatum", light limitation of growth was observed only at light in-

tensities as low as <3 |mol quanta m s" , while maximum growth rates (doubling times of 1 day) were observed between 5 and 20 |mol quanta m"

s-1. In contrast, the free-living green sulfur bacteria tested reach light satura-

tion of growth at higher light intensities (~10 |mol quanta m" s") (Overmann et al. 1991, 1992, 1998).

Subsequent studies explored the biodiversity and biogeography of photo-trophic consortia by determining 16S rRNA gene sequences of epibionts from aquatic environments worldwide (Glaeser and Overmann 2003a, 2004). Epibiont 16S rRNA gene sequences originating from 41 different consortia were obtained after sorting individual consortia by micromanipulation of samples collected in 14 different freshwater lakes. The 16S rRNA genes were amplified by a highly sensitive group-specific PCR, and the amplification products were separated by denaturing gradient gel electrophoresis (DGGE) and sequenced. Most importantly, all epibiont cells in a particular type of phototrophic consortium invariably belonged to one single phylo-type. Phylogenetic analyses further demonstrated that the epibionts of each particular type of consortium represent a distinct and novel branch within the radiation of green sulfur bacteria (Frostl and Overmann 2000; Glaeser and Overmann 2003a, 2004).

Interestingly, morphologically indistinguishable phototrophic consortia, when collected from different lakes, were found to harbor genetically different epibionts. Thus, phylogenetic analyses demonstrated that the "Chloro-chromatium aggregatum" sampled from European and North American lakes contained seven different epibiont phylotypes depending on the lake, although these consortia were identical with respect to their shape, and the arrangement and color of the epibionts (Glaeser and Overmann 2004). It was concluded that morphologically indistinguishable consortia which occur in geographically distant locations frequently harbor distinct epibionts. In addition, even epibionts with identical partial 16S rRNA gene sequences exhibit considerable differences in morphology and pigmentation and hence genetically clearly differ from each other. Therefore, phototrophic consortia are significantly more diverse than the seven different morphotypes recognized so far. The current estimate amounts to 19 different types of epibionts. Novel types of phototrophic consortia continue to be described (Overmann et al. 1998; Glaeser and Overmann 2004) and future new discoveries are most likely to be made.

It has been postulated that bacteria are ubiquitous (Beijerinck 1913; Baas-Becking 1934) and it was suggested that the high population densities of microorganisms drives a rapid, large-scale dispersal across the physical and geographical barriers (F inlay and Clarke 1999; Finlay 2002). Under these conditions, competitive exclusion of species with an identical ecological niche would be expected to result in a low overall diversity. On the contrary, if en-demism occurs among microorganisms, it would result in a significantly higher global diversity, since the latter is maintained by geographic isolation (Staley 1999). Phototrophic consortia are assumed to occupy a narrow and well-defined ecological niche (see below) and therefore represent well-suited model systems to study bacterial biogeography. It was recently demonstrated that epibionts of phototrophic consortia show a nonrandom global distribution (Fig. 3). In fact, the composition of epibionts in consortia from lakes within one geographic region (Europe or North America) was very similar, whereas only two of the 19 epibiont types known were recovered from lakes on both, the European and the North American continents (Glaeser and Overmann 2004) (Fig. 3). While many other bacteria investigated to date indeed appear to be ubiquitous, the dispersal of phototrophic consortia may be much slower than for other bacteria, at least over larger geographical distances (Glaeser and Overmann 2004). Dispersal is certainly limited by the high sensitivity of intact consortia towards molecular oxygen, which leads to a rapid disaggregation of the cell association.

Fig. 3. Biogeography of green sulfur bacterial epibionts of phototrophic consortia based on analyses of partial 16S rRNA gene sequences. Values at each study region give the numbers of lakes investigated. Each square represents a particular type of epibiont. Numbers of different epibionts detected in each study region are given in vertical columns. Squares at the same horizontal positions designate the same type of epibiont

Fig. 3. Biogeography of green sulfur bacterial epibionts of phototrophic consortia based on analyses of partial 16S rRNA gene sequences. Values at each study region give the numbers of lakes investigated. Each square represents a particular type of epibiont. Numbers of different epibionts detected in each study region are given in vertical columns. Squares at the same horizontal positions designate the same type of epibiont

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