COMUNICACIÓN LOCAL

The family of the Chlorobiaceae is a phylogenetically shallow group with high similarity values among 16S rDNA gene sequences (>90.1%/Knuc < 0.11 (Overmann 2000)), with the

sole exception being Chloroherpeton thalassium (85.5–87%) (Overmann 2001). However, within this family, a large diversity is found. But the question arises, which selection factors drive the diversification of the family Chlorobiaceae. Since their habitat is limited to the chemocline of meromictic lakes and water bodies, variables that lead to niche formation are equally limited as well.

One factor that contributes to the diversification of GSB is light. Depending on the depth of the chemocline, the turbidity or the pigmentation of organisms in the water layers

91 above the chemocline, light of different wavelength reaches the habitat of the GSB. Depending on the wavelength of the light reaching the chemocline, either green- or brown- colored types of GSB are found predominantly. The green colored types of GSB thrive in dystrophic lakes (Parkin and Brock 1980) or underneath mats of purple sulfur bacteria, where light of the blue or red wavelength range prevails (Caldwell and Tiedje 1975; Gorlenko and Kuznetsov 1971; Overmann et al. 1998a). Further distinguishing the green types of GSB is their ratio between bacteriochlorophyll (bch) c and d. Strains with high amounts of bch c grow faster under low-light conditions than mutants of the same strain only being able to produce bch d (Maresca et al. 2004).

The brown types, on the other hand, are primarily found in eutrophic lakes or at greater depth, where only light in the blue-green to green wavelength range is available. Under these conditions, brown-colored types of GSB containing high concentrations of the light-harvesting carotenoids isorenieratene and β-isorenieratene instead of chlorobactene, have a selective advantage over their green-colored counterparts (Montesinos et al. 1983). When looking at the distribution of green- and brown-colored strains among the 16S rDNA tree of GSB, clusters of green or brown types cannot be identified. The only exception is the absence of brown strains in subgroup 4a. Albeit, only of 5 of the 14 cultured strains in this subgroup, information about their pigmentation is available. The relatively even distribution of green and brown strains in the phylogenetic tree suggests that differing wavelength ranges are not a major driving force in the diversification of GSB. This is supported by the occurrence of strains with identical 16S rDNA sequences that differ in their particular color. For a green-colored GSB to become a brown-colored one, the monocyclic chlorobactene needs to be transformed to the dicyclic β-isorenieratene and bch c has to be formylated at the C7 position to be converted to bch e (Brockmann 1976). Initiating the transformation of chlorobactene to β-isorenieratene is the gene cruB, which is exclusively found in brown- colored GSB. In the genomes of the three sequenced brown-colored GSB species, it lies in a small gene cluster that includes three other genes, one of which is a candidate for the conversion of bch c to e (Figure 20.) (Maresca 2007).

92 Figure 20. Gene neighborhoods around cruB (#3) in Chl. phaeobacteroides BS-1, Chl. phaeobacteroides DSM 266T, and Pld. phaeoclathratiforme BU-1. #1. 2-vinyl bacteriochlorophyllide hydratase (bchF), #2. Hypothetical protein (radical SAM motif), #4. Isoprenylcysteine carboxyl methyltransferase (Maresca 2007).

It is therefore speculated that this cluster is easily transferred between green sulfur bacterial species and responsible for the conversion from a green- to a brown-colored strain. The even distribution of the two types of GSB we found among the 16S rDNA tree supports the hypothesis of lateral gene-transfer as reason for the switch from green- to brown-colored strains.

Interestingly, all but one green sulfur bacterial sequence found on corals form a distinct cluster in the marine group, with Prosthecochloris vibrioformis 3M being the only cultured representative. Very likely, this type of green sulfur bacterium is part of a bacterial community responsible for coral diseases. Of the five studies that contributed green sulfur bacterial sequences associated with corals, four were investigating diseased tissue (de Castro et al. 2010; Frias-Lopez et al. 2002; Myers and Richardson 2009; Pantos et al. 2003). The black band disease dissolves corals by creating a sulfide-rich microenvironment (Richardson et al. 2009). This explains why GSB are frequently found associated with corals, which are usually not located in the chemocline but in the oxic zone of the sea. The fact that all sequences were clustered together suggests that a genetic adaptation was necessary to colonialize this specific niche.

From the temperature of the habitat, a connection can be drawn from the physiology to the phylogeny. Most sequences derived from warm or hot habitats are found in group 4. But thermophilic GSB are not found in this cluster exclusively but also in group 4b and 3b.

93 Therefore, this trait is either polyphyletic or was acquired by other strains through horizontal gene transfer. This is similar to the trait of salt tolerance in GSB. Until recently, the salt requirements of GSB were believed to be a reliable criterion to differentiate between green sulfur bacterial species (Alexander et al. 2002; Imhoff 2001; Overmann 2000) and therefore are a result of niche formation within the GSB. However, as it has been shown lately (Triado- Margarit et al. 2010), salt tolerance does not seem to be restricted to species of any specific group, but is widespread among the phylogeny of GSB, and closely-related phylotypes can have dissimilar salt tolerance capacities. Thus, also salt tolerance most likely lead to the formation of the marine group, it is not a criterion that is strictly limited to one group of green sulfur bacteria. Since lateral gene transfer is responsible for the color-switch from green to brown pigmentation, one could speculate that GSB frequently exchange genetic material that leads to an extensive distribution of physiological properties. This frequent exchange would also explain the futility of physiological traits that have been used to characterise the family of the Chlorobiaceae like pigmentation, motility, cell morphology and the ability to form gas vesicles (Pfennig and Trüper 1989).