3.1.2. Chlorobiaceae

As most of the green sulfur bacteria thrive in freshwater habitats, it is not surprising that they lack possibilities of synthesis of betaine and ectoine. The marine *Chloroherpeton thalassium* is the only one that can produce betaine from glycine (Table 1 and Figure 1). However, members of the genus *Prosthecochloris* and in particular *Prosthecochloris aestuarii* are also regularly found in brackish and marine coastal or saline habitats. They might cope with marine concentrations of salt by accumulation of trehalose, as shown to occur in *Prosthecochloris vibrioformis* DSM 260 (Pfennig 6030) and *Chlorobaculum thiosulfatophilum* DSM 249 (Pfennig 6230) [13]. If grown in marine media supplied with betaine, they can also accumulate the betaine [13]. Therefore, they have limited possibilities to cope with salt stress by accumulation of trehalose and uptake of betaine from the environment, to thrive at elevated salt concentrations. It is expected that they take advantage of betaine uptake when occurring in hypersaline habitats. A gene cluster annotated as *proVopuAB,AC* in *Prosthecochloris vibrioformis* DSM 260 (Table 1) presumably is a betaine transport system. The sequence of OpuAB is clearly distinct from those found in the *Chromatiaceae* and *Ectothiorhodospira* species, as well as from the ProW sequences found in other *Chromatiaceae* and *Halorhodospira* species and its ProV sequence is an outsider of the OpuAA branch (data not shown). This indicates that components of the transport system of *Prosthecochloris vibrioformis* might be related to an ancient ancestor of both the OpuA and the ProU transport systems, both of which might have evolved from a similar ancient ancestor. It would be interesting to study their catalytic properties and the evolutionary path of both systems.
