3.1.6. Acetobacteraceae

Phototrophic Acetobacteraceae are freshwater bacteria. *Rhodopila globiformis* lacks possibilities of synthesis and transport of betaine and ectoine, and *Paracraurococcus ruber* depends on external supply of such compounds and on transport via a ProU system (type W2) for osmotic adaptation (Table 2). *Acidiphilium* species, however, can produce ectoine and hydroxyectoine and in addition have a ProU (type W3) uptake system associated with the genes of betaine biosynthesis from choline (*proXWVbetBA*). *Acidiphilium* species are adapted to life in acidic freshwater habitats and their acidophilic nature should preclude their development in neutral/basic marine habitats. Therefore, the accumulation of ectoine and possibly also betaine might play a role in adaptation to highly acidic conditions rather than to saline habitats.

#### 3.1.7. Marine and Halophilic Rhodospirillaceae

Freshwater species of *Rhodospirillaceae* including species of *Pararhodospirillum*, *Rhodospirillum*, and *Phaeospirillum* lack ectoine and betaine biosynthesis (from glycine) and only some strains of *Rhodospirillum rubrum* can synthesize betaine from choline or have a ProU transport system (Table 2). On the other hand, marine and halophilic *Rhodospirillaceae* (*Rhodospira, Rhodovibrio, Roseospira, Roseospirillum*, and *Caenispirillum* species) are genomically well equipped with possibilities of osmotic adaptation. These species are adapted to moderately and extremely high salt concentrations. In particular, the *Rhodovibrio* species tolerate more than 3 M (up to 20%) NaCl. All of them synthesize ectoine (*Caenispirillum salinarum* also hydroxyectoine) and betaine. With the exception of *Rhodospira trueperi* and *Roseospirillum parvum*, they can also transform choline to betaine (*betABI*). Different to other *Rhodospirillaceae*, in the *Rhodovibrio* species, the GMT and DMT genes are not fused and form a gene cluster with MAT and SHAase. Their GMT gene is the only example of a B-type GMT gene in *Alphaproteobacteria*, and the *ectA* gene is not included in an *ectABC* cluster, as in almost all other phototrophic bacteria producing ectoine. In addition to *betT* (multiple), marine *Rhodospirillaceae* have one or more ProU transport systems. These are related to the *proVWX* type W1 (*proU* of *E. coli*), *proXWV* (type W2), or *proXVW* (type W4) systems. In *Caenispirillum salinarum*, a type W3 ProW system is present (Table 2).

#### 3.1.8. Halophilic *Rhodothalassium salexigens*

*Rhodothalassium salexigens* is a moderate halophilic and especially salt tolerant bacterium that grows at salt concentrations exceeding 3 M (20% NaCl). The outstanding properties of *Rhodothalassium salexigens* as distinct from all other phototrophic *Alphaproteobacteria* are demonstrated by sequences of the 16S rRNA gene and of the photosynthesis reaction center and bacteriochlorophyll biosynthesis genes [46]. These are in line with its recognition as a separate genus, family and order of the *Alphaproteobacteria* [47,48]. Ectoine biosynthesis is absent (Table 2). Betaine biosynthesis from glycine is possible and GMT sequences form a distinct lineage among those of the *Alphaproteobacteria* (Figure 1). In addition, BetT and a type W1 ProU transport systems are present.
