3.1.1. Cyanobacteria

It was demonstrated that the salt-tolerance of *Cyanobacteria* was clearly related to the compatible solutes accumulated, those of the lowest tolerance (freshwater strains growing below 0.7 M NaCl) accumulate sucrose and trehalose, those of moderate tolerance (marine strains growing up to 1.8 M NaCl) accumulate glucosylglycerol, and those with the highest tolerance (strains of marine and hypersaline origin, in grea<sup>t</sup> majority classified as *Synechococcus* strains) accumulate glycine betaine or glutamate betaine [11]. Apparently, the majority of cyanobacteria adapted to marine environments count on glucosylglycerol as osmoticum [43–45]. The formation of a fused molecule with glycerol (glucosyl glycerol) as a component appears to be a clever strategy to keep at least, in part, the excellent compatible nature of glycerol but reduce leakage through the cell membrane. Those *Cyanobacteria* originating from salt lakes and hypersaline ponds (tolerance of >20% NaCl) accumulate betaine, which apparently is essential to provide sufficient protection at moderately and extremely high salt concentrations [11].

Betaine biosynthesis is found in two major phylogenetic branches of *Cyanobacteria*, but ectoine biosynthesis is absent (Table 1, Figure 1). One branch is formed by *Aphanothece halophytica* (*Halothece* PCC7418), together with other *Chroococcales*. *Aphanothece halophytica* is a characteristic inhabitant of hypersaline environments and among the most halotolerant of *Cyanobacteria*. A gene cluster of the two methyltransferases (GMT and DMT) is present in these bacteria (Table 1). The BetT present in *Aphanothece halophytica,* which lacks cholinedependent betaine synthesis, is characterized as a specific transporter of betaine [18]. A second major branch of *Cyanobacteria* includes representatives of the heterogeneous groups of the *Synechococccus* and *Prochlorococcus* species and is phylogenetically quite distinct from the first branch (Figure 1). These Cyanobacteria have *proXWV* genes included in a cluster with GMT and DMT genes (Table 1).

The two groups are also distinguished by significant difference in the G + C content of the DNA. Representative strains of the *Prochlorococcus* group have a G + C content near 50 mol% for the *Prochlorococcus marinus* (strain MIT9313: 50.7 and strain MIT9303: 50.0%) and near 59% for *Synechococcus* (strain WH8102: 59.4% and strain WH8103: 59.5%). Much lower values are present in the *Halothece/Aphanothece* group: 42.9 mol% in *Aphanothece halophytica*, 41.1 mol% in *Euhalothece natronophila*, and 42.4 mol% in *Dactylococcopsis salina*. Although systematically assigned to the *Synechococcales*, *Dactylococcopsis salina* fits very well into the *Halothece* group, according to the gene repertoire and the phylogeny of the GMT sequence (Figure 1, Table 1) as well as theG+C content, which puts a question mark to its current taxonomic affiliation.

**Figure 1.** The phylogeny of betaine biosynthesis based on the sequences of glycine/sarcosine methyltransferase GMT is shown in a neighbor-joining tree. Sequences and gene bank accession numbers are shown in the Supplementary Table S1. Bootstrap values expressed as percentages of 1000 replications are given at the branches. The bar indicates an evolutionary distance of 0.1. The following color code highlights the different systematic groups.
