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strains like Acaryochloris marina ( Dibrova, Galperin, & Mulkidjanian, 2010 ).
Compared to Na + , the export of Cl is much less well understood among
cyanobacteria (for a review, see Hagemann (2011) ).
Additional to the energy demand for ion export, cells using the 'salt-
out' strategy need energy and organic matter for the accumulation of high
amounts of compatible solutes, which are used instead of inorganic ions to
balance the osmotic potential and to maintain turgor. These compounds are
low-molecular-mass organic molecules that are highly water soluble and
usually do not carry net charge at physiological pH. Compatible solutes
can be accumulated in high (molar) amounts without negative interference
(i.e. being compatible) towards the metabolisms ( Brown, 1976 ). In addition
to the osmotic equilibrium, the compatible solutes can also exhibit direct
protective effects towards sensitive macromolecules. The protective effect
explains why often the accumulation of rather low amounts of compatible
solutes, i.e. at concentrations not making big contribution to the intracel-
lular osmotic potential, results in significant increase of salt or drought stress
tolerance (for a review, see Chen and Murata (2011) ).
3. COMPATIBLE SOLUTES
After the first description of glucosylglycerol (GG) as compatible sol-
ute in a marine Synechococcus strain ( Borowitzka, Demmerle, Mackay, & Nor-
ton, 1980 ), about 200 cyanobacteria were screened for such compounds and
their salt-resistance range (see Hagemann (2011) for a comprehensive table).
This data set revealed that a rather small spectrum of compatible solutes
is found in salt-loaded cyanobacteria, and, that a correlation between the
chemical nature of the compatible solute and the salt resistance limits exists
( Reed, Borowitzka et al., 1986 ). Accordingly, group 1 of low salt tolerance
accumulates sucrose and/or trehalose (150 examples), group 2 of moderate
halotolerance prefers GG (71 examples), and group 3 of halophilic strains
have an absolute requirement for a minimal salt concentration and syn-
thesize glycine betaine (22 examples) as characteristic compatible solute.
Unfortunately, cyanobacterial taxonomy is problematic. Species names as
well as strain numbers have been changed or mixed. Therefore, it is dif-
ficult to make a direct comparison of the strain list with compatible solutes
(see Hagemann, 2011 ) and the list of compatible solute genes derived from
genome sequences of cyanobacteria ( Table 2.1 ).
In this chapter, the author will often refer to two big groups of cyano-
bacteria, which were initially distinguished according to their RubisCO
 
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