Biology Reference
In-Depth Information
2. BASIC SALT ACCLIMATION STRATEGY
To acclimate to a quick increase in the external salinity or to live
permanently in the high salt environment, cyanobacteria as most other cells
apply the so-called 'salt-out' strategy (
Galinski, 1995
). The other strategy
is called 'salt-in' strategy, which is used by some halophilic Archaea and
Bacteria. The latter prokaryotes accumulate high internal amounts of inor-
ganic salts (especially KCl) exceeding the external salt concentration. The
accumulation of inorganic ions is metabolically cheaper to increase internal
osmotic concentrations and to ensure water uptake, turgor pressure and
growth. However, the presence of high salt concentrations in the metaboli-
cally active compartment needed an adaptation of all organic macromol-
ecules to this new environment, which seemed to be difficult to achieve
during evolution. This assumption explains why the energetic favourable
'salt-in' strategy is restricted to a few prokaryotes.
In contrast, organisms using the 'salt-out' strategy can keep the normal
set of low-salt-resistant proteins for metabolic activity but needed to find
an energetically more expensive strategy to balance the osmotic potential
difference. These organisms are characterized by almost unchanged inter-
nal ion concentration after acclimation to high NaCl concentrations. This
observation is especially true for amounts of Na
+
and Cl
−
(e.g. shown for
Synechocystis
sp. PCC 6714 by
Reed, Warr, Richardson, Moore, and Stewart
(1985)
). To keep the low internal NaCl concentration in the presence
of high external salinities, Na
+
and Cl
−
are actively pumped out from
the cells. Multiple transporters are used for Na
+
export by cyanobacteria.
Predominantly, specific members of the Na
+
/H
+
antiporter family seem
to be involved (
Elanskaya, Karandashova, Bogachev, & Hagemann, 2002
;
Inaba, Sakamoto, & Murata, 2001
;
Waditee et al., 2001
,
2002
;
Wang, Post-
ier, & Burnap, 2002
). Additionally, the Mrp system is used for Na
+
export
(
Blanco-Rivero, Leganés, Fernández-Valiente, Calle, & Fernández-Piñas,
2005
;
Fukaya et al., 2009
). These transporters receive the energy for ion
export from the proton gradient at the membrane. For many years, it was
discussed if cyanobacteria and eukaryotic algae also employ primary active
Na
+
-ATPases for ion export during salt acclimation (
Gimmler, 2000
). Only
recently, direct experimental evidence for the presence and activity of Na
+
-
ATPases of the F
1
F
0
-type was presented for
Aphanothece halophytica
(
Soon-
tharapirakkul et al., 2011
) and from genome information for some more
