Biology Reference
In-Depth Information
1. INTRODUCTION
Cyanobacteria can be found in every light-exposed habitat on
Earth. The majority of cyanobacterial strains in culture collections (e.g.
Pasteur culture collection of cyanobacteria, PCC, Paris, France) were
isolated from aquatic habitats, but many cyanobacteria are also found in
diverse terrestrial habitats such as on soils or surfaces of rocks. Beside
nutrients, light and temperature, the availability of water and the amount
of dissolved ions (total salinity) are important environmental factors
determining the occurrence of strains in specific environments. Because
total salinity and water amount are closely linked, e.g. during desiccation
of soil, the amount of water is decreasing and, in parallel, the total salt
concentration is increasing, it is not surprising that acclimation towards
drought and high salinity employs overlapping mechanisms. In both
cases, the maintenance of water and turgor pressure inside the cell is
one of the central issues during the acclimation (experimentally verified
for cyanobacteria by
Ladas and Papageorgiou (2000)
). Because water
uptake is a passive process following the water potential gradient, grow-
ing microbial cells need to establish a lower water potential inside the
cell relative to the surrounding medium, which is achieved by regulat-
ing the cellular osmotic potential via varying amounts of low molecular
compounds.
The main difference between pure water or osmotic stress and salt
stress is the additional direct ion effect on metabolic activities in the latter
case. In the nature exists large variations regarding the amount and com-
position of inorganic salts, which clearly affects cyanobacterial distribu-
tion (
Oren, 2000
). In addition to the problem that high total ion content
generally makes it difficult to maintain water and turgor inside the cell,
many ions are toxic for living cells. This direct toxicity is true not only
for heavy metals but also for any ion at nonphysiological high cellular
concentrations.
This chapter deals with cyanobacterial salt acclimation in the genomic
era. Because the annotation of genome information relies on the knowl-
edge of well-studied model organisms, the author will use here the term
high salt equivalent for an enhanced Na
+
and Cl
−
contents, the main inor-
ganic ions in the marine environment, which are usually used in laboratory
experiments to mimic salt stress.
