Unusual or Extreme Habitats - Freshwater Ecology: Concepts and Environmental Applications - page 296

(Fig. 7.2) and tertiary structures are not maintained, protein synthesis
(translation) will not occur. In organisms adapted to high temperatures,
RNA has extended regions of base pairing (C-G or A-U) to stabilize sec-
ondary and tertiary structures. The requirement for these extended regions
is less stringent in low-temperature organisms. Organisms that can with-
stand freezing include some multicellular organisms.
To survive freezing, the ability to avoid the damaging effect of ice crys-
tal formation in cells is required (Sakai and Larcher, 1987). Ice crystals
rupture the plasma membrane and destroy the integrity of the cells. Com-
pounds with antifreeze properties can allow organisms to maintain liquid
inside of cells to temperatures several degrees below freezing. Also, com-
pounds that inhibit formation of large ice crystals allow some aquatic or-
ganisms to withstand complete freezing.
Different adaptations are required for organisms living under extremes
of salinity. In these situations, ions outside the cells (such as magnesium
compounds) become highly hydrated, and water becomes limiting. Animals
and microbes can survive moderate levels of salinity by excreting excess
salt. Osmoregulation of fish has received considerable study (Eddy, 1981)
and the physiological mechanisms to control salt balance are documented.
Only organisms that are able to withstand high intracellular concentrations
of salt (above about 10% salinity) can survive in high-salinity environ-
ments. Osmotic pressure will collapse cells and drain their water if they do
not maintain an internal concentration of dissolved materials approxi-
mately equal to the external ion concentration. For example, Dunaliella, a
green alga that thrives in saline waters, synthesizes high concentrations of
glycerol to counteract the effects of increased salinity (Javor, 1989). Many
other species of algae and fungi also use glycerol to counteract osmotic
pressure in aquatic environments. Some halobacteria can accumulate up to
5 M KCl in their cells (Grant et al., 1998). Salinity also alters proteins by
increasing hydrophobic interactions.
Lack of water is a particularly severe condition for aquatic organisms.
Temporary pools, wetlands, and intermittent streams all have periods of dry-
ing. The ability to withstand desiccation is found in many groups of aquatic
organisms. An impressive example is the cyanobacterium Nostoc, which ac-
cumulates sucrose to maintain biological molecules during drying and has
been documented to withstand 107 years of desiccation (Dodds et al., 1995).
Finally, habitats with high light intensity (such as the surface layer of
a pond or lake) can be detrimental to many organisms because solar radi-
ation harms cells. Solar irradiance causes formation of free radicals that
can react with biological molecules. High-energy light, particularly UV,
causes the most damage. Compounds such as carotenoids (many types of
organisms), mycosporine-like amino acids (diatoms), flavenoids (green al-
gae and higher plants), and scytonemin (cyanobacteria) absorb damaging
light. These compounds protect organisms from damage by preventing for-
mation of the harmful free radicals (Long et al., 1994).
SALINE LAKES
Saline lakes and ponds occur in closed basins in which water leaves
primarily through evaporation. In these situations, salts are weathered