Unusual or Extreme Habitats - Freshwater Ecology: Concepts and Environmental Applications - page 295

Any aquatic habitat has a tremendous number of associated subhabi-
tats, many of which have received scant attention. Examples of subhabitats
include the inside of animal digestive systems, epiphytes on leaves, splash
zones near lake and stream edges, and groundwater upwelling zones in
streams and lakes. Such habitats will not be discussed in detail in this topic.
ADAPTATIONS TO EXTREMES
Many of the adaptations that will be discussed here are cellular or mo-
lecular, and most of the organisms that inhabit the most extreme environ-
ments are microorganisms. Microbes probably dominate because higher
plants and animals have complex multicellular systems that cannot evolve
to compensate for extremes, such as particularly high temperatures, salin-
ity, and variations in pH.
Understanding the influence of extremes in pH, salinity, and tempera-
ture requires knowledge of the structure and function of biological mole-
cules. The main influences of temperature are related to protein structure,
DNA and RNA structures, and lipid fluidity. For proteins to function, they
must maintain structure and have ample thermal energy. An enzyme not
only needs to maintain an active site in a very specific configuration but
also must be able to translate thermal energy into making or breaking
chemical bonds. Thus, enzymes have a temperature range in which they are
able to maintain structure and activity, and within that range they have an
optimum temperature for activity. Enzyme activity increases with temper-
ature up to a point, and then the enzyme starts to break down (denature).
Proteins in organisms in high-temperature environments maintain stability,
but the biochemical mechanisms are not well established (Stetter, 1998).
Factors that are correlated with thermal stability of proteins include a
hydrophobic core, reduced glycine contents, high ionic interactions, and
reduced surface area to volume ratio (Madigan and Oren, 1999).
Biological membranes need to maintain an optimum degree of fluidity
to function properly. If the membranes are excessively fluid, they will not
maintain cell integrity, and if the membranes are solid they lose biological
activity. As temperature increases, the melting point of the lipids increases
as well. Lipid melting points increase with greater proportions of single
bonds (increasing saturation), increased branching, greater length, and, in
the Archaea, ether lipids (Russell and Hamamoto, 1998). For lipids to re-
main fluid at low temperatures, the opposite properties (unsaturated, short,
and unbranched) are required.
DNA and RNA molecules also have a specific temperature range in
which they function optimally. DNA molecules of organisms adapted to
high temperatures may have more C-G bonds than those growing at lower
temperatures because C-G pairs are stabilized by three hydrogen bonds,
whereas A-T pairs only have two bonds. However, this is not a universal
adaptation because all hyperthermal organisms do not have greater pro-
portions of C-G bonds. Proteins that stabilize DNA structure may be more
important (Stetter, 1998; Madigan and Oren, 1999). RNA molecules also
need to maintain particular structures. For example, messenger and ribo-
somal RNA need portions of the molecule held in specific configuration
(secondary and tertiary structures) to function properly. If these secondary