Environmental Engineering Reference
In-Depth Information
which at less than 200 mg are more than 1 OM smaller
than hummingbirds, are the most accomplished practi-
tioners of this art (Heinrich 1993). They can reach the
thoracic temperature of 30
C required for flying even
when the surrounding temperature is near 0
C by start-
ing to shiver at temperatures as low as
2
C and persist-
ing often for more than half an hour. They repeat the
cycle after short flights that only accelerate their heat
loss. Such energy-intensive endothermy is ephemeral,
and the moths spend at least 99% of the winter inactive.
In contrast, hummingbirds drop their temperature
during cold nights to as little as half the waking level, a
quasi-ectothermic torpor that reduces otherwise exces-
sive heat loss. But when shivering themselves back to
life, they require nearly as much energy as for hovering.
Marine ectotherms regulate temperature by selecting
waters that will support optimum growth rates and swim-
ming speeds (@30
C for young carp,@10
C for trout).
The portable environments of endotherms are highly
uniform (36
C-40
C for most mammals, 38
C-42
C
for birds). Endotherms survive in environments that re-
main for most of the year well below the freezing point
and that can plunge repeatedly to below
40
C in abso-
lute terms and to chill-factor equivalents of more than
60
C. Musk oxen and polar bears prove that both her-
bivores and carnivores can thrive in extremely cold envi-
ronments, but their thermoregulatory achievement pales
in comparison with that of small birds, which maintain a
temperature gradient of about 80
C across less than 5
cm between the outside air and the core of their tiny
bodies, warmed to just above 40
C. Heat stored in an
animal should be equal to the sum of inputs generated
by internal metabolism, radiation, conduction, convec-
tion, and evaporation. This balancing does not have
to be instantaneous, but no endotherms can tolerate
large and prolonged excursions of
their core body
temperatures.
In contrast, microbes and some invertebrates have the
option of surviving extreme temperatures by becoming
almost completely dehydrated and entering a death-like
state of cryptobiosis. Cryptobiotic forms that can survive
the greatest temperature extremes belong to the phylum
Tardigrada (fig. 4.3). These tiny water bears (50 mm
to 1.2 mm), related to arthropods and nematodes, and
common in thin water films and on mosses, lichens,
and algae, survive exposures to as much as 151
C and as
little as
270
C, very close to absolute zero (Greven
1980). The highest temperatures compatible with cellu-
lar metabolism are clearly much lower, but studies of
extremophilic organisms have raised them to levels that
were thought previously impossible. Before 1960 the
record belonged to Bacillus stearothermophilus, growing
at 37
C-65
C; then it passed to hyperthermophilic
strains of Bacillus and Sulfolobus that survive at 85
C
(Herbert and Sharp 1992). During the 1980s came the
discoveries of marine hydrothermal thermophiles, grow-
ing at 95
C-105
C, and Pyrolobus fumarii, an archaeon
that grows in the walls of deep-sea vent chimneys and
tolerates 113
C (Stetter 1998).
Some hyperthermophilic enzymes are effective up
to 140
C-150
C, and autotrophic synthesis of all
protein-forming amino acids is favored in 100
C subma-
rine hydrothermal solutions compared to syntheses in
18
C warm seawater. We still do not understand why
the hyperthermophilic enzymes have optimal catalytic
activity above 100
C because they contain the same 20
amino acids as enzymes of other organisms, and there
are no gross structural differences between them and the
compounds in mesophilic prokaryotes (Zierenberg,
Adams, and Arp 2000). Higher organisms are much less
