AMER. ZOOL., 31:81-92 (1991)
What Is Cold Adaptation and How Should We Measure It?1
ANDREW CLARKE
British Antarctic Survey, NERC, High Cross, Madingley Road, Cambridge CB3 OET, UK
SYNOPSIS. Cold adaptation encompasses all those aspects of an organism's physiology
that allow it to live in polar regions. With the exception of the special case of the need
to avoid freezing, it is therefore merely a specific example of the more general temperature
compensation needed by all marine organisms. Temperature compensation is a form of
homeostasis; the extent to which a given organism has achieved this can only be assessed
in those processes which can be studied at the molecular level. Recent studies of polar
organisms, primarily fish, have indicated that compensation is not always perfect. Studies
of complex integrated processes such as growth or respiration do not necessarily give
useful information concerning cold adaptation. Growth, for example, may show compensation at the molecular level but still be slow for other reasons (for example, resource
limitation). Respiration is a particularly misleading indicator of temperature compensation,
primarily because it represents the summation of many processes each of which may react
differently to temperature. The use of respiration rate to assess temperature compensation
should be abandoned forthwith.
example, antifreeze proteins) but also I suspect because from an anthropocentric
viewpoint it appears somehow easier or
more amenable to live in tropical water
than a polar ocean. This latter point has
influenced most of the history of investigation into temperature biology and while
it is undoubtedly true for mammals and
birds, the validity for poikilothermic
organisms is questionable (as I hope to show
below).
Nevertheless there are certain physioWhat is cold adaptation?
logical challenges which are peculiar to low
In general terms cold adaptation is easy temperatures. These surround the need to
to define. It encompasses all those aspects avoid, or to tolerate, the freezing of body
of the physiology of an organism which fluids. This problem is most severe in teleallow it to inhabit the polar regions. Cast ost fish whose tissue fluids are generally
in such broad terms cold adaptation is no hyposmotic to seawater. Under normal cirmore than a specific example of the more cumstances teleost fish would freeze in seageneral adaptation any organism must have water below about — 0.7°C; polar fish avoid
to the particular thermal features of its this possibility partly by the production of
environment. Thus polar fish show "cold protein or glycoprotein antifreezes. No fish
adaptation" in the same sense that fish liv- has been shown reliably to withstand the
ing in a tropical reef show "warm adap- freezing of tissue fluids (although among
vertebrates this is known for some terrestation."
Why then do we tend to separate cold trial frogs and one species of turtle); howadaptation as something special? Probably ever, intertidal invertebrates have to withpartly because there are certain adapta- stand very much lower temperatures and
tions specific to very low temperatures (for a few species can tolerate a limited degree
of frozen extracellular water. These adaptations are a response specifically to freez1
From the Symposium on Antarctic Marine Biology ing, or the threat of freezing, and are thus
presented at the Annual Meeting of the American
not a feature of adaptation to temperature
Society of Zoologists, 27-30 December 1988, at San
in
general. I will therefore not discuss them
Francisco, California.
INTRODUCTION
In recent years there have been a number of reviews of cold adaptation (for
example, Clarke, 1983, 1987; Cossins and
Bowler, 1987; Macdonald^a/., 1987). For
this symposium it therefore seems inappropriate to attempt a further review.
Rather I have chosen to examine some of
the basic concepts and see whether this
indicates areas of ignorance or possible lines
of future research.
81
ANDREW CLARKE
Temperature
polar organisms cope with the limitations
imposed by temperature, compared with
those living in warmer waters. Since polar
oceans are at one end of the spectrum of
temperature to which marine organisms
have evolved, it is sensible to start by viewing cold adaptation as merely part of the
general problem of compensation for temperature.
TEMPERATURE COMPENSATION
n
O CO
Temperature
Temperature
FIG. 1. Temperature compensation. Top) Compensation in a temperate organism cooled to a lower temperature; the solid line shows the effect of acute cooling and the arrow shows the process of compensation.
Middle) Compensation in a temperate organism
warmed to a higher temperature; presentation as
above. Bottom) Temperature compensation as homeostasis; the bandwidth indicates that at any given temperature there may be a range of rates in different
organisms, and the slope indicates that compensation
may not be perfect.
further here. (For recent reviews see
DeVries, 1983, 1984.)
The Southern Ocean contains a rich and
diverse fauna. It is also highly productive
during the short austral summer. There is
therefore no scientific need to demonstrate cold adaptation. The scientific value
comes not in showing that organisms are
capable of performing a certain function
at polar temperatures but in understanding the mechanisms by which they achieve
this. It is also valuable to see how effectively
Historically, adaptation to low temperature has been examined by cooling an
organism that normally lives at a warmer
temperature and seeing how it behaves. A
typical experiment is shown in Figure la.
On cooling almost all physiological processes slow down, at least initially. This is
generally due to purely thermodynamic
effects (reaction rates are a positive function of temperature) and the process is
illustrated diagrammatically by the solid
line. Compensation is said to have occurred
if after a period of time the rate returns to
that exhibited before the temperature was
lowered, and this process is illustrated by
the arrow.
Usually an experimentally cooled organism will be operating more slowly, at least
initially, than a similar organism which has
evolved over millions of years to live at low
temperature. (The comparison shown in
Fig. la is precisely the experiment carried
out on the locomotor ability and respiration rate of fish. The polar fish was able to
swim more or less as well in water close to
zero as could a temperate fish in warmer
water.)
Clearly the reverse will happen if a temperate fish is warmed to tropical temperatures (Fig. lb). Initially there will be an
increase in physiological rates simply
because the temperature is higher. In this
case in comparison with a fish that has
evolved to live at tropical temperatures a
decrease in physiological rate is necessary
(rather than an elevation as in the case of
a polar species). Overall, therefore, temperature compensation should be regarded
as a form of homeostasis, a maintenance of
physiological rate in the face of the accelerating or decelerating influence of a
change in temperature. Given that at any
COLD ADAPTATION IN MARINE ORGANISMS
83
TABLE 1. Types of temperature response.
environmental temperature organisms
show a range of physiological rates (for
Definition
Response
example, in locomotor ability) associated
The adjustment of organism
with ecological or evolutionary diversity, Acute
physiology to an immediate
then a simplistic picture of temperature
change in temperature; can
compensation might be that shown in Figinclude torpor, coma (and
death).
ure lc. Compensation may thus be denned
The adjustment of organism
as the maintenance of an appropriate phys- Acclimation
physiology to a new temperaiological rate in the face of temperature
ture in the laboratory.
change.
Acclimatization* The adjustment of organism
physiology to changes in enviIt is worth pointing out that although I
ronmental temperature. This
have presented cold adaptation and warm
may be tidal, daily, seasonal
adaptation as essentially similar challenges
or inter-annual.
to physiology, this is to an extent an over- Adaptation
The evolutionary adjustment of
simplification. Although they both repreorganism physiology to environment. This can include
sent a physiological challenge in repreadjustment to a seasonal or
senting a potential disturbance of
daily fluctuation in temperahomeostasis, the nature of the challenge is
ture requiring acclimatizadifferent depending on the direction of
tion.
temperature change. In essence the prob* It should be noted that in laboratory acclimation
lem with a decreased temperature is one it is usual to modify only a single variable (e.g., temof maintaining physiological rate in the face perature) while keeping all others constant. An orof decreased cellular enthalpy (kinetic ganism undergoing acclimatization in the field is subenergy). When temperature increases the ject to coincident variation in a whole range of
variables.
problem is one of maintaining physiologi- environmental
Modified from Clarke, 1987.
cal power (that is, the ability to deliver a
regulated work output in the face of a temperature induced acceleration of rate).
functioning of organisms. These include
viscosity, solubilities of gases and solutes,
Adaptation and acclimation
and diffusion. In addition all physiological
The definition of temperature compen- processes involve a change in free energy
sation given above avoids any consider- and are therefore affected by a change in
ation of the rate of temperature change. temperature.
Compensation is an equally valid concept
The first point to be made is that if an
whether short-term or long-term temper- organism is to cope with a change in enviature change is involved. The rate of tem- ronmental temperature over evolutionary
perature change may, however, influence time, all cellular processes must evolve some
both the extent to which compensation can degree of compensation for the effects of
be achieved and the mechanisms involved. this change in temperature. At the cellular
A useful classification of temperature level this includes gene expression, conchange is given in Table 1. Acute change tractile protein function, ion pump activcan produce either an immediate compen- ity, ATP generation, synthesis of macrosation, or if the extent of temperature molecules, and intermediary metabolism.
change is too great, a response to minimize At higher levels of integration, temperadamage. At the other extreme, on the time ture compensation may also involve neural
scale of evolutionary change, sufficient time activity, muscular activity, circulatory and
may be available to allow the evolution of respiratory processes, growth, reproducsubtle tuning of physiology to temperature tion, development and behavior.
(Clarke, 1990; Clarke and Crame, 1990).
Where should we look first? Traditionally physiologists have examined enzyme
Some general principles
activity, respiration rate and contractile
A change in temperature affects many protein function, but are these the best
physical parameters that influence the places to look?
84
ANDREW CLARKE
I would suggest that in order to gain an
understanding of what is involved in temperature compensation we should look first
at those processes where it is easier to see
what is going on at the molecular level.
Only when we have a clear idea of what is
(or is not) happening here can we progress
to an understanding of the effect of temperature change on more integrated processes such as respiration, growth or
embryonic development. Although this
might appear a severely reductionist
approach, I am not suggesting that processes such as respiration can only be
understood in these terms. Rather it is that
the history of temperature physiology
shows that attempts to explain higher order
processes such as respiration or growth as
single processes responding simply to temperature have led to more confusion than
understanding (Clarke, 1987). Of course
with these higher order processes a full
picture of what is happening at the molecular level is most unlikely to provide a complete explanation. The point is that we need
to understand the molecular level first.
A particular difficulty surrounds those
processes where it is not always straightforward to distinguish direct limitation by
temperature from a rate that is slow for
ecological or other indirect reasons. This
point is considered in more detail in the
sections on growth and respiration. First,
however, I would like to consider some
recent studies where (at least in some cases)
we can begin to see what may be happening
at the molecular level. These have been
reviewed a number of times in recent years
(Clarke, 1983, 1987;Johnston, 1985; Macdonald et al., 1987) and so will only be dealt
with in outline here.
EVIDENCE FROM RECENT STUDIES
Fish brain microtubules
A good example of compensation where
changes at the molecular level have been
demonstrated clearly is that of microtubule function. Microtubules are involved
in many aspects of cellular biology, including cytoplasmic streaming and cell division. The microtubules of mammals are
cold-labile; that is, they depolymerize to
their constituent subunit proteins (tubulins) at low temperatures. The microtubules of Antarctic fish are, however, functional at these low temperatures (Williams
et al., 1985). Clearly some modification has
occurred over the course of evolutionary
time to shift the temperature range over
which the fish microtubules function.
Detailed electrophoretic studies of the
cytoplasmic microtubules of three species
of Antarctic fish have shown that the a
tubulins of the polar species contained less
glutamyl residues and a slight reduction in
overall negative change compared with
warmer water fish and the cow (Detrich
and Overton, 1986; Detrich et al., 1987).
Changes in the temperature/function
relationship thus appear to be related to
changes in protein primary structure.
Since microtubules appear to be universal constituents of cells it is not surprising
that cold-stable microtubules have also been
reported from polar foraminifera (Bowser
and DeLaca, 1985) and chlorophyte algae
(Burch and Marchant, 1983). Indeed
microtubule function is a good example of
a process where the scientific value lies not
in demonstrating activity at polar temperatures (for this can be assumed from the
high biomass and diversity of Antarctic
marine organisms), but in unravelling how
efficiently and by what mechanism this has
been achieved.
Fish white muscle
The influence of environmental temperature on the structure and function of
fish white muscle has received a great deal
of scientific attention (for a recent review
see Johnston, 1987).
The gradual lowering of seawater temperature since the early Tertiary has
induced a whole suite of evolutionary
responses in fish white muscle. At the structural level the white muscle of polar fish
generally contains higher densities of mitochondria than that of warmer water fish,
and this tends to compensate for the effects
of the low temperature on reaction and
diffusion rates (Johnston, 1987; Sidell,
1990). At the molecular level the activation free energies of muscle ATPases are
reduced only slightly in polar fish; how-
COLD ADAPTATION IN MARINE ORGANISMS
ever, activation enthalpies are tightly correlated with environmental temperature in
a range offish from tropical to polar waters
(Johnston and Goldspink, 1975; Johnston
and Walesby, 1977). The ATPases from
polar fish are far more sensitive (by a factor
of about 500) to thermal denaturation at
50°C than those from warmer water fish
(Johnston and Walesby, 1977; Cossins et
al., 1981). The differences in thermodynamic activation parameters and thermal
stability are believed to be related to differences in the tertiary structure of the
molecule, as a result of alterations in amino
acid sequence fixed by natural selection.
Enzyme activity in vitro is very sensitive to
assay conditions such as pH, ionic strength,
substrate concentration and the presence
of inhibitors or competitors. The enzyme
environment in vivo is complex and generally unknown; care must therefore be
taken in extrapolating from the behaviour
of isolated enzymes in dilute media to the
situation in the living tissue.
Comparative studies of the swimming
ability of fish from different temperatures
have suggested that the colder the water,
the lower the maximum swimming speed
it is possible to obtain (Wardell, 1980; Bennett, 1985). Studies of skinned muscle fibers
from four species of notothenioid fish from
Signy Island have revealed very little temperature compensation in the maximal rate
of contraction. That is, the maximal rates
observed in polar fish are well below those
observed in warmer water fish (Johnston
and Harrison, 1985). This inherent temperature limitation will limit maximal
swimming speeds and many polar fish do
show fairly inactive patterns of behaviour.
However, this may have as much to do with
the lack of swimbladder and a largely benthic or demersal way of life as it does with
temperature. Some polar fish are active
swimmers (for example, Champsocephalus
gunnari and the pelagic phase of Notothenia
rossii, both of which feed on krill swarms),
and all must be able to capture prey and
avoid predators (many of which are endothermic).
In contrast to maximal contraction rate,
the maximal Ca2*-activated force in the
polar fish so far examined shows a high
85
degree of compensation, being in the upper
range reported for teleost fish (when all
measurements are made at the temperature to which each species is adapted).
These differences exemplify the difficulties involved in interpreting the picture
seen at higher levels of integration even
when aspects of adaptation at the molecular level are understood. Overall it appears
we must conclude that although a substantial degree of compensation for temperature has been achieved in fish white muscle
function, this is not perfect; an element of
unavoidable temperature limitation
remains. A similar conclusion was reached
by Montgomery and Macdonald (1984,
1985) in relation to eye movement in fish.
Nervous conduction
The speed of nervous conduction in fish
peripheral nerves is a linear function of
temperature. The relationship for temperate water fish suggests that conduction
should fail at around zero, and indeed temperate fish do become comatose at these
temperatures. Polar fish, however, show
nervous conduction down to — 5°C, a clear
case of adaptation. The compensation,
however, is not complete for the conduction velocity of Dissostichus mawsoni at
-1.9°C (10-12 m/sec) is only about half
that of a temperate fish at 15°C (Macdonald, 1981). The molecular basis for these
differences is unclear.
Enzymes
Early studies of cold adaptation included
numerous investigations of the activity of
enzymes isolated from polar fish. These
suffered from the difficulties of interpretation surrounding in vitro assay conditions
discussed earlier. Nevertheless some generalizations are possible. In several cases
activation energies (a measure of enthalpy
of activation) are lower than in comparable
enzymes isolated from temperate fish,
although in other cases they are not (see
summary in Macdonald^al., 1987). These
data suggested the possibility that the
changes in activation parameters associated with changes in primary structure in
muscle ATPases may be a general phenomenon. But until further studies are under-
86
ANDREW CLARKE
taken with enzymes assayed under realistic
conditions of ionic strength, substrate concentration, and particularly pH and temperature, we cannot yet say how general.
One possible way to offset the effect of
a decreased temperature on enzyme reaction rate is to increase the amount of
enzyme present. The solvent capacity of
the cell will clearly set an upper limit to
this, and there are technical difficulties in
demonstrating unequivocally an increase
in enzyme quantity (as against activity).
However, the increase in mitochondrial
density in the white muscle offish living at
lower temperatures will help both to offset
the effects of low temperature on diffusion
phenomena (Sidell, 1990) and to increase
the amount of associated enzymes.
Conclusions
The studies outlined above indicate that
compensation for temperature at the
molecular level has been achieved in several cellular and physiological processes.
The degree of compensation, however,
varies between processes. Changes in protein tertiary structure as a consequence of
alterations in primary structure are implicated in several of these examples. These
are generally accompanied by changes in
thermodynamic activation parameters
(specifically the enthalpy of activation) and
a reduction in thermal stability. This
emphasizes the complex nature of the
interaction between enzyme activity,
enzyme stability and temperature. It is particularly important to emphasize that
although the enzymes isolated from cold
water organisms are less thermally stable
than those isolated from warmer water species, this is not necessarily the case at the temperatures to which each species is adapted to
live.
We clearly need further careful studies
of the activity of enzymes isolated from
polar fish, particularly in relation to primary structure and thermal stability. We
also need studies of the turnover of specific
proteins in vivo in fish living at different
temperatures.
It is important also to consider the immediate environment of the enzyme. In particular it would be useful to know whether
the composition and fluidity of cellular
membranes in organisms that have adapted
over evolutionary time to live at low temperatures have altered in the way predicted
from experimental acclimation studies of
eurythermal organisms. Preliminary data
suggest that they have (see Macdonald et
al., 1987 for a recent summary) but more
studies are needed.
One area of temperature compensation
that has received very little attention but
which may be important is the role of intracellular pH. It is possible that modifications
of intracellular pH may offset a substantial
degree of the fall in activity of some
enzymes as temperature decreases (particularly those whose active site is rich in histidine residues: see Clarke, 1987 for a
recent discussion).
HIGHER LEVEL PROCESSES
Growth
The extent to which growth shows compensation for temperature is a question
which highlights the peculiar difficulties
inherent in trying to unravel the particular
factors that regulate higher level processes.
The growth rate of many commercially
farmed fish slows down when the water
cools, even when excess food is present.
This slowing of growth appears to be an
unavoidable consequence of the effect of
a lowered temperature on the physiology
of these fish, although the precise mechanisms involved appear to be complex. Since
natural selection acts generally to maximize the growth rate of an organism (except
to the extent that this will prejudice future
reproductive potential) it would be
expected that if fast growth rates could have
evolved at low temperatures, they would
have done so.
In those (few) cases where the annual
growth rates of polar organisms appear to
be fast (for example, some mytophid fish,
and the sponge Mycale acerata: Dayton et
al., 1974) we can clearly conclude that
growth has evolved compensation for temperature. However, where growth rates are
slow (as they are for almost every polar
marine invertebrate studied so far: Clarke,
87
COLD ADAPTATION IN MARINE ORGANISMS
1983) we cannot conclude simply that compensation has not occurred. In the first
instance this conclusion leads us into a series
of difficulties:
1. Why should temperature compensation
for growth have evolved in some species
and not others, and
2. Why should compensation for temperature be possible in some processes (for
example, microtubule activity) but not
others (for example, growth)?
More importantly, where growth rates
in polar organisms are slow we cannot conclude that this is due necessarily to direct
limitation by temperature, for we cannot
dismiss the possibility that growth rates are
slow for some other reason.
Purely for the purposes of illustration let
us equate growth with protein synthesis. In
Figure 2 the upper trace represents the
activity of a ribosome in a temperate species at 10°C; during a set period of time
the ribosomes make 16 units of protein.
The height of the bar corresponds to the
rate at which the ribosomes make polypeptide; the gaps indicate periods when
there is no synthetic activity. Let us assume
that a similar polar species living at 0°C
produces only 8 units of protein in the same
period. This corresponds to a not unreasonable Q l o of 2. There are two possible
explanations for this slower rate. The first
is that there is something in the process of
protein synthesis which is unavoidably
linked to temperature, and even when
working flat out the polar species can produce only 8 units of protein. This is illustrated in the middle trace where the height
of the bar is reduced and there are no gaps.
An alternative explanation, however, is that
the ribosomal machinery can compensate
fully for temperature and that when it is
producing protein it operates just as efficiently as in the temperate species (and
hence the height of the bar is the same),
but that other factors dictate that only 8
units of protein are produced. These other
factors might include resource limitation,
or a reduced requirement for protein at
low temperature. It is perfectly possible, of
course, that the real explanation lies somewhere between these two extremes, in a
[Lnnnnnnn
I Q. n n n
Time
•
FIG. 2. Diagrammatic representation of polypeptide
elongation at 10°C (upper trace), at 0°C with no temperature compensation (middle trace), and at 0"C with
complete temperature compensation but with resource
limitation (lower trace). The boxes show the relative
amounts of protein synthesized per unit time.
situation somewhat analogous to that with
muscle contraction. The important point
is that it is impossible to separate these two
explanations merely by a measurement of
growth rate. One cannot look at a polar fish,
see that it grows slowly and simply conclude that
the growth rate is slowed by the low temperature.
What evidence is there that factors other
than temperature might in reality be limiting the growth rate of polar organisms?
The most telling evidence is that in many
species growth is severely seasonal; growth
is slow or nonexistent in winter and limited
essentially to the summer months. As a
result when growth is averaged over the
year, mean annual growth rates are slow.
However, when growth is actually occurring, it may proceed at a pace fully comparable with that of warmer water organisms. This evidence (which is reviewed
extensively in Clarke, 1988) suggests
strongly that the slow growth rates of many
polar marine organisms are due to a seasonal supply of food. Growth is limited by
resource availability, not low temperature.
If resource limitation is indeed the
explanation, then the obvious experiment
is to provide excess food in winter and to
see whether growth rate responds. Interestingly this rarely happens, usually because
the food is ignored rather than being
ingested but unutilized. This experiment
highlights the difficulties of much experimental work on marine organisms.
Although obvious physical factors such as
temperature, oxygen, photoperiod and
88
ANDREW CLARKE
food can be controlled, it is very difficult
to influence predictably (or control for)
factors such as hormonal state. It is quite
possible that the lack of response to excess
food in polar organisms held in simulated
winter conditions may be related to higher
level controls having switched off" appetite
and the synthetic machinery. Until we have
a more extensive understanding of these
factors we cannot interpret even such simple experiments. It is possible that similar
factors may explain why some polar fish
cease feeding in winter even when food
would appear to be available (for example,
demersal fish which feed on benthos). This
would still say nothing about the wider
adaptive value of such behaviour, however.
Respiration
Oxygen consumption has been used for
over 70 years to assess the metabolic rate
of organisms and how this varies in response
to a change in temperature. Unfortunately, although a measurement of respiration rate is technically fairly simple, the
resultant plethora of measurements have
helped very little in unravelling the mechanism of temperature adaptation. To
explain why will require a small historical
diversion.
Metabolic cold adaptation
In the second decade of this century the
great respiratory physiologist Krogh measured the resting oxygen consumption of
a goldfish as it was cooled. As the temperature fell respiration decreased. The same
pattern was shown by locomotor ability and
was essentially that illustrated diagrammatically in Figure la. Polar fish have
achieved compensation in locomotor ability by a homeostatic "elevation" of locomotor ability compared with the sluggish
rates exhibited by temperate fish cooled to
low temperature (as illustrated diagrammatically by the arrow in Fig. la). Krogh
(1916) therefore suggested, not unreasonably, that a similar "elevation" was to be
expected in the respiration of fish living
normally at polar temperatures compared
with that shown by temperate fish cooled
at low temperatures. This expected elevation has been enshrined in the concept
of metabolic cold adaptation. Early measure-
ments of the respiration rate of polar fish
did indeed suggest such an elevation, and
metabolic cold adaptation is discussed in
papers and textbooks to this day.
The concept of metabolic cold adaptation has been criticized increasingly in
recent years. These criticisms fall essentially in three main areas:
1. The early measures of respiration rate
were made with insufficient regard for
experimental protocol, and especially
the effects of stress (Holeton, 1973,
1974).
2. There can be no objective "expected"
rate at polar temperatures with which
to compare the rates actually observed
in polar fish; the "expected" rate calculated depends on the comparative
organism used and the value of Q, o used
for extrapolation (Holeton, 1974;
Clarke, 1980).
3. That the concept of compensation in
respiration is itself meaningless, and
hence any attempts to demonstrate or
disprove metabolic cold adaptation are
doomed to failure from the start
(Clarke, 1980, 1987).
I have discussed the first two criticisms
extensively (Clarke, 1980, 1983, 1987; but
for contrary views see Wells, 1978; Cossins
and Bowler, 1987) and will therefore concentrate on the third. This is a far more
fundamental objection, and one that is as
relevant to short-term acclimation experiments as it is to questions of evolutionary
adaptation.
Compensation in respiration
Most of the difficulties encountered in
attempting to interpret the relationship
between respiration and temperature can
be traced to an inappropriate view of what
respiration actually represents. Stemming
from Krogh's original suggestion, respiration has traditionally been viewed by
temperature physiologists as a discrete process and moreover one that should show a
simple pattern of compensation for temperature. A very large number of measurements of respiration rate in relation to temperature have now been made and an
COLD ADAPTATION IN MARINE ORGANISMS
enormous diversity of patterns revealed,
ranging from "over compensation"
through "perfect compensation" to "reverse" (or "paradoxical") compensation.
(For a review of these terms see Clarke,
1987 or Cossins and Bowler, 1987.)
Respiration, however, is not a discrete
process which must respond in some set
fashion to a change in circumstances. Respiration is simply a measure of instantaneous demand for ATP; it therefore repre-
89
lization of reserves and an increase in oxygen consumption. The important point is
that the system is driven by the demand
for work. There is no selective advantage
in increasing ATP production simply for
the sake of it, or in order to achieve "compensation" for temperature. Generation of
ATP utilizes resources which must be met
by an increased foot intake. Selection will
therefore act to keep this resource utilization (and hence oxygen consumption) as
sents a cost to the organism, not a benefit. Unlike low as is feasible allowing for the need to
locomotor ability, gene expression or pro- be able to respond to environmental chaltein synthesis there is therefore no selec- lenges.
tive benefit in maintaining or elevating resThere will clearly be some constraints
piration in itself. Clearly the organism will limiting the level to which basal oxygen
gain a benefit from the ATP produced (for consumption can be decreased. For examexample, in fuelling escape behavior to ple, maintenance metabolism and lifestyle
evade a predator, or in fuelling protein appear to be linked (at least in fish). Thus
synthesis for growth or gamete produc- active species tend to have higher resting
tion); however, this benefit is at the cost of oxygen consumptions and basal metaboreserves which must be utilized to fuel ATP lism cannot be lowered so far that it comproduction. In physiological terms this promises the scope for activity (that is, the
explanation is extremely simplistic for it ability to capture food or escape predaignores questions such as demand for tors). However, this is not the only considreduced pyridine nucleotide, or the main- eration affecting basal metabolism and
tenance of redox balance. Nevertheless it considerations of scope for activity do not
is broadly accurate and serves to emphasize mean that we should expect classical perthe distinction between the views of res- fect compensation in the sense of observpiration as a process which must compen- ing similar resting metabolic rates in polar,
sate for temperature, and the view of res- temperate and tropical fish (or other marine
piration as a measure of costs.
organisms).
Clearly the cellular machinery that proThis is because the ATP synthesized in
duces the ATP must itself show compen- metabolism may be used in any one of a
sation for temperature. That is, when ATP variety of physiological processes, includis required in a polar organism it must be ing ion pump activity, muscular activity,
capable of being produced at a rate com- neural activity, growth, gametogenesis,
parable with warmer water relatives. The excretion of waste products or locomotor
point is that there is no logical (or biolog- activity involved in predator escape or
ical) reason why the rate at which this ATP attracting a mate. Any one (or all) of these
is required should be the same in a warm processes may change as a response to a
and cold water organism.
change in temperature and the result will
A further point is that ATP generation be a change in oxygen consumption. It is
(and hence oxygen consumption further impossible, however, to predict in advance
down the line) is under feedback control. what this change might be, either as the
This control ensures that generally the ratio result of a short-term temperature change,
of ATP to less phosphorylated forms (the or evolutionary temperature adaptation.
cellular energy charge) is maintained at a
Particular difficulties have surrounded
high level. When the energy charge is dis- attempts to interpret the diverse seasonal
turbed by an increased demand for ATP, patterns seen in oxygen consumption. In
then glycolysis, the Krebs cycle and oxi- recent years far more coherent explanadative phosphorylation are stimulated to tions have been achieved by explaining searedress the balance. This results in a uti- sonal changes in oxygen demand as a
ANDREW CLARKE
Seasonal changes in metabolic rate
- Metabolic costs
of activity,
feeding and
growth
-0,0 = 2 -
a
-Basal
metabolism
Winter
-1.5°C
§
Summer
+O.5'C
FIG. 3. Seasonal changes in metabolic rate. The
height of the bars represents the demand for oxygen,
with basal metabolism indicated by stippling. Basal
metabolism is assumed to increase slightly in summer,
equivalent to a Q l o of 2. The increased oxygen consumption in summer required to fuel the extra
demands for feeding, growth and activity result in an
overall rise in oxygen consumption equivalent to a
Q,o of 8.
reflection of changes in synthetic activity
rather than a passive (or active) response
to temperature (Parry, 1978, 1983). This
is illustrated diagrammatically in Figure 3,
where the two bars represent the summer
and winter oxygen consumptions of a polar
organism snowing seasonal growth.
Because the respiratory costs of growth,
reproduction and other activity all increase
in summer, interpretation of the summer
to winter difference purely in terms of temperature leads to a high apparent Q10 of 8.
In this example a small increase in basal
metabolism during summer has been
assumed, corresponding to a biologically
reasonable Q 10 of 2. However, it can be
seen clearly that the differences in observed
oxygen consumption between summer and
winter are not due to an unusually high
temperature sensitivity, nor to any winter
dormancy or suppression of metabolism in
winter (all of which explanations have been
proposed for polar organisms). Rather the
difference is due to the seasonality of biology and the heterogeneous nature of respiratory demand.
Respiration and temperature
Given the severe difficulties of prediction and interpretation, the first step must
be to examine a range of organisms and
look for any large-scale patterns in the relationship between respiration and temperature. The results from four such studies
are shown in Figure 4. Although these
studies differ in the sensitivity of respira-
</}
C
o
o
g
Temperature (°C)
FIG. 4. Relationship between respiration rate and
temperature derived for different groups of marine
organisms. 1: Zooplankton (Ikeda, 1974), 2: zooplankton (Ikeda, 1985), 3: crustaceans (Ivleva, 1980), 4:
non-idotheid isopods (Luxmoore, 1984).
tion to temperature they all show a broadly
similar shape, with respiratory costs lowest
in polar waters and highest in tropical
waters.
These studies all combine data from a
large number of separate studies, pulling
together information from organisms of
different taxonomic affinity, size, habitat
and ecology. Nevertheless the pattern
remains the same whether the study
encompasses all marine invertebrates, all
crustaceans or is limited to non-idotheid
isopods. It is tempting to conclude that
these relationships are expressing something fundamental about physiology and
temperature. Whatever is dictating the size
of respiratory demand at different temperatures, living at low temperatures is
clearly less costly in terms of respiration
than living in the tropics. This difference
varies from x 4 to x 26 depending on which
relationship is considered. In energetic
terms it is more of a problem for marine
ectotherms to live in warm water than in
cold. This counter-intuitive result emphasizes the difficulties inherent in making
judgments from an anthropocentric, warmblooded point of view.
It is not clear what energetic factors are
producing the patterns shown in Figure 4.
Although low rates of metabolism are associated with low temperatures, extension of
the arguments above suggests that the low
rates are not directly caused by low temperature in the sense of direct thermodynamic rate-limitation. It is likely that the
91
COLD ADAPTATION IN MARINE ORGANISMS
TABLE 2. Temperature and respiration in marine invertebrates.
Group
Q,.
Ranee over which Q!0
calculated (deg C)
Zooplankton
Zooplankton
Crustaceans
Isopods
Gastropods
1.76
1.63
2.27
2.98
2.13
4.5 to 28.5
-1.4 to 30
0 to 30
-1.5 to 25
-1.7 to 30
low rates of oxygen demand at low temperature are related to reduced ATP
demand for protein turnover, ion pump
activity and other aspects of basal metabolism (Clarke, 1987).
The data in Figure 4 indicate values of
Q10 of between 1.63 and 2.98 (Table 2).
Contrary to previous speculation there are
no indications of particularly high temperature sensitivity (that is, a high Q10) at low
temperatures. Recent studies of individual
species acclimated carefully to a range of
temperatures within the usual range of
temperatures usually experienced in life
also indicate low Q10 values (see, for example, Davenport, 1988), although some
studies of polar zooplankton subjected to
acute temperature changes have given
higher values (Hirche, 1984). This emphasizes the care that is needed to distinguish
acute from acclimated responses to temperature.
CONCLUDING REMARKS
Simple measures of the rate of complex
physiological processes such as growth or
respiration do not necessarily give any firm
indication of whether the underlying process shows compensation for temperature.
The same difficulties extend to currently
available measures of protein synthesis.
Future studies of cold adaptation should
avoid using respiration as a tool. Oxygen
consumption is a complex summation of
many processes, each of which may react
differently to a change in temperature.
Furthermore, respiration per se is a cost to
the organism; there is thus no logical or
energetic reason for respiration to exhibit
the classical "compensatory" response to
temperature, either over evolutionary time
or during an acclimation experiment.
This is not to say that measurements of
respiration have no place in temperature
Author
Ikeda, 1974
Ikeda, 1985
Ivleva, 1980
Luxmoore, 1984
Houlihan and Allan, 1982
physiology. Far from it; studies of the latitudinal variation in respiration rate have
provided a most valuable mirror to what
may be involved in temperature compensation at the molecular level. It is the crude
use of respiration to measure temperature
compensation (both in evolutionary terms
and in acclimation experiments) that needs
abandoning. Now.
ACKNOWLEDGMENTS
My research is supported by the British
Antarctic Survey (Natural Environment
Research Council), of which I am a member. I am most grateful to NSF for funding
my attendance at this Symposium. I also
thank Dr. Bruce Sidell for valuable discussion of the physiological consequences of
warming versus cooling; any errors are
mine.
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