Geoscience Reference
In-Depth Information
acidii cation, as illustrated by the contrasting results
for tropical coral reef i shes and those from temper-
ate regions.
In this context, the intuitively appealing concept
of a 'respiration index' (RI) = log
10
(
p
O
2
/
p
CO
2
) was
recently proposed to identify the limits to meta-
zoan life based upon perceived chemical con-
straints in aerobic metabolism (Brewer and Peltzer
2009). The authors suggested that with falling
ambient oxygen and increasing CO
2
levels there is
a limiting threshold RI value below which insufi -
cient free energy change is produced from the oxi-
dative metabolism of organic matter for the
organism to be viable. Such limiting or lethal con-
ditions are those where reactions would be forced
into equilibrium by falling
p
O
2
and rising
p
CO
2
.
However, such limits are extreme and found far
beyond the actual conditions limiting animal life,
as mitochondria, the sites of oxidative metabolism,
see lower than ambient oxygen and higher than
ambient CO
2
concentrations. As a corollary, even if
such limiting thermodynamic conditions can be
dei ned for aerobic metabolism in a test tube or in
a hypothetical unicellular organism with diffusion
distances set to zero, the complexity of the meta-
zoan body plan requires consideration. In animals,
convective mechanisms (ventilation and circula-
tion) are used to bridge the distance between ambi-
ent media and the mitochondria. Ventilation and
circulation minimize diffusion gradients and com-
pensate, within limits, for a decrease in ambient
oxygen and an increase in CO
2
levels. The capacity
of convective mechanisms is specii c for a species
and inl uenced by its adaptation to lifestyles and
environmental parameters including those set by
climate conditions. Consequently, the capacities to
defend the intracellular concentrations of reactants
(e.g. oxygen or CO
2
) vary greatly between species.
These concentrations are very different from those
in the surrounding seawater. As a result of species-
specii c gas exchange capacities in large complex
bodies, limiting RI values are species-specii c val-
ues, which must be determined experimentally
and are much higher than with diffusion distances
set to zero as in the hypothetical organism above.
In general, organisms are open systems that
acquire free energy from their surroundings
and operate approximately in a steady state. The
8.4
Conclusions and perspectives
Surface seawater will continue to warm and its pH
will continue to decline during the remainder of
this century. Higher temperatures will stimulate
metabolism and require enhanced performance
from thermally constrained oxygen transport sys-
tems in cephalopods and i shes. In cephalopods in
particular, oxygen supply capacity will be impaired
by ocean acidii cation. Thus warming and acidii ca-
tion may cause ventilatory and circulatory stresses
that restrict aerobic scope and impair swimming
activity. Together these variables may reduce an
animal's ability to respond to external stimuli, leav-
ing it more vulnerable to its main predators. Ocean
warming and acidii cation will increasingly be
accompanied by expanding hypoxic zones ( Stramma
et al
. 2008 ; Seibel 2011 ). If the OMZs continue to
expand vertically, the Humboldt squid,
D. gigas
, for
example, will have to retreat to even shallower
waters at night in order to reduce any accumulated
oxygen debt and to hunt actively. Thus, acidii ca-
tion and warming of the surface ocean may create a
ceiling that will preclude these squid from entering
shallow waters, while the expanding hypoxic zone
will increase the depth below which they cannot
penetrate during their night-time recovery from
hypoxia. The synergistic effect of these three cli-
mate-related variables may be to vertically com-
press the habitable depth range of the species.
The i rst level of sensitivity of animals to extreme
temperatures, hypoxia, or ocean acidii cation
involves a limitation in the functional capacity of
tissues, including those responsible for oxygen sup-
ply to cells and their mitochondria. The sustainable
performance or i tness of an animal thus becomes
constrained once its functional capacity is limited
and/or it cannot exploit its metabolic capacity for
aerobic energy production. Therefore, understand-
ing the functional specialization of an animal to its
environment, including the environmental variabil-
ity of these factors (Pörtner 2006), involves an in-
depth understanding of the mechanisms controlling
oxygen supply and respiratory activity. Likewise,
evolutionary adaptation to various climate zones,
lifestyles, and associated exercise capacities as well
as to variable natural CO
2
levels are likely to shape
the sensitivity of marine fauna to ongoing ocean
