Geoscience Reference
In-Depth Information
decreasing oxygen binding to transport proteins
(e.g. haemoglobin or haemocyanin).
These synergies have been poorly explored to
date. The concept of oxygen- and capacity-limited
thermal tolerance (OCLTT) may provide a suitable
matrix for integrating thermal effects with those of
other climate-related factors such as CO
2
or
hypoxia (Pörtner 2010). The concept explains why,
where, and how the thermal window of perform-
ance in aquatic organisms is set and limited by the
functional capacity of those tissues involved in
oxygen uptake and distribution, i.e. mainly the
cardiocirculatory system (cf. Pörtner 2006). As a
result of capacity adjustments, the window of per-
formance equals the window of aerobic metabolic
scope (see Box 8.1). The term 'metabolic scope'
refers to the amount of energy that can be allocated
to activities beyond those required for basic
Box 8.1 Thermal windows shape and restrain energy availability
Schematic of the thermal window of a species or one of its
life stages (after Pörtner and Farrell 2008), following the
concept of oxygen- and capacity-limited thermal tolerance
(OCLTT). The graph shows how the limits (vertical lines) of
thermal tolerance shape the temperature-dependent
performance curve and its optimum (T
opt
) as rel ected in the
thermal window of aerobic scope determined in swimming
trials of nektonic animals (Fry 1971). Aerobic scope is the
difference between maximum and resting aerobic
metabolic rates. In the warmth and cold, the i rst thermal
limits experienced by the organism at pejus temperatures
(T
pej
, pejus means 'getting worse') result from the limited
capacity of tissues overall and especially of
cardiocirculation, and in addition, in invertebrates,
ventilatory organs. Finally, the limitation in oxygen supply
leads to the transition to anaerobic metabolism beyond
critical temperatures (T
crit
) and before molecular
denaturation sets in (at T
den
). These limits can be shifted by
the acclimation of functional capacity. Beyond pejus limits,
passive tolerance to temperature extremes is supported by
protective mechanisms including anaerobic metabolism,
heat shock proteins and antioxidative defence. Hypoxia is
expected to narrow the thermal window and lower
performance optima (dashed lines, green arrows). CO
2
also
causes such narrowing, but in the thermal optimum
performance decrements may occur in sensitive species or
life stages only. Energy allocation to life-sustaining
functions will become constrained once aerobic scope is
limited by temperature and its synergistic interaction with
other environmental factors.
loss of performance, abundance
capacity
T
opt
T
pej
onset of anaerobiosis
endurance
T
pej
CO
2
onset of denaturation
hypoxia
protection
T
crit
T
crit
T
den
Temperature (
T
, ºC)
passive
short
-
-
active
long
-
-
passive tolerance range
short-term tolerance
