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Over time, a biphasic response was found at the
level of the transcriptome of common eelpout. An
initial down-regulation in the message of gradient-
dependent transporters at stable capacities of Na + /
K + -ATPase was followed by an up-regulation of
Na + /K + -ATPase message and activity in the gills
during long-term acclimation. During long-term
CO 2 exposure (weeks) the gills may respond to
enhanced rates of ion regulation by increasing tran-
scription and translation levels. In line with these
observations, hypercapnia caused an increase in the
energy demand of ion regulation in isolated per-
fused gills from two species of Antarctic notothen-
ioid i shes (Deigweiher et al. 2010 ).
metabolic rates and aerobic scope remained
unchanged in North Sea cod ( Gadus morhua ; NSC)
incubated for 4 months at a p CO 2 of 3000 μatm. Gill
Na + /K + -ATPase activity, as a general indicator of
ion regulatory capacity, did not differ from control
values, suggesting that existing ion regulatory
capacities were sufi cient to maintain acid-base
homeostasis, and also that the cost of pH compen-
sation is low enough not to signii cantly affect max-
imum swimming performance. However, cod from
the Barents Sea (north-eastern Arctic Cod; NEAC)
responded by enhancing the gene expression of
Na + /K + -ATPase after 12 months of exposure to a
p CO 2 of 6000 μatm while standard and active meta-
bolic rates, critical swimming speeds, and aerobic
scope remained at control levels. However, effects
on other components of the energy budget, like
growth, cannot be excluded. A reduction in growth
due to elevated energy demand for ion and acid-
base regulation would be consistent with earlier
observations in cold-exposed NSC and NEAC. In
the cold, the expression of genes coding for aerobic
enzymes was stronger in NEAC (Lucassen et al.
2006 ), rel ecting enhanced mitochondrial ATP syn-
thesis capacity and proton leakage in this popula-
tion. The high cost of proton leakage might
contribute to lower growth of NEAC (see above).
Long-term elevated Na + /K + -ATPase capacities in
cod and eelpout under elevated CO 2 levels
( Deigweiher et al. 2008 ; Melzner et al. 2009b ) are
thus indicative of a persistent shift in the energy
budget, possibly at the expense of other factors,
such as growth.
The increased gene expression of Na + /K + -ATPase
does not necessarily indicate a response of acid-
base regulation. A high capacity of Na + /K + -ATPase
represents an enhanced driving force for those
mechanisms of acid-base regulation which exploit
the ion gradient set up by Na + /K + -ATPase. Con-
sidering their high capacity for pH homeostasis, it
is unlikely that adult teleosts will experience an aci-
dosis in their body l uids in response to increasing
seawater p CO 2 . However, they may still experience
potentially elevated costs of acid-base regulation
and an associated shift in their energy budget
( Deigweiher et al. 2008 , 2010 ). Direct evidence for
a signii cant energetic cost of acid-base regulation,
as part of the energy budget allocated to ion
8.2.2 Functional capacities, sensitivities,
and energy budget in various climates
Enhanced rates of ion regulation elicited by long-
term CO 2 exposure resemble observations during
cold adaptation. Eurythermal marine and freshwa-
ter teleost species including common eelpout dis-
play cold-compensated Na + /K + -ATPase capacity
(reviewed by Pörtner et al. 1998 , 2005b ). Conversely,
reduced capacity of this carrier appears to be an
important element of acclimatization to elevated
temperatures depending on the season or climate
zone. The cost of ion exchange contributes signii -
cantly to the standard metabolic rate, such that
thermal acclimatization probably contributes to
sustained energy efi ciency. In fact, the high costs
of eurythermal cold adaptation are likely to reduce
the energy available for growth and thus cause
lower temperature-specii c growth rates in sub-
Arctic and Arctic populations of Atlantic cod
(Pörtner et al. 2008). Under future ocean scenarios,
an elevated energy demand for ion regulation in
response to elevated p CO 2 levels may reduce the
rates and capacities of other functions, as a trade-
off in the animal's energy budget. Such constraints
may affect i tness even in high-performance spe-
cies which display a higher capacity for ion and
acid-base regulation than the more sluggish spe-
cies (Melzner et al. 2009a ).
A recent study examined the relationships
between ion regulation capacity, metabolic rate, and
swimming performance in Atlantic cod under long-
term hypercapnia (Melzner et al. 2009b ). Routine
 
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