Geoscience Reference
In-Depth Information
operative on evolutionary timescales, including
mass extinction events (Pörtner
et al
. 2004 , 2005a ;
Knoll
et al
. 2007). Many nektonic organisms are val-
ued by society for their beauty and impressive per-
formances as well as for being a relevant food source
to humans. The well-being and conservation of
these charismatic species under scenarios of global
climate change are thus of concern to society.
ously overlooked component of the global inorganic
carbon cycle (Wilson
et al
. 2009 ).
The aerobic scope of i shes can be exploited by
increasing oxygen uptake via the gills, oxygen sup-
ply to tissues through the circulatory system, or
oxygen delivery across tissue capillary beds, or a
combination of these three processes. Farrell
et al.
(2009) pointed out that increased cardiac output
and enhanced arterio-venous difference in oxygen
content both play a crucial role in meeting increased
tissue oxygen demand. The high efi ciency of acid-
base regulation in i shes supports the maintenance
of metabolic scope under elevated CO
2
tensions.
Even when the i sh is exposed to 10 000 μatm or
higher, extracellular pH will be nearly restored to
pre-hypercapnic levels within a few days (Heisler
1986), thereby supporting blood oxygen saturation
under hypercapnia. In addition, i sh have the abil-
ity to i nely regulate the intracellular pH of red
blood cells through the release of catecholamines,
triggered by decreasing blood oxygen levels in the
early phase of hypercapnia (Perry
et al
. 1989 ).
Increasing oxygen transport capacity through the
release of red blood cells stored in the spleen does
occur during exercise (Nikinmaa 2006), but not usu-
ally during hypercapnic exposure at rest (Ishimatsu
et al.
1992 ; Gallaugher and Farrell 1998 ).
Efi cient acid-base regulation under hypercapnia
causes an accumulation of bicarbonate in body l u-
ids, to higher levels in the plasma than in the intra-
cellular space. In teleosts, this is clearly paralleled
by an equimolar decrease in plasma Cl
-
. Branchial
cells that are active in acid secretion contain elec-
troneutral Na
+
/H
+
exchangers (NHE) or V-type
H
+
-ATPases, coupled energetically to apical Na
+
-
channels (EnaC). The ion gradients driving the stoi-
chiometric exchange of Na
+
(uptake) for H
+
as well
as of Cl
-
(uptake) for HCO
3
-
( Wood 1991 ; Perry
et al.
2003) are established, directly or indirectly, by the
Na
+
/K
+
-ATPase. Na
+
/K
+
-ATPase activity uses a
large fraction of the cellular and epithelial energy
budget and is tightly regulated. It has therefore
been used as a marker of the overall capacity for ion
and acid-base regulation.
Regulation of Na
+
/K
+
-ATPase activity by gene
expression under hypercapnia has been poorly
explored to date and with variable results. In devel-
oping Atlantic salmon, mRNA levels of branchial
8.2 Effects of ocean acidii cation
on i shes
8.2.1 Acid-base regulation
Among i shes, freshwater teleosts have been more
intensely studied than marine i shes with respect to
effects of hypercapnia and the mechanisms and
scope of acid-base regulation (Heisler 1986;
Claiborne
et al.
2002 ; Evans
et al.
2005 ; Marshall and
Grosell 2006). The information available for marine
i shes suggests that they are capable of maintaining
blood pH at control levels even when seawater
p
CO
2
rises above 5000 μatm (Toews
et al.
1983 ;
Larsen
et al.
1997 ; Hayashi
et al.
2004 ; Michaelidis
et al.
2007). Exposure to elevated seawater
p
CO
2
results in elevated internal
P
CO
2
and requires the
net excretion of acid to compensate for the respira-
tory acidosis in body l uids. Similar to cephalopods
(see below) or crustaceans (Wheatly and Henry
1992), the gills are the primary sites of acid-base
regulatory processes in i shes (Perry and Gilmour
2006). The capacity of acid-base regulation depends
on the organism's lifestyle and is linked to meta-
bolic and thus exercise capacity (Melzner
et al.
2009a). Branchial acid-base regulation is comple-
mented by ion exchange through the kidney and
gut. Marine teleosts drink ambient seawater to
avoid dehydration. They absorb water from the gut
to replace the water lost to the hyperosmotic envi-
ronment. The resulting enrichment of Ca
2+
in the
gut l uid leads to the precipitation of calcium car-
bonates. The bicarbonate required for this process is
secreted by the intestine, thereby leading to an acid-
ii cation of the blood plasma (Cooper
et al
. 2010 ).
The excretion of calcium carbonate (CaCO
3
) via the
gut of marine teleosts is substantial and accounts
for between 3 and 15% of the CaCO
3
produced in
the ocean. It thus represents a signii cant, previ-
