Geoscience Reference
In-Depth Information
regulation, has only recently been provided for
marine organisms. Specii cally, inhibition of trans-
membrane proton equivalent ion transport and a
reduction in oxygen demand was caused by a red-
uction in extracellular pH in a sipunculid (
Sipunculus
nudus
), in Mediterranean mussels (
Mytilus edulis
)
( Pörtner
et al.
1998 , 2000 ; Michaelidis
et al.
2005 ),
and in isolated cell preparations of two Antarctic
i shes,
Pachycara brachycephalum
and
Lepidonotothen
kempi
, under a
p
CO
2
of 10 000 μatm (Langenbuch
and Pörtner 2003). These i ndings demonstrate that
the mechanism of cellular metabolic inhibition
through extracellular acidosis also exists in marine
i shes. However, marine teleosts do not normally
allow extracellular pH to fall permanently during
long-term hypercapnia, such that oxygen uptake
remains unchanged at the whole-organism level.
The stimulation of acid-base regulation may even
cause a stimulation of whole-organism energy
turnover. Fishes indeed maintain, or transiently
increase, oxygen uptake when exposed to elevated
p
CO
2
at rest (Ishimatsu
et al.
2008 ). However, CO
2
may also affect the mode of energy metabolism.
From changes in metabolic enzyme activities in the
skeletal and heart muscles, Michaelidis
et al.
( 2007 )
concluded that a shift from aerobic to anaerobic
metabolism occurred in the marine i sh
Sparus aura-
tus
exposed to a
p
CO
2
of 5000 μatm for 10 days. This
may indicate a loss in aerobic scope, which may
compromise processes that depend on aerobic
scope, such as growth. As a corollary, it must be
emphasized that a clear comprehensive picture
does not yet exist. At the CO
2
levels expected dur-
ing ocean acidii cation scenarios and lower than 10
000 ppmv, metabolic depression may not occur even
in the invertebrates, but metabolic stimulation may
occur instead due to the stimulation of ion and
acid-base regulation.
While the stoichiometric energy cost of some
individual carriers has been quantii ed, the total
cost of acid-base regulation in a cell or whole organ-
ism has not yet been estimated. It depends upon the
rates of proton-equivalent ion exchange at cellular
and whole-organism levels and on the fractional
contribution of various carriers to net ion l ux. A
high cost of acid-base regulation may inl uence the
energy budget and reduce the aerobic scope availa-
ble for other functions. However, available studies
have not found a signii cant inl uence of
p
CO
2
below 10 000 μatm on somatic growth rates in
marine teleosts, as shown by Fivelstad
et al.
( 1998 )
in post smolts of
Salmo salar
studied for 6 weeks, by
Foss
et al.
(2003) in juveniles of
Anarhichas minor
studied for 10 weeks, or by Foss
et al.
( 2006 ) in juve-
niles of
Gadus morhua
studied for 9 weeks as well as
by Munday
et al
. (2009c) in juveniles of tropical
marine i shes. In all of these studies seawater pH
remained unbuffered, no sodium bicarbonate salt
was added, and all were carried out in full-strength
seawater. Thus seawater carbonate chemistry
resembled extreme ocean acidii cation. The afore-
mentioned studies were, however, performed in
dense monoculture conditions with
ad libitum
feed-
ing. The effects of long-term exposure to elevated
CO
2
on ecologically relevant parameters, such as
individual i tness, competition, and predation inl u-
ences have not been experimentally addressed yet.
The extended duration of many growth experi-
ments (30-275 days; Ishimatsu
et al.
2005 ) may over-
come the pitfalls associated with acute studies and
allow for acclimatory responses, which compensate
for CO
2
-induced shifts in energy budget. Such com-
pensation was observed in a study that recorded
growth rates of juvenile spotted woli sh (
Anarhichas
minor
) on a weekly basis at a
p
CO
2
of 10 000 μatm.
Despite a 20% decrease in growth during the i rst 3
weeks of exposure, specii c growth rates returned
to control values for the remainder of the 10-week-
long trials (Foss
et al.
2003 ). Differing acclimation
patterns between short- and long-term exposures
have also been documented in expression levels of
acid-base relevant ion transporters in gills of the
eelpout
Z. viviparus
over a time course of 6 weeks at
a
p
CO
2
of 10 000 μatm (Deigweiher
et al.
2008 ). To
further elucidate interspecies variability and accli-
mation plasticity in teleosts, studies that examine
baseline transporter expression proi les and distin-
guish between acute and long-term responses to
ocean acidii cation are necessary.
Recent studies on acutely exposed tropical cardi-
nal i shes from coral reefs have documented a sur-
prisingly high sensitivity to ocean acidii cation.
Aerobic scope and critical swimming speed
decreased signii cantly in both
Ostorhinchus doeder-
leini
and
Ostorhinchus cyanosoma
after 1 week of
acclimation to 1000 μatm (Munday
et al.
2009a ).
