Geoscience Reference
In-Depth Information
community responses in various ecosystems
( Pörtner and Farrell 2008 ).
Temperature is the most pervasive environmen-
tal factor which affects all levels of biological organ-
ization ( Box 8.1 ). The specii c effects of ocean
acidii cation are then also affected by temperature
and exacerbated at thermal extremes. An overarch-
ing hypothesis is that disturbances in acid-base sta-
tus play a key role in mediating the specii c
physiological effects of elevated CO
2
tensions
( Pörtner 2008 ; Melzner
et al.
2009a ). The mecha-
nisms of acid-base regulation strive to compensate
for such disturbances, and thereby the organism
escapes the detrimental effects of ambient stressors
on physiological functions. Some of these mecha-
nisms differ between animals breathing air and
those breathing water. In general, all water-breath-
ing animals require much higher ventilation vol-
umes than air breathers for access to sufi cient
amounts of oxygen. As a result, CO
2
release is very
efi cient and diffusion gradients for CO
2
between
body l uids and ambient media are about 30-fold
lower than in air breathers. Acid-base regulation in
air breathers may involve large respiratory adjust-
ments in body l uid
P
CO
2
. This process plays only a
minor role in water breathers, where acid-base reg-
ulation largely occurs through branchial ion trans-
port processes ( Heisler 1986 ; Claiborne
et al.
2002 ).
The initial compensation of acid-base disturbances
occurs through the pre-existing trans-epithelial ion
transport mechanisms. On longer timescales further
regulation may involve the dynamic modulation
of the content and functional properties of ion
transport proteins, including the mRNA and pro-
tein expression of specii c isoforms and their post-
translational modii cation.
The capacity for acid-base regulation will deter-
mine the extent of compensation of acid-base status
and the potential for shifts in crucial physiological
functions or their balance. Extracellular acid-base
status is generally more susceptible to environmen-
tally induced changes, whereas intracellular pH is
much more stable and thus better protected.
Extracellular rather than intracellular pH has been
demonstrated to cause shifts in physiological func-
tioning at the molecular (within membranes), cel-
lular, tissue, and systemic levels (e.g. Reipschläger
and
Accordingly, extracellular pH (pH
e
) is thought to
play a crucial and integrating regulatory role in
shaping the short- or long-term whole-organism
response to environmental stressors such as ele-
vated CO
2
( Pörtner 2008 ; Fig. 8.1 ). By lowering
extracellular pH, ocean acidii cation may act on
performance at various levels, such as somatic (and
shell or skeleton) growth, reproduction, and behav-
iour. This includes an impact on calcii cation and,
thereby, the structural stability, shape, or size of the
(exo- and endo-) skeleton. The compensatory accu-
mulation of bicarbonate can ameliorate this impact
(see below). The ecological success of marine organ-
isms facing ocean acidii cation may thus largely
depend on the degree to which they can maintain
pH homeostasis and their capacities to regulate
acid-base status. The associated energetic costs are
likely to comprise a signii cant fraction of an organ-
ism's energy budget, and are thus related to the rate
of energy metabolism. Low-performance, especially
hypometabolic, marine invertebrates, which are
characterized by a low capacity to compensate for
disturbances in extracellular ion and acid-base sta-
tus, are probably more sensitive than high-perform-
ance species with a high capacity for ion and
acid-base regulation (Seibel and Walsh 2001).
Other specii c effects of CO
2
may occur, either
independently or linked to acid-base status in ways
that are still unknown. For example, CO
2
-induced
acidosis leads to the accumulation of adenosine,
which acts as an inhibitory neurotransmitter in
nervous tissue and can cause metabolic depression
under extreme hypoxia or hypercapnia (Reips-
chläger
et al
. 1997). The metabolism of other amino
acid neurotransmitters is probably also inl uenced,
via the sensitivity of carboxylation and decarboxy-
lation reactions to changes in cellular or mitochon-
drial CO
2
or bicarbonate levels (e.g. Hardewig
et al.
1994 ; Mühlenbruch 2004 ; Stark 2008 ; Fig. 8.1 ).
Functional responses to these changes have not
been explored to date. At the level of the organism,
olfactory disturbances may occur and lead to atypi-
cal behaviours such as those reported for tropical
i shes (Munday
et al
. 2009b ).
The acid-base balance and associated physio-
logical functions of organisms respond acutely to
environmental change. On longer timescales,
however, organisms may acclimate by changing
Pörtner
1996 ;
Pörtner
et al
.
1998 ,
2000 ).
