Geoscience Reference
In-Depth Information
is the decrease in the saturation state of calcium
carbonate of about 20% between 1766 and 2007
and a projected further potential decline of about
40% by 2100 (Table 1.1). Consequently, the CaCO
3
saturation horizon (the depth at which the satura-
tion is 1) was, depending on the CaCO
3
mineral
considered, 80 to 200 m shallower in 1994 than in
pre-industrial times (Feely
et al.
2004 ). Recent
changes are most dramatic in surface waters of
high-latitude areas where seasonal undersatura-
tion of aragonite has been reported. Model projec-
tions indicate that undersaturation of aragonite
will become a widespread feature of the Southern
( Orr
et al.
2005 ) and Arctic ( Steinacher
et al.
2009 )
oceans within the next decades (see Chapter 3).
2
+
−
Ca
+
2HCO
→+
CaCO
CO
+
H O
(1.2a)
3
3
2
2
2
+
−
Ca
+
CO
→
CaCO .
(1.2b)
3
3
Both parameters decrease with increasing ocean
acidii cation, which can trigger a decline in the cal-
cii cation rate. In contrast, the reverse reaction of
dissolution of calcium carbonate is favoured by the
decrease in the CaCO
3
saturation state generated by
ocean acidii cation.
Photosynthesis is another process which can
be directly affected by changes in the carbonate
system. The ultimate source of inorganic carbon is
carbon dioxide according to Eqs 1.3a and 1.3b
(depending on the source of nitrogen):
+
2
−
1.3 The biological and biogeochemical
processes that are potentially affected
106CO
+
16NH
+
HPO
+
106H O
(1.3a)
2
4
4
2
+
=
C
H
O
N
P
+
106O
+
14H
106
263
110
16
2
−
2
−
Changes in the carbonate chemistry of seawater
can have a wide range of effects, some of which
may be mediated through disturbances in the
acid-base status of affected organisms. The extra-
cellular pH of body l uids in animals and the
intracellular pH of various organs or unicellular
organisms are usually tightly regulated, but the
capacity of regulatory mechanisms can be over-
whelmed. pH plays a key role in many physiolog-
ical processes such as ion transport, enzyme
activity, and protein function. Many intracellular
enzymes are pH-sensitive and display a pH opti-
mum around the physiological range (Madshus
1988). For example, the activity of phosphofruc-
tokinase, a key enzyme in the glycolytic pathway,
exhibits a 10- to 20-fold reduction when pH
decreases by as little as 0.1 units below the physio-
logical pH optimum (Trivedi and Danforth 1966).
Other direct effects of ocean acidii cation could
occur when one or several reactant(s) in a physio-
logical process, such as calcii cation and photo-
synthesis, is a carbon species. Calcii cation is often
described by Eq. 1.2a, which may give the wrong
impression that it could be stimulated by ocean
acidii cation as the concentration of bicarbonate
rises. However, the ultimate reaction at the site
where CaCO
3
is precipitated is controlled by the
concentration of carbonate ions (Eq. 1.2b), and
hence the CaCO
3
saturation state
106CO
+
16NO
+
HPO
+
122H O
(1.3b)
2
3
4
2
+
+
18H
=
C
H
O
N
P
+
138O .
106
263
110
16
2
However, CO
2
is often in very limited supply in sea-
water and several organisms have developed car-
bon-concentrating mechanisms (CCMs; Reinfelder
2011). CCMs use bicarbonate, which is available in
large amounts in seawater, to concentrate inorganic
carbon intracellularly. The increase in CO
2
and
bicarbonate with increasing ocean acidii cation
could therefore have signii cant effects on photo-
synthesis. It can be stimulated by elevated CO
2
in
species lacking a CCM or when the CCM is not
operating optimally. It is also possible that even
bicarbonate users favour the use of CO
2
over HCO
3
-
because CCMs are energetically costly to operate
(see Chapter 6 ).
Ocean acidii cation could also have indirect
effects on many biological processes. For example,
photosynthesis requires nutrients other than CO
2
(Eqs 1.3a and 1.3b), such as nitrogen, phosphorus,
or iron (not shown in the equations). Lower ocean
pH has an impact on the thermodynamics and
kinetics of metals and some nutrients in seawater,
resulting in changes in their speciation, behaviour,
and fate (e.g. Millero
et al.
2009 , and Chapter 6 ).
These changes could affect the availability and tox-
icity of metals to marine organisms. For example,
chemical equilibria (Millero
et al.
2009 ) suggest an
