O
OCEAN ACIDIFICATION, EFFECTS ON
CALCIFICATION
Joan A. Kleypas
National Center for Atmospheric Research, Boulder,
CO, USA
Synonyms
Lowered pH of seawater
Definition
Aragonite: A calcium carbonate (CaCO 3 ) mineral with an
orthorhombic crystal lattice structure; a polymorph of
calcite.
Calcification: The process by which an organism secretes
calcium carbonate.
Calcite. A calcium carbonate (CaCO 3 ) mineral with
a trigonal (rhombohedral) crystal lattice structure;
a polymorph of aragonite.
High-Mg calcite: Calcite containing at least 4 mol%
MgCO 3 .
Ocean acidification: The decrease in the pH of the Earth’s
oceans caused by the uptake of carbon dioxide from the
atmosphere.
Rhodoliths: Coralline algae that grow in rounded, freeliving forms that are unattached to the substrate.
Saturation state: A measure of the thermodynamic potential of a mineral to precipitate or dissolve.
Introduction
Technically, the term “ocean acidification” refers to any
process that causes a decrease in seawater pH. Today,
however, the term refers almost exclusively to the process
by which oceanic water absorbs carbon dioxide from the
atmosphere, causing a decrease in ocean pH and changes
in other chemical properties of seawater. Ocean acidification is not a consequence of climate change, but the two
share a common cause: increasing concentration of carbon
dioxide in the atmosphere. Ocean acidification can affect
coral reefs in multiple ways: effects on biogeochemical
processes, physiology of organisms, and even such things
as sound transmission, have been demonstrated. However
the best-documented effects are those on organisms that
secrete calcium carbonate, most notably corals, coralline
algae, and foraminifera, which are major contributors to
reef growth.
Chemistry of ocean acidification
The chemical reactions involved in ocean acidification are
well documented, albeit somewhat complicated. Carbon
dioxide reacts with seawater to form carbonic acid:
CO 2 þ H 2 O ! H 2 CO 3 :
(1)
Most of the carbonic acid dissociates quickly to hydrogen
ions (a contributor to acidity) and bicarbonate ions:
H 2 CO 3 ! HCO 3
À
þ H
þ
;
(2)
and some of the bicarbonate also dissociate to hydrogen
ions and carbonate ions:
HCO 3
À ! CO 3
2À
þ H
þ
:
(3)
When carbon dioxide is added to seawater, the relative
proportions of the carbonic acid complex (dissolved CO 2
and H 2 CO 3 ), bicarbonate, and carbonate, shift to maintain
charge balance in seawater. This comprises the buffering
capacity of seawater to maintain pH. For example, the
addition of carbon dioxide to seawater causes an increase
in bicarbonate ion concentration, a decrease in carbonate
ion concentration, and a decrease in pH. The following
David Hopley (ed.), Encyclopedia of Modern Coral Reefs, DOI 10.1007/978-90-481-2639-2,
# Springer Science+Business Media B.V. 2011
OCEAN ACIDIFICATION, EFFECTS ON
CALCIFICATION
Joan A. Kleypas
National Center for Atmospheric Research, Boulder,
CO, USA
Synonyms
Lowered pH of seawater
Definition
Aragonite: A calcium carbonate (CaCO 3 ) mineral with an
orthorhombic crystal lattice structure; a polymorph of
calcite.
Calcification: The process by which an organism secretes
calcium carbonate.
Calcite. A calcium carbonate (CaCO 3 ) mineral with
a trigonal (rhombohedral) crystal lattice structure;
a polymorph of aragonite.
High-Mg calcite: Calcite containing at least 4 mol%
MgCO 3 .
Ocean acidification: The decrease in the pH of the Earth’s
oceans caused by the uptake of carbon dioxide from the
atmosphere.
Rhodoliths: Coralline algae that grow in rounded, freeliving forms that are unattached to the substrate.
Saturation state: A measure of the thermodynamic potential of a mineral to precipitate or dissolve.
Introduction
Technically, the term “ocean acidification” refers to any
process that causes a decrease in seawater pH. Today,
however, the term refers almost exclusively to the process
by which oceanic water absorbs carbon dioxide from the
atmosphere, causing a decrease in ocean pH and changes
in other chemical properties of seawater. Ocean acidification is not a consequence of climate change, but the two
share a common cause: increasing concentration of carbon
dioxide in the atmosphere. Ocean acidification can affect
coral reefs in multiple ways: effects on biogeochemical
processes, physiology of organisms, and even such things
as sound transmission, have been demonstrated. However
the best-documented effects are those on organisms that
secrete calcium carbonate, most notably corals, coralline
algae, and foraminifera, which are major contributors to
reef growth.
Chemistry of ocean acidification
The chemical reactions involved in ocean acidification are
well documented, albeit somewhat complicated. Carbon
dioxide reacts with seawater to form carbonic acid:
CO 2 þ H 2 O ! H 2 CO 3 :
(1)
Most of the carbonic acid dissociates quickly to hydrogen
ions (a contributor to acidity) and bicarbonate ions:
H 2 CO 3 ! HCO 3
À
þ H
þ
;
(2)
and some of the bicarbonate also dissociate to hydrogen
ions and carbonate ions:
HCO 3
À ! CO 3
2À
þ H
þ
:
(3)
When carbon dioxide is added to seawater, the relative
proportions of the carbonic acid complex (dissolved CO 2
and H 2 CO 3 ), bicarbonate, and carbonate, shift to maintain
charge balance in seawater. This comprises the buffering
capacity of seawater to maintain pH. For example, the
addition of carbon dioxide to seawater causes an increase
in bicarbonate ion concentration, a decrease in carbonate
ion concentration, and a decrease in pH. The following
David Hopley (ed.), Encyclopedia of Modern Coral Reefs, DOI 10.1007/978-90-481-2639-2,
# Springer Science+Business Media B.V. 2011