This document discusses how rising atmospheric CO2 levels are causing ocean acidification, which affects marine life and ecosystems. Since the industrial revolution, CO2 emissions have increased atmospheric CO2 levels from 180-300 ppm to over 380 ppm currently. The ocean absorbs over 30% of this CO2 annually, which reacts to form carbonic acid and lowers ocean pH. This decreases carbonate ion concentrations by 16%, threatening calcifying organisms. Corals and plankton are especially at risk, as reduced carbonate saturation can inhibit their growth and shell formation. While some species show adaptations, overall declining carbonate levels threaten marine food webs and ecosystems humans rely on for food, income, and coastal protection from erosion and storms

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Will OceanAcidificationAffect the Deposition of Marine Biogenic Carbonates?
Introduction
Since the beginning of the industrial revolution during the 19th
century, mankind has been releasing
CO2 into the atmosphere in quantities that have since been rising with the expansion of the human
population (Website 1). The link between increasing atmospheric CO2 (chiefly from industry) and
global warming is well established and accepted in many fields of science, although in more recent
times it has become apparent that Earth’s oceans are undergoing change coincident with rising
levels of atmospheric CO2 (Bradshaw, 2007).
Over the past two centuries, Earth’s oceans have been a major sink for anthropogenic CO2 (Fabry et
al., 2008). This process has been altering the chemistry of seawater, ultimately making it more
acidic (Website 2). Not only does this lower the pH of the water, it affects marine life, particularly
shelled organisms, by reducing the amount of carbonate ions (CO3
2-
) available, and lowers the
saturation states of essential carbonates such as aragonite and calcite (Kleypas et al., 2006).
This essay aims to inform the reader of the negative effects that rising atmospheric CO2 is having on
our oceans, and ultimately how that is affecting certain marine species, and entire ecosystems that
rely on a steadily decreasing supply of CO3
2-
in an increasingly acidic environment. How humanity
is affected in turn by ocean acidification will be covered.
Anthropogenic CO2 and its interaction with Earth’s oceans
Humanity’s continual and widespread burning of fossil fuels since the advent of industrialization,
some 150-200 years ago (Website 1), along with cement production and deforestation has caused a
steep rise in atmospheric CO2 (Kleypas et al., 2006, Sarmiento & Siegenthaler, 1993). Various
dating techniques including deep-ice coring have shown that atmospheric CO2 concentrations have
remained within 180-300 ppmv (parts per million by volume) over the 650,000 years (approx.) that
preceded man’s industrial endeavours (Fabry et al., 2008, Kleypas et al., 2006, Siegenthaler et al.,
2005).
Due to exponential population growth and the resulting increase in human activity, today’s
atmosphere contains upwards of 380 ppmv CO2 (Duarte et al., 2013, Fabry et al., 2008) – 22
million tons of which is taken up by the ocean daily (Website 3). It has been estimated that this
figure could surpass 500 ppmv by 2050, and reach 800-1000 ppmv by the beginning of the 22nd
century (Kleypas et al., 2006). Only ~50% of anthropogenic CO2 remains in the atmosphere; ~30%
is absorbed by the ocean on a year-to-year basis, with the remaining ~20% subject to terrestrial
sinks (Kleypas et al., 2006, Website 4).
Ocean acidification and oceanic carbonate chemistry
When atmospheric CO2 mixes with the ocean’s surface waters, it reacts, forming carbonic acid
(H2CO3) (Fabry et al., 2008). The formation of H2CO3 lowers the pH of seawater, ultimately
causing ocean acidification (OA) (Duarte et al., 2013, Kleypas et al., 2006). In addition to this, the
H2CO3 breaks down into a H+
ion and a bicarbonate ion (HCO3
-
), the H+
ion then bonding to the
carbonate (CO3
2-
) present in the ocean, forming yet more HCO3
-
, as is demonstrated in figure 1

2. Magnus McFarlane 2198456m
(Fabry et al., 2008). It is these key processes of ocean acidification that are causing detrimental
effects to the functionality and survival of many marine organisms, mainly calcifiers that rely on the
ocean’s supply of CO3
2-
(Kroeker et al., 2013).
As a result of increased dissolved inorganic carbon (DIC)
from anthropogenic CO2 over the last two centuries, the
mean pH value of surface waters has decreased by 0.1
(Chierici & Fransson, 2009, Fabry et al., 2008). Although a
seemingly minor figure, this equates to a 30% rise in H+
,
compared to pre-industrial values (Fabry et al., 2008). In
turn, OA has lowered concentrations of oceanic carbonate
ions by 16% over the past two centuries, and this has led to
decreased levels of CaCO3 saturation (Shamberger et al.,
2011), affecting the growth of organisms that rely on CaCO3
(Kleypas et al., 2006).
Corals and their response to OA in coastal and open
ocean reef systems
Coral reefs provide wide-spanning habitats for marine ecosystems in both pelagic and benthic
environments (Duarte et al., 2013). Many calcifiers such as corals, macroalgae, molluscs,
echinoderms and benthic foraminifera co-exist within reef systems (Duarte et al., 2013, Kleypas et
al., 2006). Corals exist symbiotically with various algae, which aid corals in forming vast CaCO3
reefs by providing nutrients necessary for coralline skeletogenesis. (Cunning et al., 2015) It has
been estimated that a 50% increase in CO2, which may occur within the next 100 years, can cause
up to 40% decrease in reef production from corals and their symbionts (see figure 2) (Langdon et
al., 2003).
Increased DIC, and therefore decreased saturation of CaCO3, mainly in the forms of calcite and the
metastable carbonate polymorph, aragonite, increases the dissolution of carbonate structures of
corals and other calcifying taxa (Fabry et al., 2008). In corals, especially small colonies, this can
inhibit survival, rate of growth and repair (Atkinson & Langdon, 2005).
In addition to dissolution, an acidic environment is optimal for fleshy algae, which is a competitor
of coralline algae (Kamenos & McCoy, 2015). Fleshy algae does not require CaCO3, and thus has
increased survivability in areas of low carbonate saturation, in comparison to coralline algae
(Kamenos & McCoy, 2015)
An experiment on the deep-ocean cold-water
scleractinian coral, Lophelia pertusa,
conducted by Hennige et al., (2015), over a
period of 12 months, has however shown that
adaptation is possible within coral structures.
Under conditions simulating predicted future
acidification levels and increased temperature,
L. pertusa altered its usual calcifying patterns
within a period of 4 weeks, with no major
changes following this initial time scale, for
the remainder of the 12 months, ultimately
showing rapid acclimatization to advanced
Figure 1: The oceanic carbonate system, showing the
bicarbonate cycle. (Image taken from website 5)
Figure 2: Effects of increased CO2 on a benthic coral community
(Image from website 6).

3. Magnus McFarlane 2198456m
OA influences. The outer aragonite skeleton of cold-water corals is still prone to dissolution over
time however, which can affect the integrity of deep-ocean reef structures over periods of more than
12 months (Hennige et al., 2015).
According to Shamberger et al., (2011) net ecosystem calcification (NEC) of coral reef
communities will reach zero when atmospheric carbon (and therefore OA) is 3 times that of pre-
industrial values, and that corals would then cease to be major calcifiers, as they are today.
How planktic calcifiers are affected by OA
Transport of CaCO3 through the water column in pelagic oceans, from photic zones to deep sea
beds, is chiefly controlled by three main CaCO3-generating planktons; foraminifera,
coccolithopores and euthecosomatous pteropods (Fabry et al., 2008). Of these, foraminifera and
coccolithopores form calcite tests (shells), whilst euthecosomatous pteropods secrete more soluble
aragonite tests (see figure 3) (Fabry et al., 2008). These organisms die, causing a steady stream of
CaCO3 shell particles to sink to the ocean floor, depositing marine sediments and oozes, which are
important providers of information into the history of Earth’s marine environments (Schiebel &
Hemleben, 2005).
Controlled experiments carried out by Engel et al. (2015), showed that the high-magnesium calcite
shells of Amphisorus hemprichii and Heterostegina depressa were affected by a high-temperature,
low-pH environment, proving dissolution under OA conditions. However, several other species
within the experiment showed resilience to low pH, seemingly being affected more so by the
increased temperatures of the water.
Beare et al. (2013) found that coccolithopores, foraminiferans and echinoderm larvae of the mid
North Sea have increased in numbers over the past ~60 years, indicating adaptability in gradually
higher temperatures and decreasing pH levels. Pteropods and bivalve larvae were susceptible to
these changes however, and their numbers have declined.
Observations have revealed that foraminifers possess the ability to raise pH levels internally as they
calcify, which aids in their survival. (Fujita et al., 2011) However, this process requires additional
energy which puts strain on their life cycle (Fujita et al., 2011). Pelagic echinoderm larvae can also
alter their internal pH levels when seawater pH is lowered (Beare et al., 2013)
It has been predicted that photosynthesising
microalgae, which require CO2 for growth, could
flourish due to increasing anthropogenic CO2 (Beare
et al., 2013, Fujita et al., 2011). Algal symbiont-
bearing foraminifera are aided significantly, as their
photosynthesising symbionts provide increased
nutrients under higher CO2 conditions, and work in
discarding metabolites which interfere with
calcification processes (Fujita et al., 2011).
Symbiont-bearing foraminifera may still suffer from
the effects of lowered CO3
2-
saturation, long-term
(Fabry et al., 2008).
Figure 3: Dissolution of pteropod shell due to
undersaturation of aragonite, over 45 days (Image from
website 7).

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Molluscs and other marine calcifiers
Reef systems are home to many other calcifying organisms, such as bivalves, echinoderms and
crustaceans (Kleypas et al., 2006). These animals are abundant in benthic coastal reefs; their shells
adding to the carbonate structures laid down by corals (Fabry et al., 2008).
In a study which simulated seawater affected by 740 ppmv atmospheric CO2, calcification rates of
two adult bivalve species, a mussel and an oyster, fell by 25% and 10%, respectively (Fabry et al.,
2008). Worst affected however, are the larvae and early-stage invertebrates, as their skeletons are
more fragile as they are developing (Byrne et al., 2011). Biomineralization, or shell-formation of
larval bivalves, urchins and sea snails are susceptible to the damaging effects of OA as initial
development utilises amorphous CaCO2, which is more soluble than calcite, aragonite, and high-
magnesium carbonates (Fabry et al., 2008).
Not much is known about how crustaceans will cope with increasing OA, although Whiteley (2011)
postulated that crustaceans such as crabs, lobsters and prawns could be under threat due to their
inability to buffer the acidity by using HCO3
-
ions, like the foraminifers and echinoderms.
How ocean acidification affects humans
Coral reefs are extremely important socially and economically for coastal settlements, providing
food, revenue from tourism and protection from rising sea levels and storm waves (Fenner, 2012,
Hernandez-Delgado, 2015). Upwards of 800 million people inhabit coastal terrain, many of whom
rely on reefs for food and income - the estimated worldwide figure for coral reef services totalling
at 375 billion US$ yearly (Fenner, 2012). OA has caused phase shifts in species, with macroalgae
dominating over corals, leading to a reduction in numbers of fish, as their habitats are taken
(Fenner, 2012). Reduced production of corals, and therefore reduced numbers of fish, molluscs and
other marine resources, could cost millions their incomes and their livelihood (Fenner, 2012).
Conclusion
Anthropogenic CO2 is the main driver of
OA, which in turn is the main driver of
carbonate undersaturation in both coastal and
open-ocean environments (Fabry et al., 2008,
Kleypas et al., 2006). Lowered pH is having
noticeable effects on several marine
calcifying species, causing shell dissolution
and phase shifts within benthic ecosystems
(Fabry et al., 2008). Increased burning of
fossil fuels will cause surface water pH to
drop further, and a continuing decrease of
CO3
2-
ions (see graph 1) (Fabry et al., 2008).
Although some corals and calcifying plankton show resistance to OA, there is evidence that overall,
these marine carbonate producers are likely to decline, causing a decrease in deposition and
affecting food webs, and, indirectly, human populations worldwide.
umol
kg-
1/pH
Pre-
Industrial
Present 2xCO2 3xCO2
Change –
Pre-
industrial
to 3xCO2
pCO2 280 380 560 840 200%
H2CO3 9 13 18 25 178%
HCO3
- 1739 1827 1925 2004 15%
CO3
2- 222 186 146 115 -48%
DIC 1970 2026 2090 2144 8.8%
pH 8.16 8.05 7.91 7.76 -0.4
Graph 1: Past, present and future concentrations (umol kg-1
)
of carbonate species, partial-pressure CO2 (pCO2), DIC, and
change in pH units, in response to elevated atmospheric
CO2. Adapted from Fabry et al., 2008.

5. Magnus McFarlane 2198456m
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