# Ocean acidification

 Definition of Ocean acidification: The process whereby atmospheric carbon dioxide dissolves in seawater producing carbonic acid, which subsequently lowers pH of surrounding seawater; widely thought to be happening on a global scale. This is the common definition for Ocean acidification, other definitions can be discussed in the article

This article presents an introduction to ocean acidification and summarizes results of studies on the impact of ocean acidification on a few common calcifying marine organisms.

## Ocean acidity

The unit for measuring ocean acidity is the $pH$. $pH$ is a measure of hydrogen ion $H^+$ activity. It is estimated from the approximate formula

$pH \approx - log_{10} ([H^+]) , \qquad (1)$

where $[H^+]= c_{eq}(H^+)$ is the equilibrium concentration (measured in number of moles per liter) of $H^+$ ions at a given temperature ($25 ^oC$). A solution is neutral if $pH=7$, acidic if $pH\lt 7$ and basic if $pH\gt 7$.

The average acidity of ocean surface waters was $pH=8.15$ in pre-industrial times. Due to the increase in the atmospheric $CO_2$ concentration, the amount of $CO_2$ dissolved in the ocean has also increased. It is estimated that about 30% of the yearly emitted $CO_2$ is absorbed by the oceans[1]. The uptake of $CO_2$ has raised the acidity (decreased the $pH$) of the ocean surface waters in 2020 to about $pH=8.05$. This is equivalent to an increase of hydrogen ion activity of about 26%.

The average acidity of ocean surface waters is expected to decrease by 0.14–0.43 units (i.e., a decrease in $pH$ from about 8.15 to about 7.7 - 8) with a concurrent increase of +2 oC and +4 oC in sea surface temperature by 2100[2].

Ocean acidification is not only due to uptake of atmospheric $CO_2$. Other processes also contribute to acidification, such as calcification, decomposition of organic material, nitrification in surface water (promoted by sewage discharge) and oxidation processes in sediments[3]. Acidity is not directly related to the presence of dissolved oxygen, because $O_2$ and $H^+$ do not react to produce $H_2O$.

## Negative feedback to acidification

Alkalinity plays a major role in ocean chemistry, in $CO_2$ storage and in calcium carbonate precipitation and dissolution. The presence of calcium carbonate $CaCO_3$ provides a feedback mechanism that mitigates ocean acidification due to $CO_2$ uptake. Alkalinity is defined as the excess of proton acceptors (typically anions) over donors (typically free protons $H^+$) in seawater[4]. Alkalinity and $pH$ are in general positively correlated. Alkalinity can be determined by measuring the quantity of a strong acid that must be supplied to convert all anions in uncharged species. Seawater alkalinity is largely determined by the concentration of carbonate ions $CO_3^{2-}$ and $HCO_3^{-}$, as other anions in seawater have much lower concentrations[4].

The dissolution of $CO_2$ in water corresponds to the chemical balances

$CO_2+H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons HCO_3^- + H^+ \rightleftharpoons CO_3^{2-} +2H^+ . \qquad (2)$

The increased concentration of free hydrogen ions by dissolution of $CO_2$ decreases the $pH$ thus increases the acidity. As part of the hydrogen ions combine with carbonate ions $CO_3^{2-}$ present in the seawater to form bicarbonate $HCO_3^-$, the seawater alkalinity decreases. Fewer carbonate ions remain available for marine calcifying organisms to produce calcium carbonate $CaCO_3$ shells.

Dissolution of calcium carbonate in seawater can follow different pathways[5],

$CaCO_3 \rightleftharpoons Ca^{2+}+CO_3^{2-} ; \quad CaCO_3+H_2O \rightleftharpoons Ca^{2+}+HCO_3^{-} +OH^- ; \quad CaCO_3+H_2O+CO_2 \rightleftharpoons Ca^{2+}+2HCO_3^{-} . \qquad (3)$

The carbonate ions $HCO_3^-$ and $CO_3^{2-}$ produced by dissolution can combine with hydrogen ions. Dissolution of calcium carbonate thus decreases the concentration of free hydrogen ions and therefore counteracts the increase in acidity due to $CO_2$ uptake[6].

Increased alkalinity of the ocean's surface waters due to dissolution of $CaCO_3$ (or due to other causes) influences the chemical balances (2) and (3) such that part of the dissolved $CO_2$ is converted into more stable bicarbonate and carbonate molecules. Chemical equilibria are restored by the absorption of more $CO_2$ into the ocean, thus reducing the concentration of atmospheric $CO_2$. Other processes, such as anaerobic mineralization of organic matter and denitrification, also increase alkalinity and thus play the same role[4].

Conversely, calcifying organisms that extract carbonate ions from seawater promote acidification. The same applies to the aerobic breakdown of organic matter in which electrons are absorbed in oxygenation reactions, causing reduction in alkalinity and $pH$ [4].

Ocean surface waters are typically supersaturated with calcium carbonate $CaCO_3$ due to high concentrations of calcium ions. The solubility of calcium carbonate increases when the saturation state decreases. The saturation state of calcium carbonate decreases with decreasing concentration of carbonate ions. It also decreases with increasing depth because the greater solubility of $CO_2$ at low temperature and high pressure raises the $pH$. This explains why calcifying organisms do not occur at great depths in the ocean. The dissolution of $CaCO_3$ in the deep ocean raises alkalinity and $pH$. This buffer mechanism of calcium carbonate protects seawater $pH$ against significant change from uptake of atmospheric $CO_2$ [5].

Calcium carbonate in the oceans occurs in two crystalline forms (polymorphs): aragonite and calcite. Aragonite is much more soluble than calcite. Organisms that produce aragonite are therefore more vulnerable to changes in ocean acidity than those that produce calcite. The dissolution of aragonite in the deep sea releases alkalinity and raises $CaCO_3$ saturation states, thus providing a buffer against dissolution of calcite deposits[7].

## Influence of ocean acidification on a few bivalve species

### Oyster Magallana gigas and mussel Mytilus spp.

 Magallana gigas Mytilus edulis

Mytilus spp. and Magallana gigas together account for almost half of global mollusc production within the aquaculture industry. As the ocean’s $pH$ decreases, the extent of the effect of ocean acidification is dependent on the shell structure and composition of the organism. Both the mussel Mytilus species (spp.) and the oyster Magallana gigas form calcite layers, but Mytilus spp. also forms aragonite on the inner shell layer. Although the two polymorphs share the same chemical formula, the different atomic arrangement of aragonite increases susceptibility to ocean acidification, compared to calcite.

Experiments were conducted to examen the interactive effects on Mytilus spp. and Magallana gigas of $pH$ (8.1 versus 7.7), temperature (12 versus 14 oC) and feeding (control versus extra feed) in a full factorial experimental design. The following observations were reported (Mele et al., 2023[8]):

"When seawater temperature rises, Mytilus spp. appear to rely on metabolically sourced carbon for shell calcite potentially from extrapallial fluid (fluid from outside the mantle) rather than from mantle tissue or from the feed under ocean acidification. The altered biomineralization pathway in Mytilus spp. into the shell calcite layer, is sufficient to maintain the growth of the shell, as well as its thickness and hardness. On the other hand, Mytilus spp. increases environmentally sourced carbon for aragonite under low $pH$ conditions. This response is sufficient to maintain and increase shell thickness in high water temperature scenarios. Low $pH$ also affects M. gigas from a feeding and nutrient perspective shown by variation in mantle nitrogen isotopes, but biomineralization pathway is maintained along with growth.

Previous research has shown that increasing food supply to mollusks during ocean acidification experiments can limit shell corrosion and increase shell growth. However, plankton blooms would be more beneficial to M. gigas, as this study shows overall better shell performance and resilience than Mytilus spp."

### White furrow shell Abra alba

Abra alba

A study of Vlaminck et al. (2022[9]) on the physiological response of the white furrow shell Abra alba to three $pH$ treatments ($pH = 8.2; \, pH = 7.9; \, pH = 7.7$) showed no $pH$ effect on survival.

"However, lowered respiration and calcification rates, decreased energy intake (lower absorption rate) and increased metabolic losses (increased excretion rates) occurred at $pH \sim 7.7$. These physiological responses resulted in a negative Scope for Growth and a decreased condition index at this $pH$. This suggests that the physiological changes may not be sufficient to sustain survival in the long term, which would undoubtedly translate into consequences for ecosystem functioning."

### Peruvian scallop Argopecten purpuratus

Argopecten purpuratus

Along the Peruvian coast, natural conditions of low $pH$ (7.6–8.0) are encountered in the habitat of the Peruvian scallop Argopecten purpuratus, due to the nearby coastal upwelling. During 28 days, scallops (initial mean height = 14 mm) were exposed to two contrasted pH conditions: a control with unmanipulated seawater presenting $pH$ conditions similar to those found in situ ($pH = 7.8$) and a treatment, in which CO2 was injected to lower the $pH$ to 7.4.

"At the end of the experiment, shell height and weight, and growth and calcification rates were reduced about 6%, 20%, 9%, and 10% respectively in the low $pH$ treatment. Mechanical properties, such as microhardness were positively affected in the low $pH$ condition and crushing force did not show differences between $pH$ treatments. Final soft tissue weights were not significantly affected by low $pH$. This study provides evidence of low $pH$ change shell properties increasing the shell microhardness in Peruvian scallops, which implies protective functions" (Cordova-Rodriguez et al., 2022[10]).

## Influence of ocean acidification on coccolithophores

Emiliania huxleyi

Coccolithophores are unicellular organisms belonging to the marine phytoplankton community. The most common species is Emiliania huxleyi, ubiquitous in temperate, subtropical and tropical oceans. Coccolithophores are covered with calcium carbonate scales called coccoliths. Coccoliths from dead coccolithophores contribute to the ocean's carbon sink. Dead coccolithophores sink slowly and a large fraction is probably mineralized before reaching the ocean floor. Coccoliths and coccospheres are more likely to be transported to depth when incorporated within fecal pellets or marine snow[11]. A major fraction of carbonate in ocean surface waters (close to 90%) consists of coccolithophores but their contribution to the $CaCO_3$ stock buried in deep sea sediments is probably not larger than the contribution of the much less abundant foraminifera[12].

Krumhardt et al. (2019[13]) studied the sensitivity of coccolithophore growth and calcification to increasing CO2 both regionally and on a global scale using the Community Earth System Model (CESM) version 2.0[14]. This model was validated by comparison with satellite-derived ocean data of particulate inorganic carbon, a compilation of coccolithophore biomass estimated from shipboard measurements, a global compilation of coccolithophore calcification rates and estimates of globally integrated annual upper ocean calcification rates.

The model results show that: "increasing CO2 stimulates the growth of coccolithophores in some regions (North Atlantic, Western Pacific, and parts of the Southern Ocean), allowing them to better compete for resources with other phytoplankton functional types in the model. As CO2 increases in the upper ocean, however, calcification is impaired. Most regions of the ocean show vast declines in pelagic calcification, with some regions (in the Southern Ocean and North Pacific) being subject to almost no calcification by coccolithophores at end-of-the-century CO2 levels. Though CO2 stimulates growth in some areas, coccolithophores in general are projected to be more lightly calcified under future, high CO2 conditions."

The findings of Ziveri et al. (2023)[12] suggest that calcium carbonate production by coccolithophores and carbon export to the deep sea are not strongly coupled. Several other processes play a role such as changes in the ability to export $CaCO_3$ out of the photic zone due to changes in grazing, particle aggregation, the organic/inorganic carbon ratio of the aggregates, or changes in the relative abundance of foraminifera to coccolithophores/pteropods. Decrease in calcification by coccolithophores generates a negative feedback to acidification. The reduced export of alkalinity enables additional dissolution of CO2, decreasing the atmospheric concentration of this greenhouse gas.

## Other impacts of ocean acidification

Ocean acidification will likely affect marine ecosystems globally. Most studies up to now have focused on single species. Some studies conducted near shallow volcanic seeps showed that algal communities transplanted on recruitment tiles in these CO2-enriched waters became dominated by turf algae with lower biomass, diversity and complexity, a pattern consistent across seasons. Algal communities recovered after being transplanted back to non-enriched conditions[15]. A comprehensive open-access review of possible impacts of ocean acidification on marine benthic ecosystems is provided by Somma et al. (2023[16]).

## Related articles

Ocean carbon sink
Effects of global climate change on European marine biodiversity
Greenhouse gas regulation
Blue carbon revenues of nature-based coastal protection

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 The main author of this article is Job DronkersPlease note that others may also have edited the contents of this article. Citation: Job Dronkers (2023): Ocean acidification. Available from http://www.coastalwiki.org/wiki/Ocean_acidification [accessed on 24-02-2024] For other articles by this author see Category:Articles by Job Dronkers For an overview of contributions by this author see Special:Contributions/Dronkers J