Global warming, an important topic of our daily life. CO2 in the atmosphere plays a major role when looking at Climate change. Due to the industrial revolution and our days fossil burning industry, the earth`s atmospheric CO2 will increase more and more in the next 200 years. By increasing the amount of CO2 in the atmosphere, the partial pressure of CO2 (pCO2) will increase as well (Caldeira and Wickett, 2003). This enhancement will cause the ocean pH to drop (Acidification) from 8.4 to 7.4 (Figure 1). More acidic water will lead to a lot of changes for marine ecosystems. It is known, that a lower pH will affect calcifying organisms and other biological processes (Orr et al 2005). But a more acidic pH level can also change the speciation of organic and inorganic metals in ocean surface waters. Major factor is the decrease of hydroxide (OH-) and carbonate (CO32-) concentrations, which can result in changing solubility, adsorption, toxicity or the rates of redox reactions. Hydroxide and Carbonate are forming strong complexes with in surface water dissolved divalent (Baes and Mesmer,1976; Byrne et al., 1988; Millero and Hawke, 1992) and trivalent (Millero,1992; Millero et al., 1995; Cantrell and
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Byrne, 1987; Millero 2001b) metals. In the next 200 years these anions are expected to decrease by a high percentage. This loss of ions can have a great impact on speciation of dissolved metals (Byrne, 2002; Millero, 2001a,b).
Figure 1: expected pH and pCO2 changing
Source: M i l lero, Woosley, DiTrolio, Waters, 2009) and (Caldeira and Wickett 2003)
2. Effect on Inorganic Metal Speciation
It is important to take a look on different inorganic metals, how the speciation could change and how it could affect the ocean environment. Millero, Woosley, Benjamin DiTrolio, and Water studied the effect of ocean acidification on metal speciation in 2009.
Five groups of trace metals are dissolved in seawater, classified by their dominant ligand. Groups are: Hydrolyzed (OH-): e.g. Fe 3+, Carbonate (CO32-): e.g. Cu2+,Chloride (Cl-), Free and Mixed. Only metals that form strong complexes with hydroxide and carbonate will have changes in speciation when the pH in the ocean drops. To examine the effect of pH on certain metal speciation, you can use the ionic Pitzer (1991) interaction model. This model depends on the stability constant (β), for forming a complex in H2O (Millero and Pierrot, 1998, 2002). By looking at hydroxide, the complex forming is shown as a stepwise hydrolysis of the metal. Carbonate is different, because of a TCO32- constant (Total carbonate constant) to free carbonate. Due to this ratio the complexation of carbonate can be shown with the total ion concentration.
Mn+ + iH2O = M(OH)i(n - i) + iH+
βi = [M(OH)i(n - i)] [H+]i / [Mn+]
CO3βk = [M(CO3)k]/([Mn+] [CO32-]Tk)
Only the formation of strong complexation, are shown in the pitzer interaction model. The model illustrates the effect of the main compounds of seawater on metals. Therefore the determination of stability constants in the ocean is possible. The changes in speciation are mainly affected through decreasing concentrations of OH- and CO32-.(Figure 2)
Figure 2: Dropping OH- and CO32- concentration
Sources: Millero and Pierrot, 1998, 2002
Looking at the example copper (Cu2+), a metal that represents many other metals, by speciation and the changes of CO32-. Copper is a metal that forms strong complexes with carbonate. A change in pH will strongly affect this metal, that results in an increase of its free ionic form. The future increase is about 30% (Figure 3). Free copper is toxic to organisms (Steeman-Nielsen band Wium-Anderson, 1970; Sunda and Ferguson, 1983).
Metals that form a strong complex with hydroxide, such as aluminium (Al3+) are not affected by the increase of their free forms. But there will be a different change, a shift to less hydroxides per metal ion ( i.e. Al(OH)+4 -> Al(OH)3). The most significant change will be the Al(OH)3 complex with an increase of 36%. Other examples are lead and yttrium, placed in the mixed group of metals. Lead can form complexes with either chloride or carbonate. When the pH level of the ocean drops, the free form of lead will increase by 10%. Yttrium has more speciation's, therefore the free form will increase by 7% (Cantrell and Byrne, 1987).
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Figure 3: Copper speciation in time Source: Millero and Pierrot, 1998, 2002
Ocean acidification will also affect estuarine systems, when low pH (7.4) water mixes with lower pH (6) from river waters, it could change biogeochemical processes in the system (Hofmann et al., 2009). For example copper in its free form can be more toxic as normal. Another important topic is the solubility. The solubility of trivalent metals depend on the pH, some are more soluble in acidic or basic conditions. The location of the minimum is somewhere in between the two pH areas. This minimum can help to determine, if the solubility of a trivalent metal will increase or decrease. Iron(III) will increase in solubility by 40%( Liu and Millero, 2002 )(Figure 4), it can change biogeochemical cycles with Iron(III) as micronutrient (Brand, 1991) or the primary production (Martin, 1990).
Figure 4: Solubility of Iron(III) in SW Sources: Liu and Millero, 2002
Overall acidification of the world oceans could have a harmful effect on the primary production, through an increase of free ionic copper, or it can lead to a stimulation through dissolved iron(III) (Millero; Woosley; Benjamin DiTrolio; and Water 2009).
3. Effect of Iron and Copper on Marine Phytoplankton and Primary Production
Iron, most sensitive to pH (Shi 2010), is a key element in surface water biogeochemical cycles and a limited nutrient for primary production (Barbeau, 2006). Phytoplankton (single-celled algae) need iron for many inner cellular functions and processes such as electron transfer, nitrate reduction and most important the synthesis of chlorophyll, a pigment for photosynthesis.(Bowie et al. 2001) The natural anthropogenic addition of iron stimulates primary production in the ocean high nutrient-low chlorophyll zones (HNLC). Carbon fixation, also a characteristic of iron additions depends on organic ligands, natural or produced by bacteria. Organic ligands control the stabilization or the solubility of iron (III) at a neutral pH (Jones, 2011). Phytoplanton growth in high nutrient-low chlorophyll zones can be stimulated by iron addition (Buma et al. 1991, De Baar et al. 1990), therefore the increase of iron in the ocean can lead to an enhancement of the biological pump. A higher primary production also results in a higher carbon fixation, which may mitigate climate change (cooling effect) (Martin, 1990). This information was summarized by Martin in the 'Iron Hypothesis'.
Copper is a trace metal, such as Iron, it is essential (small amount) for the most living organisms on earth, but when the concentration increases it can become toxic. Copper concentrations can either have a positive or negative effect on phytoplankton and therefore for primary production (Coale, 1991). Even low concentrations of copper (II) ions can inhibit the phytoplankton activity severely. Copper can inhibit growth (Aliotta et al., 1983; Baker et al., 1983; Kessler, 1986) and interfere with cellular processes such as photosynthesis, respiration or cell division (Stauber
and Florence, 1987; Ahmed and Abdel-Basset, 1992; Guanzon et al., 1994). The free ions (Cu2+) are highly attracted to the ligand site on the cell membranes. A bonding can lead to a ion transfer blockade.
The approximate environmental level (addition) of copper (1.6x10-8 M) can cause a reduced phytoplankton growth (Davey et all, 1973; Steemann, Nielsen and Wium-Anderson, 1971). The pH is one major factor that controls the total copper concentration (Sunda and Guillard, 1976; Anerson and Morel, 1978; Sunda and Lewis, 1978). A decrease in pH can result in an increase in copper (or every metal, e.g. Uranium) toxicity (Rai et al., 1993, 1994). The increase is due to the predominance of the free ionic form of metals (Starodub et al., 1987;
Rai et al., 1993, 1994). On the other hand, some studies show a decrease of metal toxicity with a decrease in pH (Steemann Nielsen and Kamp-Nielsen, 1970). The studies are related to a reduced metal uptake through H+ competition at the membrane.
Overall the most studies show that high concentrations of copper (free ionic, as Cu2+) are toxic for phytoplankton and thus causes a negative effect on primary production.
A decrease in ocean pH level (acidification) from 8.4 to 7.4 in the next 200 years will lead to many changes, not only for calcifying organisms. A lower pH can also effect dissolved metals and their speciation. This effect can cause a concentration increase or decrease of different types of metals. Most important are metals that are sensitive to pH such as iron and copper, which both form complexes with either hydroxide or carbonate.
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Iron(III), is one of the main speciation's of Iron, that will increase by 40% in the next 200 years (Liu and Millero, 2002). It is known that iron(III) has a positive effect on phytoplankton growth and therefore on primary production. Higher primary production can lead to a higher carbon fixation and a mitigation of climate change (Martin, 1990).
Another limited micronutrient is copper, in low concentrations important for the most organisms on earth. A dropping pH can cause the concentration of the free ionic form (Cu2+) to enhance. Free ionic copper in high concentration has a toxic effect on phytoplankton and other organisms. For example, copper is also used as an algaecide in garden ponds (Fitzgerald and Faust, 1963).
Climate change accompanied by ocean acidification will have many impacts on marine biology and chemistry, but it is still not known how these changes will actually affect life on earth in the future.