in climate change due to global warming, although CO2 shows ... Change of chemical balance ... complexes with organic ligands in Dissolve Organic Matter.
2011 Seventh International Conference on Natural Computation
Chemical modeling of marine trace metals Effects of ocean acidification to marine ecosystem
Katsumi Hirose Department of Material and Life Sciences Faculty of Science and Technology, Sophia University Tokyo, Japan Abstract—Chemical thermodynamic model is introduced to evaluate ecological effects of ocean acidification, which is predicted from increasing ocean uptake of carbon dioxide. The chemical model contains metal complex formation with two types of dissolved organic ligands in seawater. Concentration responses of copper and iron species in seawater to pH decrease were simulated. Free copper ion concentration, whose level is closely related to toxicity to phytoplankton, shows no response to ocean acidification as buffering effects by organic ligands. Ocean acidification leads to increase of bio-available iron (organic iron complex) in seawater, which causes increasing marine primarily production and following export flux of carbon into ocean interior. Chemical model regarding bioactive trace metals suggests presence of a negative feedback to the rising atmospheric CO2. Keywords-chemical model; marine system; trace metal; organic ligand; copper; iron
I.
INTRODUCTION
Large amounts of pollutants have been released in the earth environment due to increasing human economic activities. Anthropogenic pollutants contain many kinds of chemical substances, which exhibit highly toxicity in ecosystem such as Hg and As. Typically, increasing emission of carbon dioxide (CO2), which is directly related to economic activities, results in climate change due to global warming, although CO2 shows less direct toxicity for biota. A significant part of CO2 released in the atmosphere is adsorbed in seawater. Since CO2 dissolved in seawater behaves an acid, marine ecological effects due to ocean acidification is current concern in the field of marine biogeochemistry [1]. Model simulation of the carbon cycle in the earth system [1,2] predicted that the ocean would be acidified at pH7.9 by 2100 as a result of anthropogenic CO2 uptake. Ocean acidification also significantly affects chemical balances, especially behavior of bioactive trace metals, in marine chemical system [3]. Change of chemical balance between trace metal species may lead serious effects to marine ecosystem including biological diversity. Therefore, to predict future ecological effects in the marine environment, it is necessary to construct ecological modeling including chemical processes. In this paper, we describe chemical modeling of marine trace metal (copper (Cu) and iron (Fe)) system including complex formation with two kinds of natural organic ligands and discuss about response of chemical balance of bioactive trace metal species to ocean acidification.
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II.
BACKGROUND
A. Chemical speciation of trace metals in seawater The common chemical forms of bioactive dissolved metals such as Copper (Cu), Iron (Fe) and Zinc (Zn) in seawater are complexes with organic ligands in Dissolve Organic Matter (DOM) [4]. According to speciation studies of Dissolved Organic Ligands (DOL) bound with Cu, Fe and Zn in seawater, two classes of DOL coexist in seawater [5,6]. These are classified as type-I (DTPA: diethylenetrinitrilopenta-acetic acid) and type-II (EDTA: ethylendiaminetetra-acetic acid) ligands [4,7,8]. Fe reacts mainly with the type-1 ligand in DOM [9]. A presently unknown issue is why Cu reacts with two types of dissolved organic ligands while Fe reacts with the stronger ligand in the marine environment. This is closely related to metal toxicity and bioavailability of Cu and Fe [4]. Although our understanding of the chemical properties of organic ligands in oceanic DOM is limited, the ability of organic ligands to react with metals in seawater may be summarized as follows. A strong ligand, L1, which reacts with hard metal ions such as Fe(III) and thorium by electrostatic interaction, is detected in DOM, Particulate Organic Matter (POM) [4] and microorganisms, bacteria [10], and has lower concentrations of 1 to 3 nM in DOM [11], whereas a weaker ligand (type-II, L2, which reacts with Cu and Zn, is detected only in DOM and exhibits higher concentrations of 10 to 100 nM in DOM [4,5]. B. Chemical modeling of trace matal system in seawater The concept of free metal ion concentration in seawater is essential to a better understanding of the ecological roles of trace metals in seawater. The free metal concentration is controlled by organic and inorganic complex formation, as is the total metal concentration. The free metal ion concentrations are ecologically important factors because metal toxicity and deficiency for marine organisms are directly related to levels of free metal ions in seawater. In order to clarify the variations in the free metal ion concentration under the conditions of seawater, we have constructed a chemical equilibrium model including two organic ligands, L1 and L2, in DOM, as well as inorganic complex formation with hydroxide, carbonates, and others. Seawater is chemically a very complicated system because it contains all naturally occurring elements with different concentration levels along with unidentified organic materials
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[12]. The presence of living organisms makes it difficult to construct chemical models of marine ecological systems. However, concentrations of major ions and their constitutions are generally uniform in open surface waters. Taking into account pH of surface waters, a chemical equilibrium model based on chemical thermodynamics can be constructed for the seawater system. Historically, chemical models that include interactions with inorganic ligands and minerals have been introduced to explain the concentration levels of elements dissolved in seawater, in which earlier attempts were conducted by Sillén [13]. However, the chemical equilibrium model appeared inadequate to explain the behavior of the trace metals because of the presence of active particle transport due to biological activity. There has been little attention on the chemical equilibrium model including complex formation with organic ligands in DOM, although knowledge on dissolved organic ligands has accumulated during the past two decades [4]. However, ocean acidification requires introduction of chemical processes in marine ecological modeling. In order to construct a chemical model of the marine system, we consider chemical equilibriums between the a+ 2+ 2+ 3+ bioactive trace metal ions (M : Cu , Zn , Fe ) and organic bc(Li ) and inorganic (Ii : OH-, CO32-, and others) ligands in seawater. The chemical equilibriums are written as follows: a+
M
+
Lib-
↔
MLi(a-b)+
Ma+ + Iic- ↔ MIi(a-c)+ (a-b)+
(1) (2)
(a-c)+
where MLi and MIi show organic and inorganic complexes with the metal ion, respectively. The conditional stability constants of organic and inorganic metal complexes in seawater (KMLi and KMIi) are defined as follows: a+
KMLi = [M Li’]([ M ][Li’]) (a-c)+
KMIi = [MIi
a+
-1
c-
]([ M ][ Ii ])-1
(3) (4)
where [Li’] denotes the total concentration of ligand not bound by other metal ions; [Li’] = Σ[LiHj(b-j)+]. [MLi’] shows the total concentration of metal complexes, generally including (a+j-b)+ protonated species such as MHjLi . The total concentration of a metal in seawater (CM) is given by the following equation:
CM = [M] + [ML1’] + [ML2’] + [MI1] + [MI2] (5) where [M] denotes the free ion concentration of the metal in seawater, in which charges of metal ions, ligand and complexes are abbreviated. ML1 and ML2 represent metal complexes with dissolved organic ligands in seawater. M1I1 and MI2 represent metal complexes with inorganic ligands; e.g., Cu forms complexes with carbonate and hydroxide [14], which are major anionic species in seawater. Formation of inorganic complexes of trace metals depends of pH because most of the inorganic ligands behave as a kind of base.
The total ligand concentrations in seawater are expressed by the following equation:
CLi = [Li] + [LiH] + [LiH2] + [CaLi] + [MgLi] + [MLi’] + [MlLi’] = [Li’]αLi(H,Ca,Mg) + [MLi’] i=1,2 (6) where αLi(H, Ca,Mg) is the side reaction coefficient of a ligand with proton and metal ions other than M; αLi(H,Ca,Mg) = 1 + 2+ 2+ KHLi[H+] + KH2Li[H+]2 + KCaLi[Ca ] + KMgLi[Mg ] + KMlLi[Ml’], where KHLi and KH2Li are the proton-addition constants of natural organic ligands at the first step and the second step, respectively. From Equations 5 and 6, we can derive a general description of the relationship between total metal and ligand concentrations in seawater by using the conditional stability constants of metal complexes under the conditions of seawater.
CM = [M] + [ML1’] + [ML2’] = [M]αM(I) + KML1CL1[M](αL1(H,Ca,Mg) + KML1[M])-1 + KML2CL2[M](αL2(H,Ca,Mg) + KML2[M])-1 (7) Here, αM(I) is the side reaction coefficient for complex formation of metal ion with the inorganic ligand; αM(I) = 1 + ΣKMIi[I]. The free inorganic ligand concentrations are directly related to hydrogen ion concentration. The free metal ion concentration in seawater can be calculated numerically as a function of total metal, ligand concentrations and pH from Equation 7. III.
RESULTS AND DISCUSSION
We applied a basic data set to simple chemical model calculation, which includes conditional stability constants of natural organic complexes, proton-addition constants of EDTA and DTPA, typical metal concentrations (Cu: 2 nM, Fe: < 1 nM), and ligand concentrations in seawater (L1: 1 nM, L2: 20 nM). These data were cited from Hirose [8] and Kotrly and Sucha [15]. A range of pH change (8.3 to 7.5) to predict for ocean acidification [1,2] was used for model simulation. A. Effects of pH change on reactivity of ligands Reactivity of the dissolved organic ligands to metal ions depends on pH because the dissolved organic ligands are a kind of base. However, there is little information on acid-base properties of the natural organic ligands (i.e. proton-addition constants). Speciation study of trace metals in seawater revealed that two types of organic ligands coexist in seawater: EDTA type (type-II) and DTPA type (type-I). We assume that dissolved natural organic ligands have the same protonaddition constants as EDTA and DTPA. Since both EDTA and DTPA have two proton-addition constants near pH=8 (EDTA: 9.95 and 6.97, DTPA: 9.48 and 8.26) [15], the free organic ligand concentrations are expected to change in the pH range of 8.5 to 7.5. In order to simulate response of the free ligand concentrations to pH change, it is important to elucidate complex formation reactions between organic ligands and
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major ions because most of the organic ligands react with Calcium (Ca) and Magnesium (Mg) ions [7], which are major components of seawater. We simulated the free ligand concentration change corresponding to pH change (8.5 to 7.5) in presence of Ca and Mg. The result indicates that there is no change of the free ligand concentration because of ligand buffer by Ca and Mg. This implies that reactivity of the organic ligands does not change in the pH range of 8.5 to 7.5 even if proton-addition constants of natural ligands slightly differ from EDTA and DTPA. B. Effect of pH change for trace metal species distributions We simulated changes of the free metal ion concentrations of typical bioactive metals, i.e., copper and iron, to predicted pH change in ocean. To elucidate ecological behaviors of copper, which shows biological toxicity in its higher concentration, we calculated pH dependency of the free copper ion (Cu2+) concentration for two cases; one is no presence of organic ligands, and another is presence of two organic ligands. For inorganic complex formation of copper, dominant copper species is a carbonate complex as shown in Fig. 1 even if pH decreases from 8.5 to 7.5 accompanied with increasing total inorganic carbon. The Cu2+ concentration increases to biological toxic level with decreasing pH. This finding suggests that copper toxicity for phytoplankton growth occurs at decreasing pH under absence of any dissolved organic ligands. In case of the presence of two organic ligands, the Cu2+ concentration was calculated as a function of pH. The result is shown in Fig. 2. We found there is very little change of the Cu2+ concentration in the pH range of 8.5 to 7.5, which is under a typical metal buffer. A similar result on the chemical role of the dissolved organic ligands was obtained by Millero et al. [3].
(Fe3+) concentration in seawater is primarily controlled by ion product of Fe(OH)3 because of extremely low solubility of ferric compounds in seawater. The thermodynamics on solubility of insoluble hydroxide compounds suggests that the Fe3+ concentration increase with decreasing pH. In this case, it is important to examine pH dependency of bio-available Fe concentration (organic Fe complex) because the only Fe3+ concentration is too low to support phytoplankton growth. We simulated response of the concentration of the organic Fe3+ complex to pH change in the presence of Cu. The result is shown in Fig. 3. The concentrations of the organic Fe3+ complex increase from 0.2 nM to 1 nM against pH decrease of 8.3 to 7.5. This finding suggests that acidification is more favorable to Fe uptake by phytoplankton because acidification enhances amounts of the bio-available Fe(III). Increasing Fe bioavailability due to ocean acidification was supported by a coastal seawater mesocosm experiment [18]. This result suggests a possibility that ocean acidification mitigates iron deficiency in high nutrient and low chlorophyll regions of open
Figure 2. pH dependency of logarithmic Cu2+ concentration in coexistence of organic ligands
Figure 1. pH dependency of each inorganic Cu species in seawater.
A representative trace metal in the marine environment is Fe, which is one of the most important bioactive elements for phytoplankton growth [16]. In contrast to Cu, Fe deficiency occurs in the wide areas of the open ocean [17]. In particular, Fe is a limiting factor of primary production of phytoplankton in the high-nutrient low chlorophyll (HNLC) regions. Free Fe
Figure 3. pH dependency of total Fe concentration (organic Fe complex)
ocean. Mitigation of iron deficiency would enhance primarily production of phytoplankton in the HNLC region and lead to
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increase following carbon export flux in ocean interior, which may be a negative feedback of increasing atmospheric CO2. IV.
CONCLUSION
Increasing carbon dioxide in atmosphere causes not only climate change but also serious influences to ecosystem. Especially, to evaluate ecological effects from ocean acidification by CO2 uptake is one of current concerns. To predict ecological effects of ocean acidification, it is essential to construct modeling involving chemical processes. Chemical model on trace metal in seawater reveals that ocean has a chemical mechanism that the free metal concentration such as Cu2+ is maintained at an optimal level for phytoplankton growth even if predicted acidification of seawater occurs. Furthermore, acidification may enhance primarily production as a result of increasing bio-available Fe. Generally, natural system contains complicated chemical processes in addition to physical and biological processes. In order to predict future ecological effects due to increasing emission of anthropogenic pollutants, physical and ecological modeling containing chemical processes is required. ACKNOWLEDGMENT K.H. thanks P. Povinec to provide an opportunity to present our work. REFERENCES [1] [2]
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