Metals cycling

Complexation of metals has important interactive effects on biological availability and photochemical reactivity [117,152,165,196-225]. Iron, copper and manganese are essential micronutrients whose free metal ion concentrations in water, and hence biological availabilities, are affected by complexation or, in the case of metal oxides, by redox transformations. Complexation reduces biological availability by reducing free metal ion concentrations and dissolved iron [202] and copper [212,213] are quantitatively complexed by organic ligands. Solar UVR can interact with these processes by inducing direct photoreactions of the complexes, by enhancing redox reactions between the ligands and metal oxides, or by indirect photoreactions in which photochemically-produced ROS react with the complexes. Because both iron and manganese are limiting nutrients in parts of the sea [153,197], these photoreactions may play an important role in global biogeochemical cycles. To illustrate these possibilities, photoreactions involving iron, copper and manganese are discussed here.

5.5.1 Iron

In oxygenated water at circumneutral pH, Fe(m) (Fe3 + and complexes thereof) is extremely insoluble. The nature of the association between iron and DOM is imprecisely known [153], but interactions of the Fe(m) with DOM are known to increase its solubility and long-term retention in the water column. In addition to formation of soluble complexes, Fe(m) apparently can also associate with colloidal iron oxides and hydroxides thus producing high concentrations of filterable (0.4 fim) iron that reach over 10 micromolar in some coastal estuaries [59]. Whatever the exact nature of the iron complexes, field studies have provided ample evidence that their photoreduction occurs rapidly in most oxygenated natural waters [59,117,198,203-205,207-209,223,226], Photoreaction of Fe(m) results in production of Fe(n) (Fe2+ and its complexes) plus oxidized ligand. The Fe(n) produced in such photoreactions is stable in acidic environments [203], but it is rapidly oxidized in circumneutral or basic natural waters, including seawater [153,207,224]. Rapid hydrolysis of the resulting Fe(m) then occurs, followed by slow formation of iron hydroxides and eventually unreactive iron (hydroxy)oxides. This sequence of photochemically-related events can transform Fe(m) into colloids, as demonstrated in a study of the photoreactions of the Fe in a coastal river [59]. Photoreactions and subsequent colloid formation may explain the high concentrations of colloidal iron that has recently been identified in the surface ocean [152]. The net effect of the UV-induced photoreactions, however, is to enhance steady-state concentrations of biologically available (and chemically reactive), soluble species of Fe(in) and Fe (n).

A recent study has clearly demonstrated the importance of Fe(m) photoreactions in controlling the biological uptake of iron in seawater [206]. Certain soluble complexes of iron with various strong organic ligands such as sid-erophores (i.e., microbial iron(m) binding ligands) are photoreactive and photoreactions of these iron complexes enhance the biological availability of the iron [207] (Figure 10).

Fe(III)

Fe(III)L

Fe Colloids

Biotic assimilation

Specific bacterial uptake

Figure 10. Schematic summary of siderophore-mediated photochemical cycling of iron in seawater [206]. Fe(m)L represents a photoreactive iron(m)-siderophore complex, L+ is the oxidized ligand photoproduct, and Lt is another chelating ligand. The Fe(n) and its initial oxidation product, Fe(m) are readily assimilated by marine organisms but Fe(m) also can be readily hydrolyzed to Fe hydroxides which can then slowly polymerize to form unreactive, non-bioavailable iron(hydroxy)oxides. [Adapted from Barbeau et al. [206], Figure 4, p. 411, Copyright 2001, Nature.']

Moreover, iron also participates in the CDOM photoreactions and ROS reactions that were previously discussed. That iron plays a significant role in the photochemistry of coastal rivers was demonstrated by the inhibiting effects on DIC photoproduction of the addition of ligands (fluoride and desferal) that formed unreactive complexes with the iron [59] (Figure 11). Complexes of Fe(ni) with organic carboxylate moieties in CDOM or with acids such as oxalic acid in cloudwater are involved in a complex array of U V-initiated reactions that affect ROS dynamics (Figure 12). For example, iron appears to play a role in the photochemical production of OH radicals in natural waters through reactions between Fe(n) and hydrogen peroxide (H202), two reactive compounds that are produced by UV-induced photoreactions of CDOM [165,227], Light-induced photoreactions of iron complexes also contribute to production of Fe(n) in atmospheric cloud droplets, thus enhancing the biological activity of the iron in wet deposition to the sea [117,153,225,228].

Several possible mechanisms are available for UV-induced photoreactions of iron complexes. First, direct photoreactions involving ligand-to-metal charge transfer are likely to be one of the most important mechanisms for photoreaction [117,198,224]. Second, iron complexes can be reduced by photochemically-produced superoxide [207-209], Superoxide ions are formed via the photo-reduction of molecular oxygen by CDOM and it is one of the most concentrated radicals in seawater and is the precursor to hydrogen peroxide [Chapter 8], Superoxide-induced reduction of Fe(m) is an important mechanism in certain lakes [207], However, the fact that Fe(n) photoproduction can be more rapid in oxygen-free water than in air-saturated water in acidic estuaries [59] or model systems with well-defined organic acid complexes of Fe(m) [117] indicates that direct photolysis of Fe(m) is likely to be a dominant mechanism for Fe(n) photoproduction in many aquatic systems.

5.5.2 Copper

Copper is an essential trace element that is widely distributed in freshwaters and the sea. Human activities can release large amounts of copper into aquatic environments and there is concern about its potentially toxic effects on aquatic organisms. In the aquatic environment, copper is present predominantly as Cu(n) (Cu2+ and its complexes), a major fraction of which is complexed by organic substances of biological origin [153,210-213]. Terrestrial humic substances quantitatively complex copper in coastal waters [213], whereas strong ligands produced by marine organisms such as cyanobacteria, likely in response to Cu stress, chelate Cu in the open ocean [153,212]. Certain organocopper complexes are known to photoreact efficiently on absorption of UVR [214,215] and surface maxima in vertical profiles of Cu(i) (Cu+ and its complexes) in the upper layers of the Atlantic Ocean are consistent with a photochemical mechanism for Cu(n) reduction [210,211]. Natural organic substances such as amino acids and amines form complexes with copper on the surfaces of microorganisms via inner sphere types of coordination, and direct photolysis of such complexes

Figure 11. Retarding effects of added desferal (0.3 mM) on the photochemical production of DIC in a coastal river of the southeastern United States [59]. DIC concentration without (•) and with (o) added desferal. The effect is attributed to formation of unreactive Fe(m)-desferal complexes.

Figure 11. Retarding effects of added desferal (0.3 mM) on the photochemical production of DIC in a coastal river of the southeastern United States [59]. DIC concentration without (•) and with (o) added desferal. The effect is attributed to formation of unreactive Fe(m)-desferal complexes.

Fe(III)

Fe(III)

hv

-►

Fe(CDOM)

Prod.

Figure 12. UV-initiated reactions involving iron carboxylate complexes and ROS. Such reactions play an important role in controlling ROS and biologically available iron concentrations in surface waters and in condensed phases of the troposphere [59,117,207]

by solar UVR may contribute to the sunlight-induced reduction of Cu(ii) in natural waters [216]. Copper has a high affinity for aquatic biota and these photoreactions on cell surfaces may contribute to the biological damage caused by copper in polluted ecosystems [216]. Photoreactions of Cu(n)-humic complexes in polluted freshwaters also could produce biologically harmful concentrations of Cu+ and Cu2+ by stripping away the ligands that help minimize its free ion concentration.

Like iron complexes, copper complexes have been shown to be an important sink for photochemically-generated superoxide in seawater and, based on the high reactivity of Cu(n) complexed by cyanobacterial-derived ligands, it is likely that redox reactions with superoxide significantly influence Cu redox speciation in the ocean [221,222]. These reactions also have important effects on the steady state concentrations of superoxide in seawater, reducing the concentration by at least an order of magnitude compared to previous estimates that ignored the reactions with copper complexes [222],

5.5.3 Manganese

Like iron and copper, manganese is an essential trace element [153] that also is involved with several biologically significant redox processes in the environment. Manganese concentrations in natural waters range from < 1 nM in sea water up to several /iM in polluted natural waters. Manganese occurs in three oxidation states in the natural environment. In fresh waters, Mn(n) occurs mainly as the dissolved, uncomplexed species, but may adsorb to particulates. In sea water, chloride and sulfate complexes become important species, but the free Mn2+ still predominates. Manganese(m) and (iv) occur as insoluble oxides, referred to here as MnOx. Manganese(m) disproportionates to Mn(n) and Mn(iv) in both acid and alkaline solutions; at neutral pH it may be stable as the oxide of mixed oxidation state Mn304, or in colloidal suspension as MnOOH (manganite).

The interconversion of the various oxidation states of Mn in natural waters is influenced by UVR through its effects on reactions involving ROS [Chapter 8] and natural phenols, photoinduced charge transfer reactions, and microbial processes. The oxidation of Mn2+ is slow at pH < 8.5 in the absence of a catalyst. The oxidation of Mn(n) is faster on metal oxide surfaces than in homogeneous solution in the pH range of 8 to 9 [217], and its oxidation also can be biologically mediated in the environment [153]. In comparison to bacteria-free waters, the oxidation rate of Mn(n) in seawater is increased dramatically by catalysis on bacterial surfaces. However, even with such catalysis, its half-life still is of the order of weeks to months in open ocean waters [153].

In the absence of a catalyst, Mn(iv), Mn(m) and Mn(n) are interconverted by ROS such as H202 and 02~ through a complex set of redox reactions [218-220,229]. Manganese oxides also are reduced by humic substances, probably through reactions with phenolic moieties [231] and the reaction rate is enhanced by light [196,226,230-234]. As a consequence of this reduction of MnOx, there is a surface maximum of soluble Mn(n) in the open ocean that helps to increase its availability to phytoplankton. In addition, biologically-labile low-molecular-weight organic products are produced from the oxidation of the humic substances [196], The high Mn(n)/MnOx ratios in the upper layer of the sea have been attributed to both UV photoinhibition of Mn(n) oxidizing microorganisms and photoreduction of the manganese oxides [232]. The photoreduc-tion of MnOx by adsorbed aquatic humic substances is not greatly affected by removal of dioxygen, indicating that reduction primarily occurs via charge transfer from excited states of the sorbed humic substances on the oxide surface [231]. Although little is known about the nature of the oxidized substances resulting from these reactions, it is likely that the initial products are free radicals such as substituted phenoxyl radicals that interact with dioxygen to produce ROS.

These redox reactions of Mn have important effects on its oxidation state, solubility and biological availability in natural waters. As noted above, Mn(iv) and Mn(m) exist as insoluble oxides and Mn(iv) is the most thermodynamically stable form of manganese. Thus, as expected, Mn(iv) is the major form of Mn at great depths in the sea. In surface layers of the sea, however, almost all of the Mn exists as Mn(n).

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