CHAPTER 3 . Photooxidation of Dissolved Organic Matter
Iron catalyzes the light-induced oxidation of humic substances where both Fe(III)
and HO· (formed in the Fenton reaction) act as oxidants. A redox cycle compared to
the one depicted in Fig. 3.4 should be a common phenomenon in iron-rich sunlit surface waters, since both Fe(II) and hydrogen peroxide are rapidly produced by photochemical reactions involving humic substances. Questions which remain to be answered are:
1. Does photooxidation of humic substances via ligand-to-metal charge-transfer reactions of Fe(III)-humate complexes lead to the formation of bioavailable carbon
compounds, as does the iron-independent photooxidation of humic substances?
2. Are Fe(III)-humate complexes major Fe(III) species in the euphotic zone of surface
waters?
Regarding the catalytic effect of iron on the light-induced oxidation of colored dissolved organic matter (eDOM), a relevant question is the following: What are the combined effects of increased solar UV radiation, due to stratospheric ozone depletion,
and of iron on eDOM photo oxidation? Increased UV radiation is expected to accelerate the redox-cycling of iron and in turn the oxidation of eDOM. If thereby biologically available carbon compounds are formed, these will be back-transformed to CO2
and water through respiration. As a consequence, UV light will penetrate deeper into
surface waters (see Eq. 3-4), and hence impact aquatic organisms, e.g. bacterio- and
phytoplankton. It has been demonstrated (Karentz et al. 1994) that fresh water and
marine bacteria are impacted by changes in solar UV radiation, and action spectra
indicate that UV-B radiation (280-315 nm) is mainly involved (Calkins and Barcelo 1982).
This has been confirmed by field studies which indicate that current levels of solar
UV radiation reduce bacterioplankton growth in the upper ocean (Herndl et al. 1993).
References
Andreae MO, Ferek RJ (1992) Photochemical production of carbonyl sulfide in seawater and its emission to the atmosphere. Global Biogeochem Cycles 6:175-183
Averett RC, Leenheer JA, McKnight DM, Thorn KA (1990) Humic substances in the Suwannee River,
Georgia: Interactions, properties, and proposed structures. U.S. Geological Survey Open File Report No. 87-557, Denver, Colorado
Blough NV (1997) Photochemistry in the sea-surface microlayer. In: Liss PS, Duce R (eds) The sea
surface and global change. Cambridge University Press, Cambridge, pp 383-424
Blough NV, Green SA (1995) Spectroscopic characterization and remote sensing of nonliving organic
matter. In: Zepp RG, Sonntag C (eds) Role of nonliving organic matter in the earth's carbon cycle.
Wiley-Interscience, New York
Blough NV, Zepp RG (1995) Reactive oxygen species in natural waters. In: Foote CS, Valentine JS (eds)
Reactive oxygen species in chemistry. Chapman and Hall, London, pp 280-333
Blough NV, Zafiriou ~C, Bonilla J (1993) Optical absorption spectra of waters from the Orinoco River
outflow: Terrestrial input of colored organic matter to the Caribbean. J Geophys Res 98:2271-2278
Bruccoleri A, Pant BC, Sharma DK, Langford CH (1993) Evaluation of primary photoproduct quantum yields in fulvic acid. Environ Sci Technol 27:889-894
Bushaw KL, Zepp RG, Tarr MA, Schulz-Jander D, Bourbonniere RA, Hodson RE, Miller WL, Bronk
DA, Moran MA (1996) Photochemical release of biologically available nitrogen from aquatic dissolved organic matter. Nature 381:404-407
Calkins J, Barcelo J (1982) Action spectra. In: Calkins J (ed) The role of solar ultraviolet radiation in
marine ecosystems. Plenum Press, New York, pp 143-150
Crumbliss AL (1991) Aqueous solution equilibrium and kinetic studies of iron siderophore and model
siderophore complexes. In: Winkelmann G (ed) CRC Handbook of microbial iron chelates. CRC
Press, Boca Raton, pp 177-233
Iron catalyzes the light-induced oxidation of humic substances where both Fe(III)
and HO· (formed in the Fenton reaction) act as oxidants. A redox cycle compared to
the one depicted in Fig. 3.4 should be a common phenomenon in iron-rich sunlit surface waters, since both Fe(II) and hydrogen peroxide are rapidly produced by photochemical reactions involving humic substances. Questions which remain to be answered are:
1. Does photooxidation of humic substances via ligand-to-metal charge-transfer reactions of Fe(III)-humate complexes lead to the formation of bioavailable carbon
compounds, as does the iron-independent photooxidation of humic substances?
2. Are Fe(III)-humate complexes major Fe(III) species in the euphotic zone of surface
waters?
Regarding the catalytic effect of iron on the light-induced oxidation of colored dissolved organic matter (eDOM), a relevant question is the following: What are the combined effects of increased solar UV radiation, due to stratospheric ozone depletion,
and of iron on eDOM photo oxidation? Increased UV radiation is expected to accelerate the redox-cycling of iron and in turn the oxidation of eDOM. If thereby biologically available carbon compounds are formed, these will be back-transformed to CO2
and water through respiration. As a consequence, UV light will penetrate deeper into
surface waters (see Eq. 3-4), and hence impact aquatic organisms, e.g. bacterio- and
phytoplankton. It has been demonstrated (Karentz et al. 1994) that fresh water and
marine bacteria are impacted by changes in solar UV radiation, and action spectra
indicate that UV-B radiation (280-315 nm) is mainly involved (Calkins and Barcelo 1982).
This has been confirmed by field studies which indicate that current levels of solar
UV radiation reduce bacterioplankton growth in the upper ocean (Herndl et al. 1993).
References
Andreae MO, Ferek RJ (1992) Photochemical production of carbonyl sulfide in seawater and its emission to the atmosphere. Global Biogeochem Cycles 6:175-183
Averett RC, Leenheer JA, McKnight DM, Thorn KA (1990) Humic substances in the Suwannee River,
Georgia: Interactions, properties, and proposed structures. U.S. Geological Survey Open File Report No. 87-557, Denver, Colorado
Blough NV (1997) Photochemistry in the sea-surface microlayer. In: Liss PS, Duce R (eds) The sea
surface and global change. Cambridge University Press, Cambridge, pp 383-424
Blough NV, Green SA (1995) Spectroscopic characterization and remote sensing of nonliving organic
matter. In: Zepp RG, Sonntag C (eds) Role of nonliving organic matter in the earth's carbon cycle.
Wiley-Interscience, New York
Blough NV, Zepp RG (1995) Reactive oxygen species in natural waters. In: Foote CS, Valentine JS (eds)
Reactive oxygen species in chemistry. Chapman and Hall, London, pp 280-333
Blough NV, Zafiriou ~C, Bonilla J (1993) Optical absorption spectra of waters from the Orinoco River
outflow: Terrestrial input of colored organic matter to the Caribbean. J Geophys Res 98:2271-2278
Bruccoleri A, Pant BC, Sharma DK, Langford CH (1993) Evaluation of primary photoproduct quantum yields in fulvic acid. Environ Sci Technol 27:889-894
Bushaw KL, Zepp RG, Tarr MA, Schulz-Jander D, Bourbonniere RA, Hodson RE, Miller WL, Bronk
DA, Moran MA (1996) Photochemical release of biologically available nitrogen from aquatic dissolved organic matter. Nature 381:404-407
Calkins J, Barcelo J (1982) Action spectra. In: Calkins J (ed) The role of solar ultraviolet radiation in
marine ecosystems. Plenum Press, New York, pp 143-150
Crumbliss AL (1991) Aqueous solution equilibrium and kinetic studies of iron siderophore and model
siderophore complexes. In: Winkelmann G (ed) CRC Handbook of microbial iron chelates. CRC
Press, Boca Raton, pp 177-233