Chapter 3
Photooxidation of Dissolved Organic Matter:
Role for Carbon Bioavailability and for the
Penetration Depth of Solar UV-Radiation
B. Sulzberger
3.1
Introduction
Most of the solar radiation that reaches land or water is converted into thermal
energy, but a significant part, especially that in the ultraviolet and visible region,
is diverted into photochemical and photobiological processes that affect the global
carbon cycle. The most prominent photobiological process on the earth's surface
is biological photosynthesis. Terrestrial vegetation and marine algae use the solar
energy to convert annually approximately 100 Gt (gigatons) of carbon in the form
of atmospheric carbon dioxide (C02) into organic matter (Zepp 1994). When plants
and algae die, the resulting non-living matter is transformed by various biological and chemical processes that either convert it back to CO2 (and other trace carbon gases) and water or to biologically refractory organic substances. The refractory organic matter is a mixture of substances, including litter and more refractory
compounds, a large portion of which consists of humic substances (Thurman 1985).
The term "humic substances" is usually used to refer to the organic matter that has
been isolated from natural waters or from soils using well-defined techniques (Frimmel and Christman 1988; Huber and FrimmeI1994). Humic substances make up
the largest single class of dissolved organic matter (DOM), accounting for 30 to
60% of the DOM in most natural waters (Thurman 1985). [The term "dissolved organic matter, DOM" is here used as synonym of "dissolved organic carbon, DOC."] The
term "colored dissolved organic matter (CDOM)" is used for the fraction of DOM
that is colored (Blough and Green 1995) and includes humic substances. Based
on Orinoco River data, Blough et al. (1993) estimated that only about 65% of the total
DOM absorbs solar radiation and is subject to direct photochemical reactions
(see Table 3.1).
Oceanic DOM is mainly of marine origin (Hedges et al.1992), although about 1 Gt of
carbon in the form of organic carbon reaches the ocean from terrigenous sources each
year (Meybeck 1982; Sarmiento and Sundquist 1992) (Fig. 3.1). On the time scale of
estuarine flushing, riverine DOM appears to be resistant to biological oxidation and
to pass through estuarine systems with little net loss or chemical alteration (Mantoura
and Woodward 1983). This requires a non-biological sink of terrestrial DOM or a
mechanism of converting it to a biologically labile form. There is growing evidence
that photo oxidation is such a mechanism and, perhaps, the rate limiting step of terrestrial DOM removal from the ocean (Kieber et al. 1990; Mopper et al. 1991; Miller and
Zepp 1995).
Photooxidation of Dissolved Organic Matter:
Role for Carbon Bioavailability and for the
Penetration Depth of Solar UV-Radiation
B. Sulzberger
3.1
Introduction
Most of the solar radiation that reaches land or water is converted into thermal
energy, but a significant part, especially that in the ultraviolet and visible region,
is diverted into photochemical and photobiological processes that affect the global
carbon cycle. The most prominent photobiological process on the earth's surface
is biological photosynthesis. Terrestrial vegetation and marine algae use the solar
energy to convert annually approximately 100 Gt (gigatons) of carbon in the form
of atmospheric carbon dioxide (C02) into organic matter (Zepp 1994). When plants
and algae die, the resulting non-living matter is transformed by various biological and chemical processes that either convert it back to CO2 (and other trace carbon gases) and water or to biologically refractory organic substances. The refractory organic matter is a mixture of substances, including litter and more refractory
compounds, a large portion of which consists of humic substances (Thurman 1985).
The term "humic substances" is usually used to refer to the organic matter that has
been isolated from natural waters or from soils using well-defined techniques (Frimmel and Christman 1988; Huber and FrimmeI1994). Humic substances make up
the largest single class of dissolved organic matter (DOM), accounting for 30 to
60% of the DOM in most natural waters (Thurman 1985). [The term "dissolved organic matter, DOM" is here used as synonym of "dissolved organic carbon, DOC."] The
term "colored dissolved organic matter (CDOM)" is used for the fraction of DOM
that is colored (Blough and Green 1995) and includes humic substances. Based
on Orinoco River data, Blough et al. (1993) estimated that only about 65% of the total
DOM absorbs solar radiation and is subject to direct photochemical reactions
(see Table 3.1).
Oceanic DOM is mainly of marine origin (Hedges et al.1992), although about 1 Gt of
carbon in the form of organic carbon reaches the ocean from terrigenous sources each
year (Meybeck 1982; Sarmiento and Sundquist 1992) (Fig. 3.1). On the time scale of
estuarine flushing, riverine DOM appears to be resistant to biological oxidation and
to pass through estuarine systems with little net loss or chemical alteration (Mantoura
and Woodward 1983). This requires a non-biological sink of terrestrial DOM or a
mechanism of converting it to a biologically labile form. There is growing evidence
that photo oxidation is such a mechanism and, perhaps, the rate limiting step of terrestrial DOM removal from the ocean (Kieber et al. 1990; Mopper et al. 1991; Miller and
Zepp 1995).