293
Photoinduced and Microbial Degradation
fractions (<0.10 μm) in surface layers and 8 % in deeper layers (Table 1). However,
DOM with molecular fractions of <5 kDa is not altered at all under dark incubation
(Table 1). These results can suggest four important features about microbial degradation of DOM in natural waters. First, molecular fractions of <0.1 μm in both lake
surface and deeper waters are labile to microbial degradation, but deeper waters are
much labile than surface waters. It is also suggested that autochthonous DOM in
surface waters, typically produced during the summer stratification period, is less
susceptible to microbial processes. It can be noted that the DOC concentration in
Lake Biwa waters is autochthounously produced (45 %) during the summer stratification period, as estimated from the summer DOC values (135 μM C in August)
that are higher than the winter ones (~93 μM C in January) during vertical mixing
(Mostofa et al. 2005). Second, DOM with molecular weight <5 kDa in both surface
and deeper layers is not susceptible to microbial degradation.
2.3 Mechanisms of the Photoinduced Degradation of DOM
in Natural Waters
The HO
• radical plays a significant role in photoinduced degradation of DOM
in natural waters (Zepp et al. 1992; Southworth and Voelker 2003; Zafiriou et al.
1984; Zika 1981; Voelker et al. 1997). Photoinduced degradation of DOM generally occurs upon direct absorption of UV and visible sunlight by functional groups
in DOM, which are optically detected either as chromophores in CDOM or fluorophores in FDOM. Evidence has been provided that an electron transfer from functional groups on DOM can lead to the photoinduced formation of H 2 O 2 in aqueous
solution (Eqs. 3.13–3.18, see also chapter “Dissolved Organic Matter in Natural
Waters”) (Mostofa and Sakugawa 2009; Senesi 1990). H 2 O 2 subsequently leads to
the generation of HO
•
, by direct photochemistry or by Fenton/photo-Fenton/photoferrioxalate reaction systems. These processes can be involved into the photo transformation of DOM in natural waters. Therefore, a general mechanistic scheme for
photoinduced degradation of DOM can be depicted as follows (Eqs. 2.1–2.4):
First, H 2 O 2 is formed photolytically through production of O 2
•− ion by the release
of electrons from DOM chromophores or fluorophores, due to solar effects (Eq. 2.1)
that have been discussed earlier in chapter “Photoinduced and Microbial Generation
of Hydrogen Peroxide and Organic Peroxides in Natural Waters”. Subsequent irradiation converts H 2 O 2 into HO
• either photolytically (Eq. 2.2), or via Fenton and
(2.1)
DOM + O 2 + H 2 O + H
+ hυ
→ H 2 O 2 + DOM
•+ + O 2 + OH
−
(2.2)
H 2 O 2
hυ
→ 2HO
•
(2.3)
DOM
•+ + HO
• hυ
→
DOM
•+ HO
• ∗
(2.4)
DOM
•+ HO
• ∗ hυ
→ LMWDOM + DIC + CO 2 + other byproducts
Photoinduced and Microbial Degradation
fractions (<0.10 μm) in surface layers and 8 % in deeper layers (Table 1). However,
DOM with molecular fractions of <5 kDa is not altered at all under dark incubation
(Table 1). These results can suggest four important features about microbial degradation of DOM in natural waters. First, molecular fractions of <0.1 μm in both lake
surface and deeper waters are labile to microbial degradation, but deeper waters are
much labile than surface waters. It is also suggested that autochthonous DOM in
surface waters, typically produced during the summer stratification period, is less
susceptible to microbial processes. It can be noted that the DOC concentration in
Lake Biwa waters is autochthounously produced (45 %) during the summer stratification period, as estimated from the summer DOC values (135 μM C in August)
that are higher than the winter ones (~93 μM C in January) during vertical mixing
(Mostofa et al. 2005). Second, DOM with molecular weight <5 kDa in both surface
and deeper layers is not susceptible to microbial degradation.
2.3 Mechanisms of the Photoinduced Degradation of DOM
in Natural Waters
The HO
• radical plays a significant role in photoinduced degradation of DOM
in natural waters (Zepp et al. 1992; Southworth and Voelker 2003; Zafiriou et al.
1984; Zika 1981; Voelker et al. 1997). Photoinduced degradation of DOM generally occurs upon direct absorption of UV and visible sunlight by functional groups
in DOM, which are optically detected either as chromophores in CDOM or fluorophores in FDOM. Evidence has been provided that an electron transfer from functional groups on DOM can lead to the photoinduced formation of H 2 O 2 in aqueous
solution (Eqs. 3.13–3.18, see also chapter “Dissolved Organic Matter in Natural
Waters”) (Mostofa and Sakugawa 2009; Senesi 1990). H 2 O 2 subsequently leads to
the generation of HO
•
, by direct photochemistry or by Fenton/photo-Fenton/photoferrioxalate reaction systems. These processes can be involved into the photo transformation of DOM in natural waters. Therefore, a general mechanistic scheme for
photoinduced degradation of DOM can be depicted as follows (Eqs. 2.1–2.4):
First, H 2 O 2 is formed photolytically through production of O 2
•− ion by the release
of electrons from DOM chromophores or fluorophores, due to solar effects (Eq. 2.1)
that have been discussed earlier in chapter “Photoinduced and Microbial Generation
of Hydrogen Peroxide and Organic Peroxides in Natural Waters”. Subsequent irradiation converts H 2 O 2 into HO
• either photolytically (Eq. 2.2), or via Fenton and
(2.1)
DOM + O 2 + H 2 O + H
+ hυ
→ H 2 O 2 + DOM
•+ + O 2 + OH
−
(2.2)
H 2 O 2
hυ
→ 2HO
•
(2.3)
DOM
•+ + HO
• hυ
→
DOM
•+ HO
• ∗
(2.4)
DOM
•+ HO
• ∗ hυ
→ LMWDOM + DIC + CO 2 + other byproducts