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In-Depth Information
with the results of field observations that the DOM fluorescence decreases with an
increase of H
2
O
2
concentration over the course of the day in marine surface waters
(Obernosterer et al.
2001
). An alternative possibility is that also other species differ-
ent from HO
•
may induce the transformation of DOM. Interestingly, the generation
rate of HO
•
was largely unable to account for the photoinduced mineralization of
acidified lake-water or filtered groundwater DOM (Vione et al.
2009
).
In addition, photodegradation of FDOM depends on several key factors such
as sunlight incident doses, water chemistry, DOM contents, mixing regime and so
on (White et al.
2003
; Ma and Green
2004
; Mostofa et al.
2011
; Reche et al.
1999
).
Moreover, the key factors that affect the FDOM photobleaching are: (1) Solar radia-
tion, (2) Water temperature, (3) Effects of total dissolved Fe and photo-Fenton reac-
tion, (4) Occurrence and quantity of NO
2
−
−
ions, (5) Molecular nature of
DOM, (6) pH and alkalinity of the waters, (7) Dissolved oxygen (O
2
), (8) Depth of the
water, (9) Physical mixing in the surface mixing zone, (10) Increasing UV-radiation
during ozone hole event, and (11) Global warming. These factors are similar to those
affecting the photoinduced degradation of DOM (see the chapter Photoinduced and
Microbial Degradation of Dissolved Organic Matter in Natural Waters).
and NO
3
3.3 Microbial Degradation of FDOM in Natural Waters
Microbial degradation by autotrophs (plants, algae and bacteria) and heterotrophs
(animals, fungi and bacteria) induces changes in FDOM in the deeper layers of natural
waters (rivers, lakes, and oceans), pore waters and soil waters. The effects have been
highlighted in field observations and experimentally under dark incubation. Microbial
processes thus alter the fluorescence properties of various FDOM such as fulvic acid,
humic acid, aromatic amino acids and FWAs (DAS1 and DSBP) (Fig.
5
) (Mostofa et
al.
2010
,
2007a
,
2005b
,
2007b
,
2011
; Ma and Green
2004
; Moran et al.
2000
).
Fulvic Acid and Humic Acid of Terrestrial Plant Material Origin
The microbial degradation can alter the fulvic acid-like fluorescence intensities
at peaks A-, C- and M-regions and their excitation-emission (Ex/Em) peak posi-
tions in natural waters (Ma and Green
2004
; Moran et al.
2000
; Mostofa et al.
2007b
,
2011
; Yamashita and Tanoue
2008
). Allochthonous fulvic acid (C-like)
fluorescence is increased by approximately 3-81 % in rivers due to microbial
degradation, for an incubation period ranging from hours to 13 days (Table
4
).
Allochthonous fulvic acid (M-like) fluorescence is entirely decomposed micro-
bially after 12 days of dark incubation at room temperature (Table
4
). In lakes,
the fulvic acid-like fluorescence is increased by up to 31 % in molecular frac-
tions <0.1
μ
m and 102 % in <5 kDa fractions in surface waters. In deeper
DOM fractions the corresponding increases are 0 and 20 % under dark incuba-
tion for 12 days (Table
4
) (Mostofa et al.
2011
). In contrast, fulvic acid-like
