Environmental Engineering Reference
In-Depth Information
fluorescence is decreased microbially (Table
4
) (Garcia et al.
2005
). In estuarine
water, the decrease in fulvic acid-like fluorescence is relatively high: 11 % at
peak A, 1-12 % at peak C, and 1-12 % at peak M in replicate samples during
a 51 days incubation period (Table
4
) (Moran et al.
2000
). In Mediterranean Sea
samples, DOM fluorescence is either increased (0.4-8 %) after 8 h dark incuba-
tion in lagoon water, or decreased (2 %) in coastal water (Table
4
) (Abboudi et
al.
2008
). Upon microbial processing, the fluorescence of standard SRFA does not
change significantly after 10 h incubation (Table
4
). A fluorescence increase after
a 9-day incubation period has been detected in fulvic (8 %) and humic acid (6 %)
extracted from Göta River (Table
4
). These results may lead to the hypothesis that
there are several characteristic chemical and optical features of fulvic acid and
FWAs in natural waters, which can be classified as: (i) Fluorescent compounds,
particularly fulvic and humic acids (C-like) in stream, are typically recalcitrant to
microbial degradation. Microbes are not capable of decomposing the fluorophores
in the longer wavelength region, particularly the peak C-region in allochthonous
fulvic and humic acids. Fluorophores of humic substances at peak C-region are
mostly composed of aromatic molecules associated with functional groups having
extensive
π
-electron systems, or with specific repeating functional groups in the
carbon matrix of peak C-region, which are highly recalcitrant to microbial degra-
dation (Mostofa et al.
2009a
,
2010
; Malcolm
1985
; Geller
1986
; Münster
1991
).
(ii) Allochthonous fulvic acid (M-like) is highly labile to biological degradation,
thus it has opposite behavior than allochthonous fulvic acid (C-like). (iii) Under
dark incubation, the increase of fulvic acid-like fluorescence is typically higher in
surface lake water compared to the deeper layers. This finding allows the hypoth-
esis that surface photo-bleached fulvic acid is highly labile to microbial changes,
which can lead to a significant increase in fulvic acid-like fluorescence. (iv)
Finally, the decrease in fulvic acid-like fluorescence is insignificant (0-5 %). This
fluorescence may presumably result from the decomposition of other fluorescent
components present in the same peak position.
Autochthonous Fulvic Acid (C-like) of Algal Origin
The fluorescence intensities of peaks A and C of autochthonous fulvic
acid (C-like), produced during the microbial assimilation of algal biomass
(Algae
+
river waters) during long-term dark incubation (180 days), were maxi-
mal at the 4th day of incubation. The intensities then became lowest at the 20th
day, gradually increased until the 70th day and increased again after the 80th day
till the 180th (Fig.
6
). This result suggests that the microbial processes can alter
rapidly the fluorophores bound at peaks A- and C-regions of autochthonous fulvic
acid (C-like) during the initial 20 days of incubation. It follows a slower microbial
alteration of fluorophores from 20 to 70 days, and an even slower alteration from
80 to 180 days. From 20 days onward, fluorescence intensities of both peaks grad-
ually increase. This result can be interpreted by the ratios of fluorescence intensi-
ties of peak A divided by peak C of autochthonous fulvic acid (C-like). Such a
