Environmental Engineering Reference
In-Depth Information
molecular fractions <0.1
μ
m (68 % in surface water and 5 % in deep water) and
<5 kDa (51 % in surface water and 28 % in deep water), and in estuaries (23-
112 %) (Table
4
). Upon microbial processing, the standard tryptophan fluores-
cence does not change significantly after 10 h incubation (Table
4
). From these
results it can be concluded that there are several characteristic phenomena con-
cerning microbial degradation of tryptophan-like components in natural waters.
First, tryptophan-like components are microbiologically labile but microbial deg-
radation is a relatively slow process whilst photodegradation is rapid (Moran et
al.
2000
; Mostofa et al.
2007b
; Baker and Inverarity
2004
). Second, an increase in
tryptophan-like fluorescence in filtered samples and a decrease in unfiltered sam-
ples can be rationalized considering that the filtration processes may deactivate
or hinder the bacterial activity. Therefore, if the fluorescence intensity decrease
in unfiltered samples may be due to the microbial degradation of tryptophan, the
increase in filtered samples might be the result of the binding of tryptophan-like
components to humic substances (Volk et al.
1997
). Interestingly, an increase of
fulvic acid-like FI is typically observed under dark incubation (Mostofa et al.
2007b
) and in deep lake or seawaters (Hayase and Shinozuka
1995
; Mostofa et
al.
2005b
). Finally, photo-bleached tryptophan-like DOM is resistant to microbial
processes in natural waters. It has been shown that 70 % of the dissolved amino
acids (DAA) and dissolved carbohydrates (DCHO) associated with the humic frac-
tion are consumed by microbial degradation in natural waters (Rosenstock and
Simon
2003
).
3.3.1 Mechanism for Microbial Degradation of Fluorophores in FDOM
The microbial degradation of high molecular weight (HMW) DOM such as ful-
vic and humic acids (humic substances) of vascular plant origin and autochtho-
nous fulvic acid of algal origin can increase the fluorescence intensities at both
peak A- and C-regions. It is generally considered that the peak A-region is linked
with aliphatic moieties and functional groups with less aromaticity, whilst the peak
C-region is characterized by high aromaticity and functional groups with repeated
structural units. Therefore, microbial degradation can effectively modify the ali-
phatic part of HMW DOM, which can enhance the fluorescence intensity mostly
at peak A-region. The microbial increase of fluorescence intensity of HMW DOM
is the result of changes in the molecular structure by several pathways.
Firstly, microbes can degrade aliphatic carbon (e.g. carbohydrates) or the
functional groups of macromolecules such as fulvic and humic acids of vascular
plant origin, as well as autochthonous fulvic acids of algal or phytoplankton ori-
gin, with subsequent release of a variety of end products such as CH
4
, CO
2
, DIC,
PO
4
3
−
, NH
4
+
