Environmental Engineering Reference
In-Depth Information
than a few
μ
m because it occupies a minimum between two different trans-
port regimes (Kaiser and Benner
2009
; Kepkay
1994
; Wells and Goldberg
1993
). In fact, Brownian motion dominates transport of smaller colloids to
bacteria, whilst larger particles are primarily transported to bacteria by turbu-
lent shear (Kepkay
1994
).
(ii) Photoinduced and microbial processes that involve DOM, including fulvic
and humic acids, can produce biologically labile LMW organic substances
(e.g. organic acids) in natural waters (Moran and Zepp
1997
; Carrick et al.
1991
; Kieber et al.
1989
,
1990
; Corin et al.
1996
; Mopper et al.
1991
; Allard
et al.
1994
; Mopper and Stahovec
1986
; Backlund
1992
). These LMW
organic compounds are important intermediates of the conversion of organic
substances such as carbohydrates, fats and proteins into CH
4
and CO
2
in
aqueous media (Smith and Oremland
1983
; Evans
1998
; Xiao et al.
2009
;
Wellsbury and Parkes
1995
).
(iii) The absorption of natural sunlight is greatly dependent on the molecular size
of DOM and has a high biogeochemical importance in natural waters. For
example, fulvic and humic acids (humic substances) can absorb both vis-
ible and UV radiation (Kieber et al.
1990
; Kramer et al.
1996
; Sadtler
1968
;
Strome and Miller
1978
). Many low molecular weight organic acids photo-
generated from large CDOM or FDOM can only absorb in the UV-C range,
with no absorption of UV-B, UV-A or visible radiation (Carrick et al.
1991
;
Kieber et al.
1990
; Mopper et al.
1991
; Sadtler
1968
). Further details are
provided in the DOM degradation chapter (see chapter
“
Photoinduced and
4.3 Autochthonous Fulvic Acids and their Differences
with Allochthonous Fulvic Acids
The key component of autochthonous DOM is variously termed as marine humic-
like substances (Coble
1996
), sedimentary fulvic acid (Hayase et al.
1987
,
1988
) or
marine fulvic acids (Malcolm
1990
), which is contradictory. However, recent stud-
ies show that the two fluorescent components are primarily produced under either
photoinduced or microbial respiration (or assimilation) of algal (phytoplankton)
biomass (Mostofa et al.
2009b
; Zhang et al.
2009
; Stedmon and Markager
2005a
).
PARAFAC modeling of EEM spectra of algal-originated DOM suggests that the
fluorescence peaks and the images of the first fluorescent component are similar
to those of allochthonous fulvic acid (Fig.
1
). On the other hand, the fluorescence
peaks and the images of the second fluorescent component are similar to those of
marine humic-like substances (Coble
1996
). However, the fluorescence intensity and
the peak positions of the first fluorescent component are quite different from EEM
spectra of standard fulvic acid, which justifies their being denoted with a new name.
To avoid the difficulties of indicating the two algal-originated fluorescent compo-
nents and considering the similarities of their EEM images with allochthonous
