Photoinduced and Microbial Degradation of Dissolved Organic Matter in Natural Waters - Photobiogeochemistry of Organic Matter - page 290

distinguished by a red shift of the fluorescence peak compared to the upper sur-
face layer (Yoshioka et al. 2007 ; Mostofa et al. 2005 ; Hayase and Shinozuka 1995 ).
These autochthonous substances are also identical to the organic substances produced
experimentally upon photoinduced and microbial assimilations of algae (Mostofa et
al. 2009b ; Fu et al. 2010 ). This hypothesis is supported by the features of autochtho-
nous fulvic acid extracted from POM in sea waters or sediment pore waters, which
typically show the fluorescence peak at longer wavelength regions (see also chapter
“ Fluorescent Dissolved Organic Matter in Natural Waters ”) (Li W et al., unpublished
data; Komada et al. 2002 ; Burdige et al. 2004 ; Managaki and Takada 2005 ; Calace et
al. 2006 ; Parlanti et al. 2000 ). Another possible explanation is that photolabile DOM in
the surface water layers is probably quickly mineralized by sunlight, which leaves only
the more photolytically refractory substances near the surface, while photolabile DOM
in the deeper layers is more protected from mineralization by the lower sunlight inten-
sity. Interestingly, groundwater DOM has been found to be significantly more suscepti-
ble to photo mineralization than surface lake water DOM (Vione et al. 2009 ).
2.2 Microbial Degradation of DOM in Natural Waters
Microbial actions can decompose the DOM, estimated as dissolved organic carbon
(DOC) concentration, in natural waters. This has been verified in experiments con-
ducted on waters under dark conditions. Microbial activity can decrease DOC concen-
trations either slowly or rapidly depending on the DOM sources during the incubation
period (Fig. 1 g-i). The initial DOC concentration, amount of DOC changes and its
percentage (%), as well as other experimental conditions in microbial degradation
experiments are presented in Table 1 . It is demonstrated that the decrease of DOC con-
centration because of microbial activity for various natural waters is approximately
0-8 % in stream waters during 12-13 days, 1-85 % in downstream river waters dur-
ing hours to 10 days, 0-8 % in lake waters during hours to 70 days, 8-23 % in estua-
rine waters during 51 days, 5-10 % in seawaters during 14 days of incubation period
under dark conditions (Table 1 ) (Mostofa et al. 2007 ; Moran et al. 2000 ; Bertilsson
and Allard 1996 ; Mostofa et al. 2005 ; Miller and Moran 1997 ; Mostofa and Sakugawa
unpublished data ; Borisover et al. 2011 ; Winter et al. 2007 ). From the results of
microbial degradation (Table 1 ) it is possible to generalize several key features com-
monly observed in natural waters: First, downstream DOM, particularly in sewerage-
impacted rivers is significantly labile to microbial degradation. Second, upstream
DOM is typically recalcitrant to microbial degradation (Fig. 1 g). Third, microbial deg-
radation is typically a slow process for the mineralization of DOM in natural waters,
except for downstream rivers with sewage effluents. For example, DOC mineralization
rapidly occurs in unfiltered and filtered dark samples (32-85 %) of downstream riv-
ers where DOM is mostly fluxed from untreated sewerage effluents near urban areas
(Table 1 ; Fig. 1 i) (Mostofa et al. 2007 ; Mostofa and Sakugawa unpublished data).
The results of microbial degradation on molecular size fractions of DOM
demonstrate that DOC mineralization is approximately 2 % for large molecular