Photoinduced and Microbial Degradation of Dissolved Organic Matter in Natural Waters - Photobiogeochemistry of Organic Matter - page 301

Various functional groups widely differ for their reaction rate constants with
HO

• •

photolytically
formed from different organic compounds are much varied (Table 1 : chapter
“ Photoinduced and Microbial Generation of Hydrogen Peroxide and Organic
Peroxides in Natural Waters ” ; Table 2 : chapter “ Photoinduced Generation of
Hydroxyl Radical in Natural Waters ”). Variations in the production rates depend
on the chemical nature of the functional groups bonded to each organic compound.
Therefore, it can be concluded that the functional groups have an important impact
both on the photoinduced production of HO

(Fig. 2 ). Similarly, the production rates of H 2 O 2 and HO

• •

reaction with organic
compounds. Both issues are very significant for the photoinduced processes that
involve dissolved organic matter in surface waters.

and on the HO

2.5 Mechanisms of Microbial Degradation of DOM in Natural Waters

The organic matter in wastes and biomass is diagenetically altered by complex micro-
bial processes into various kinds of organic substances such as long-chain fatty acids,
C 3 to C 5 organic acids, alcohols, aromatic compounds, humic substances (fulvic and
humic acids) of terrestrial plant origin, autochthonous fulvic acid of algal origin,
acetate, formate, methanol, CO 2 , H 2 , as well as minor products. These processes take
place in waters, in soil environments or in sediment pore waters of lake and marine sys-
tems (Mostofa et al. 2009a ; Conrad 1999 ; Lovley 2006 ; Li W et al., unpublished data;
Burdige et al. 2004 ; Yang and Guo 1990 ; Leenheer and Croue 2003 ).
The functional groups of organic substances and the minor components may
be subsequently converted into CO 2 , methane and other products by fermentative
microorganisms and Fe(III)-reducing microorganisms. These processes take place
with simultaneous reduction of an array of electron acceptors, including oxygen,
H 2 , nitrate, manganese oxides, Fe(III) oxides, sulfate, H 2 S, and humic substances
in water (Lovley 2006 ; Lovley et al. 1996 ; Nagase and Matsuo 1982 ; Jetten et al.
1992 ; Coleman et al. 1993 ; Roden and Wetzel 1996 ; Pelmenschikov et al. 2002 ;
Keppler et al. 2006 ; Itoh et al. 2008 ; Reguera et al. 2005 ).
Fe(III)-reducing microorganisms, commonly Geobacter species in temperate
environments (Lovley et al. 2004 ), and Fe(III)-reducing archaea in warm environ-
ments (Kashefi et al. 2004 ) metabolize the fermentation products and the func-
tional groups in organic substances. They are oxidized to CO 2 , with Fe(III) oxides
serving as the electron acceptor (Lovley 2006 ; Lovley et al. 1996 ). The mecha-
nism for CO 2 formation from Fe(III) oxide in the presence of Geobactor spp. is
depicted (Fig. 3 ) (Lovley et al. 1996 ):
The general reactions for microbial Fe(III) reduction coupled with the oxida-
tion of fermentation products such as acetate (Eq. 2.24 ) and hydrogen (Eq. 2.25 )
are described below (Eqs. 2.24 , 2.25 ) (Coleman et al. 1993 ; Lovley 1991 ).

(2.24)

4Fe 2 O 3 + CH 3 COO − + 7H 2 O → 8Fe 2 + + 2HCOL 3 − + 15OH −

Fe 2 O 3 + H 2 + H 2 O → 2Fe 2 + + 4OH − (2.25)