Dissolved Organic Matter in Natural Waters - Photobiogeochemistry of Organic Matter - page 4

(fulvic and humic acids) of terrestrial origin, autochthonous fulvic acids of phy-
toplankton or algal origin, carbohydrates, sugars, amino acids, proteins, lipids,
organic acids, phenols, alcohols, acetylated amino sugars, and so on. On the other
hand, POM includes plant debris, detritus, living organisms, bacteria, algae, phy-
toplankton, corals, coral reefs, and so on. DOM is considered as the larger pool of
organic matter in a variety of waters, which can include more than 90 % of total
organic matter (Thurman 1986 ; Kececioglu et al. 1997 ).
2.1 Biogeochemical Functions of OM (DOM and POM)
DOM of both allochthonous and autochthonous origin can play multiple functions
in photoinduced, chemical, microbial and geochemical processes in natural waters.
They can be classified as follows:
(1) Photoinduced functions of DOM. Irradiated DOM can produce H 2 O 2
(Mostofa and Sakugawa 2009 ), which in turn can produce the strong oxidiz-
ing agent hydroxyl radical (HO • ), either directly by photoinduced dissocia-
tion (H 2 O 2 + hv → HO • ) or by the photo-Fenton reaction. These processes
are involved in the photoinduced degradation of organic compounds (Vione
et al. 2006 , 2010 ; Zellner et al. 1990 ; Zepp et al. 1992 ; Farias et al. 2007 ).
DOM undergoes rapid photoinduced decomposition by natural sunlight, and
this process is less efficient in waters with high contents of DOM and more
efficient with high DOM concentrations (Moran et al. 2000 ; Ma and Green
2004 ; Vähätalo et al. 2000 ; Mostofa et al. 2007b ; Vione et al. 2009 ). DOM
can thus control redox and photo-Fenton reactions in natural waters (Voelker
and Sulzberger 1996 ; Voelker et al. 1997 , 2000 ; Kwan and Voelker 2002 ;
Jeong and Yoon 2004 ; Wu et al. 2005 ; Vione et al. 2006 ; Nakatani et al. 2007 ).
The biogeochemical functions of H 2 O 2 and HO • are discussed in details in
chapters “ Photoinduced and Microbial Generation of Hydrogen Peroxide
and Organic Peroxides in Natural Waters ”, “ Photoinduced Generation of
Hydroxyl Radical in Natural Waters ” .
(2) Microbial functions of OM (DOM and POM). DOM and POM are decom-
posed biologically by microorganisms in natural waters (Moran et al. 2000 ;
Mostofa et al. 2007a ; Ma and Green 2004 ; Lovley and Chapelle 1995 ;
Hopkinson et al. 2002 ; Coble 2007 ; Koschorreck et al. 2008 ; Lønborg et al.
2009a , b ; Lønborg and Søndergaard 2009 ). This process can produce new
autochthonous DOM or nutrients in water (Mostofa et al. 2009b ; Zhang et
al. 2009 ; Kim et al. 2006 ; Weiss et al. 1991 ; Harvey et al. 1995 ; Yamashita
and Jaffé 2008 ; Fu et al. 2010 ; Li et al. 2011 ), so that DOM is responsible
for the maintenance of the microbial loop in natural waters (utilization of
DOC by bacteria, consumption and decomposition of bacteria by protozo-
ans and release of dissolved organic compounds and CO 2 ) (Sherr and Sherr
1989 ; Carrick et al. 1991 ; Jones 1992 ; Tranvik 1992 ). Bioavailable carbon