275
Photoinduced and Microbial Degradation
the key factors that may significantly affect the photoinduced and microbial activities in
waters. Finally, this study summarizes the various photoproducts of DOM and depicts
their importance on biogeochemical cycles in the aquatic environments.
1.1 Biogeochemical Functions of DOM for Photoinduced
Processes in Natural Waters
Photoinduced reactions caused by solar radiation can produce various biogeochemical alterations of DOM that can be listed as follows (Mostofa et al. 2011):
(1) Photoinduced generation of free radicals, which are susceptible to induce photoinduced degradation of DOM in aqueous media. The free radicals sources
include: (i) Photolysis of NO 2
− and NO 3
− ions inducing production of
hydroxyl radical (HO
• ) in waters (Zafiriou and True 1979a, b; Takeda et al.
2004; Vione et al. 2006; Minero et al. 2007) (ii) Generation of free radical
species such as superoxide ion (O 2
•−
), hydrogen peroxide (H 2 O 2 ), organic
peroxides (ROOH) and HO
• by photolysis of CDOM or FDOM in waters
(Cooper et al. 1988; Moore et al. 1993; O’Sullivan et al. 2005; Mostofa and
Sakugawa 2009); (iii) Photoinduced production of HO
• by the photo-Fenton
(Zepp et al. 1992; White et al. 2003) as well as the photo-ferrioxalate/H 2 O 2
reactions (Safarzadeh-Amiri et al. 1996, 1997; Southworth and Voelker 2003).
(2) Production of new organic substances by photorespiration or assimilation
of particulate organic matter (POM: ca. algae or phytoplankton) and high
molecular weight DOM. These processes have a deep impact on the carbon cycling and include: (i) Photo-respiration or assimilations of algae that
can produce autochthonous fulvic acid and other organic substances as well
as nutrients in the aquatic environments (Mostofa et al. 2009b; Thomas and
Lara 1995; Fu et al. 2010). (ii) Photoinduced degradation of chlorophyll a
with production of new organic substances; this process is typically occurring in the photic layer of natural lake and seawater (Rontani 2001; Cuny
et al. 2002). (iii) Photoinduced assimilation or degradation of algal biomass
in surface waters under natural sunlight, which may produce new DOM or
FDOM species in the aquatic environments (Rochelle-Newall and Fisher 2002;
Thomas and Lara 1995; Fu et al. 2010; Henrichs and Doyle 1986; Biddanda
and Benner 1997; Carrillo et al. 2002; Mostofa KMG et al. unpublished).
(iv) Microbial assimilation or degradation of algal biomass or phytoplankton; in vitro experiments have shown that under dark incubation these processes may produce new DOM or FDOM (Rochelle-Newall and Fisher 2002;
Yamashita and Tanoue 2008; Fu et al. 2010; Biddanda and Benner 1997;
Mostofa KMG et al. unpublished; Yamashita and Tanoue 2004; Stedmon and
Markager 2005). (v) Photoinduced transformation of high-molecular weight
DOM into low-molecular weight organic substances; in some cases the process can lead to complete mineralization (Moran and Zepp 1997; Dahlén et al.
1996; Ma and Green 2004; Vähätalo and Järvinen 2007; Vione et al. 2009).
Photoinduced and Microbial Degradation
the key factors that may significantly affect the photoinduced and microbial activities in
waters. Finally, this study summarizes the various photoproducts of DOM and depicts
their importance on biogeochemical cycles in the aquatic environments.
1.1 Biogeochemical Functions of DOM for Photoinduced
Processes in Natural Waters
Photoinduced reactions caused by solar radiation can produce various biogeochemical alterations of DOM that can be listed as follows (Mostofa et al. 2011):
(1) Photoinduced generation of free radicals, which are susceptible to induce photoinduced degradation of DOM in aqueous media. The free radicals sources
include: (i) Photolysis of NO 2
− and NO 3
− ions inducing production of
hydroxyl radical (HO
• ) in waters (Zafiriou and True 1979a, b; Takeda et al.
2004; Vione et al. 2006; Minero et al. 2007) (ii) Generation of free radical
species such as superoxide ion (O 2
•−
), hydrogen peroxide (H 2 O 2 ), organic
peroxides (ROOH) and HO
• by photolysis of CDOM or FDOM in waters
(Cooper et al. 1988; Moore et al. 1993; O’Sullivan et al. 2005; Mostofa and
Sakugawa 2009); (iii) Photoinduced production of HO
• by the photo-Fenton
(Zepp et al. 1992; White et al. 2003) as well as the photo-ferrioxalate/H 2 O 2
reactions (Safarzadeh-Amiri et al. 1996, 1997; Southworth and Voelker 2003).
(2) Production of new organic substances by photorespiration or assimilation
of particulate organic matter (POM: ca. algae or phytoplankton) and high
molecular weight DOM. These processes have a deep impact on the carbon cycling and include: (i) Photo-respiration or assimilations of algae that
can produce autochthonous fulvic acid and other organic substances as well
as nutrients in the aquatic environments (Mostofa et al. 2009b; Thomas and
Lara 1995; Fu et al. 2010). (ii) Photoinduced degradation of chlorophyll a
with production of new organic substances; this process is typically occurring in the photic layer of natural lake and seawater (Rontani 2001; Cuny
et al. 2002). (iii) Photoinduced assimilation or degradation of algal biomass
in surface waters under natural sunlight, which may produce new DOM or
FDOM species in the aquatic environments (Rochelle-Newall and Fisher 2002;
Thomas and Lara 1995; Fu et al. 2010; Henrichs and Doyle 1986; Biddanda
and Benner 1997; Carrillo et al. 2002; Mostofa KMG et al. unpublished).
(iv) Microbial assimilation or degradation of algal biomass or phytoplankton; in vitro experiments have shown that under dark incubation these processes may produce new DOM or FDOM (Rochelle-Newall and Fisher 2002;
Yamashita and Tanoue 2008; Fu et al. 2010; Biddanda and Benner 1997;
Mostofa KMG et al. unpublished; Yamashita and Tanoue 2004; Stedmon and
Markager 2005). (v) Photoinduced transformation of high-molecular weight
DOM into low-molecular weight organic substances; in some cases the process can lead to complete mineralization (Moran and Zepp 1997; Dahlén et al.
1996; Ma and Green 2004; Vähätalo and Järvinen 2007; Vione et al. 2009).