Environmental Engineering Reference
In-Depth Information
et al.
2000
; Amon and Benner
1996
; Vähätalo and Wetzel
2004
; Miller and
Moran
1997
). In essence, during the photoinduced process various species among
which are the superoxide radical ion (O
2
•
−
•
and peroxides (H
2
O
2
and
ROOH) are generated either in surface waters or in aqueous solutions during lab-
oratory irradiation experiments (Takeda et al.
2004
; Moore et al.
1993
; Mostofa
and Sakugawa
2009
; Southworth and Voelker
2003
; Goldstone et al.
2002
). The
photogenerated reactive species are involved into the photoinduced degrada-
tion of DOM in waters. Simultaneously, these species can inhibit or deactivate
the activity of catalase, peroxidase and superoxide dismutase associated with
bacterial cells, particulate organic matter and DOM (Moffett and Zafiriou
1990
;
Tanaka et al.
1985
; Serban and Nissenbaum
1986
; Zepp et al.
1987
). Bacterial
cells can protect themselves from harmful oxidizing species such as H
2
O
2
, O
2
), the HO
•
−
•
and HO
by adjusting the level of their enzymes (Chance et al.
1979
). Therefore,
microbial degradation is expected to take place to a negligible extent during the
photoinduced degradation of DOM in aqueous media.
The bacterial growth shows seasonal variations, reaching the maximum
during spring to early summer and decreasing greatly during the summer sea-
son when the water temperature exceeds 25.5 °C in lakes (Zhao et al.
2003
;
Darakas
2002
). Natural sunlight or UV-exposure can decrease the bacterial pro-
duction by 15-80 %, which considerably inhibits the formation and biodegra-
dation of DOM in natural surface waters (Amon and Benner
1996
; Bertilsson
and Tranvik
1998
; Benner and Ziegler
1999
; Naganuma et al.
1996
; Tranvik
and Kokalj
1998
). However, other studies have found that solar exposure can
enhance the bacterial growth by about 35-200 % (Lindell et al.
1996
; Wetzel
et al.
1995
; Bushaw et al.
1996
; Miller and Moran
1997
; Herndl et al.
1993
;
Lindell and Rai
1994
; Reitner et al.
1997
; Jørgensen et al.
1998
; Moran and
Hodson
1994
). Moreover, bacterial growth is usually observed in deeper waters
(Benner and Ziegler
1999
). The increase or the decrease of bacterial growth
by sunlight depends on two key factors: (i) The production of reactive species
(H
2
O
2
, ROOH, HO
•
) and of mineralization products such as CO
2
, CO and DIC;
(ii) The concentration level and molecular nature of DOM, the concentration
of total dissolved iron for photo-Fenton reaction, water temperature, dissolved
oxygen, physical mixing etc. Water temperature during the summer season is
merely regulated by natural solar radiation, which can lead to high generation of
free radicals and mineralization products in natural waters (Moore et al.
1993
;
Mostofa and Sakugawa
2009
; Zafiriou et al.
1984
; Zika
1981
; Obernosterer
et al.
2001
; Fujiwara et al.
1993
; Sakugawa et al.
2000
). The free radicals, as
strong oxidants and depending on their concentration, may have several deadly
impacts on many stages of cell metabolism, including those involved into the
induction of programmed cell death (Samuilov et al.
2001
). This hypothesized
effect is supported by the observation of an inhibition by UV radiation of the
activity of microorganisms in water (Herndl et al.
1993
; Lund and Hongve
1994
;
Karentz et al.
1994
), and of the deadly impact of UV radiation on bacterial cells
or microorganisms in the aquatic environment (Qian et al.
2001
; Randall et al.
2005
). Qian et al. 2001; Sunlight often affects bacterial growth merely in the