Environmental Engineering Reference
In-Depth Information
cyanobacterium at 10 times lower concentrations than green alga and diatom, and a
strong light-dependent toxicity can enhance the difference (Drábková et al.
2007
).
Second, indirect effects can be operational by which UV or strong light can
produce a significant amount of strong oxidizing agents. For instance, HO
•
can be
photolytically generated in the presence of H
2
O
2
(photo-Fenton raction or direct
photodissociation), hydrogen peroxide being produced by DOM (of both algal
and terrestrial origin). The hydroxyl radical can also be photoproduced by other
chemical species such as NO
2
−
−
(see the chapters
“
Dissolved Organic
Degradation of Dissolved Organic Matter in Natural Waters
”
for a detailed descrip-
tion). The HO
and NO
3
•
radical would subsequently react with the functional groups present
in the cells of aquatic microorganisms. The indirect effect may significantly affect
waters with high contents of DOM and POM, which are usually associated to ele-
vated production of photo- and microbial products and, as a consequence, to high
photosynthesis and high primary production. Moreover, it has been shown that
the production of HO
•
during an ozone hole (151 Dobson units) is enhanced by at
least 20 %, mostly from nitrate photolysis and to a lesser extent from DOM pho-
toinduced reactions, in Antarctic seawater. Similar results have been observed for
Arctic water (see chapters
“
Photoinduced and Microbial Generation of Hydrogen
description) (Rex et al.
1997
; Qian et al.
2001
; Randall et al.
2005
).
Note that cyanobacteria (or phytoplankton) can produce autochthonous DOM
including autochthonous fulvic acids, which are very efficient in the production of
H
2
O
2
(and of HO
•
as a consequence under irradiation). Regeneration of autoch-
thonous DOM and nutrients
NO
3
−
, NO
2
−
, PO
4
3
−
AND NH
4
+
occurs during the
photoinduced and microbial assimilation of cyanobacteria or phytoplankton, and
simultaneously also from the photoinduced degradation of DOM in natural waters
(see chapter
“
Dissolved Organic Matter in Natural Waters
”,
“
Photoinduced and
for detailed description). High solar irradiation generally induces the production
of large amounts of H
2
O
2
and HO
•
−
−
in aque-
ous media (see also the chapters
“
Photoinduced and Microbial Generation of
Generation of Hydroxyl Radical in Natural Waters
”
) (Mostofa and Sakugawa
2009
; Takeda et al.
2004
). Moreover, light plays a significant role in the cycling of
terrestrially-derived DOM and (to a certain extent) of autochthonous DOM. It can
potentially increase metabolism of both terrestrially and microbially derived DOM
in natural waters (Hiriart-Baer et al.
2008
). Low light levels, due to increased
CDOM, do not have significant effects on the benthic microfloral community at
mid-shelf locations (Darrow et al.
2003
).
, from DOM or NO
2
and NO
3
