Environmental Engineering Reference
In-Depth Information
3.2.1 Mechanism of DCM Formation
It has been shown that DCM is generally developed in clear water at low tem-
perature. The main effect of these conditions result is the penetraton of radiation
into deep water, in which case photosynthesis can enhance the primary production
and produce the DCM in deeper water. The mechanism behind DCM formation is
presumably that H
2
O
2
and HCO
3
−
produced in the DCM water layer are suscepti-
ble to take part in phytoplankton photosynthesis. It has been shown that DIC (dis-
solved CO
2
, H
2
CO
3
, HCO
3
, CO
3
2
−
) is mostly produced from particulate organic
matter (POM: e.g. algae or cyanobacteria) and DOM microbiologically in natu-
ral waters as well as under in situ experiments (Ma and Green
2004
; Finlay et al.
2009
; Stets et al.
2009
; Jiao et al.
2010
). Correspondingly, most H
2
O
2
can be pro-
duced either from algae (cyanobacteria or phytoplankton or biota) or from DOM,
by several biological or photochemical processes (see also chapter
“
Photoinduced
Natural Waters
”
for more references and description) (Palenik et al.
1987
; Palenik
and Morel
1988
; Cooper and Lean
1992
; Sarthou et al.
1997
; Croot et al.
2005
;
Mostofa and Sakugawa
2009
; Zepp et al.
1987
; Angel et al.
1999
; Wentworth et al.
2000
; Wentworth et al.
2001
; Moreno
2012
; Moffett and Zafiriou
1990
). Such pro-
cesses are: (i) extracellular phenomena, (ii) biological processes such as glycolate
oxidation during photorespiration, (iii) enzymatic reduction of oxygen at the cell
surface, and (iv) microbial degradation of DOM under dark incubation. Most phy-
toplankton cells have the enzyme superoxide dismutase (SOD), which can catalyse
the conversion of superoxide to H
2
O
2
. This is one of the many biological reactions
that produce H
2
O
2
in seawater (Croot et al.
2005
).
In a field study, dark production of H
2
O
2
was highest at 40-60 m depth and the
corresponding DCM was detected at 90 m. The finding suggests that photosynthe-
sis, which causes the DCM may reduce the dark production of H
2
O
2
at 90 m depth
(Palenik and Morel
1988
). Simultaneously, the increase in pigment production
caused by phytoplankton under the low-light conditions of the DCM layer (Steele
1964
; Hobson and Lorenzen
1972
; Kiefer et al.
1976
) may lead to high contents
of H
2
O
2
and contribute to DCM formation. Note that pigments made up of Chls
can rapidly absorb light energy upon irradiation. Radiation absorption can excite
an electron to form the superoxide radical anion (O
2
•
−
) and then H
2
O
2
(see chap-
H
2
O
2
concentration increase at the depth of the Chl maximum is possibly due to
biological production (Croot et al.
2005
). The formation of H
2
O
2
by phytoplankton
in the DCM layer can be supported by the observation that
Chattonella marina
, a
harmful algal bloom species, is capable of producing reactive oxygen species (ROS)
including O
2
•
−
, H
2
O
2
, and HO
−
•
at levels 100 times higher than those produced by
most algae (Marshall et al.
2002
; Oda et al.
1998
). ROS are often produced as by-
products of various metabolic pathways localized in mitochondria, chloroplasts,
and peroxisomes (see also chapter
“
Photosynthesis in Nature: A New Look
”
) (Apel
and Hirt
2004
). The presence of the cyanobacterium
Microcystis
sp. can produce
