Environmental Engineering Reference
In-Depth Information
In this modified mechanism, the electron is originated mostly from photoin-
duced excitation of both P680 and P700 (Eq.
5.4
). Dissolved O
2
in water is thus
reduced photolytically, differently from the results of earlier studies. The dispro-
portionation of O
2
−
to H
2
O
2
and O
2
is catalyzed by superoxide dismutase (Eq.
5.6
). H
2
O
2
is then reduced to H
2
O by ascorbate (AsA), a process that is catalyzed
by ascorbate peroxidase (APX). AsA is oxidized to monodehydroascorbate radi-
cal, MDA (Eq.
5.7
).
Additional electron pathways in chloroplasts that protect the photosynthetic
apparatus from photo-oxidative stress are the Mehler reaction, xanthophyll
cycle-dependent energy, the cyclic electron flow around PSI, the cyclic electron
flow within PSII, and antioxidant metabolism (Mehler
1951
; Heber et al.
1978
;
Verhoeven et al.
1997
; Miyake and Yokota
2001
; Miyake et al.
2002
; Hirotsu et
al.
2004
). Nitrate assimilation is referred to as alternative electron flow (Makino
et al.
2002
). The Mehler reaction implies that the photoreduction of O
2
at PSI can
produce superoxide radical
,
which disproportionates to H
2
O
2
(Mehler
1951
; Asada
2006
). It is estimated that the maximum rate of O
2
photoreduc-
tion is approximately 7.5 mmol
O
2
•−
(
mol Chl
)
−
1
s
−
1
(30 mol (mg Chl)
−
1
h
−
1
)
in washed thylakoids, which corresponds to 5-10 % of the rate of total electron
transport (Asada et al.
1974
). It has also been observed that the
O
2
•−
reduction
rate can reach a maximum around 2.0 kPa O
2
(Heber and French
1968
; Takahashi
and Asada
1982
).
O
2
•−
5.2 CO
2
Forms Used in Phytoplankton Photosynthesis
CO
2
and DIC
(
CO
2
,H
2
CO
3
, HCO
3
−
, and CO
2
3
)
can be produced either photo-
lytically or microbially from both DOM and POM (e.g. alage or phytoplankton)
in natural waters (see also chapter
“
Photoinduced and Microbial Degradation of
on Biogeochemical Cycles in Natural Waters
”
) (Jones
1992
; Jansson et al.
2000
; Meili et al.
2000
; Grey et al.
2001
; Hernes and Benner
2003
; Tranvik
et al.
2009
; Ballaré et al.
2011
; Zepp et al.
2011
; Miller and Zepp
1995
; Graneli
et al.
1996
; Granéli et al.
1998
; Bertilsson and Tranvik
2000
; Ma and Green
2004
; Xie et al.
2004
; Fu et al.
2007
). This production varies seasonally and spa-
tially depending on several factors such as contents of DOM and POM, solar
intensity, water temperature and other geological and environmental conditions
(White et al.
2010
).
Gaseous CO
2
is rapidly dissolved in waters (Liu et al.
2010
):
CO
2
+
H
2
O
↔
H
2
CO
3
↔
H
+
+
HCO
3
−
↔
2H
+
+
CO
2
−
(5.8)
3
where the reaction (Eq.
5.8
) is an equilibrium mixture of dissolved carbon
dioxide ([CO
2
]
aq
), carbonic acid (H
2
CO
3
), bicarbonate
HCO
3
−
and carbon-
ions with the pKa of 6.3 and 10.3 for
H
2
CO
3
↔
H
+
+
HCO
3
−
AND HCO
3
−
↔
H
+
+
CO
3
2
−
,
respectively (Liu et al.
2010
; Appelo and Postma
CO
3
2
−
ate
