Environmental Engineering Reference
In-Depth Information
4.1 Debates/Questions Regarding O
2
-Releases
from PSI and PSII
Some key issues on the debate concerning the details of electron- and O
2
-release
from PSI and PSII will be discussed in the following parts.
First, an electron is released upon excitation by light, either producing the
charge-separated state P680
+
H
A
−
from Chl molecules (P680), or accompanied by
no charge separation (or by considerable protein relaxation) (Dashdorj et al.
2004
;
Germano et al.
2004
; Rappaport and Diner
2008
; Müller et al.
2010
; Takahashi
et al.
1987
; Periasamy et al.
1978
). Accordingly, after release of an electron by
PSI or PSII upon excitation by light, is it possible to accept the same component
of PSI or PSII? From the point of view of aquatic humic substances (fulvic and
humic acids) or CDOM (DOM or FDOM, fluorescent dissolved organic matter),
the answer is no. The secondary component (dissolved O
2
in water) can accept
the electron to produce super oxide radical anion (O
2
•
−
) and then H
2
O
2
(Eqs.
3.36
-
3.40
). The detailed mechanism for H
2
O
2
production from DOM (or FDOM
or CDOM) is extensively discussed in chapter
“
Photoinduced and Microbial
Second, which and how many Chl molecules are taking part to the primary
donor sites in PSI and PS II? (Stewart et al.
2000
; Jordan et al.
2001
; Diner and
Rappaport
2002
; Li et al.
2006
; Müller et al.
2010
; Durrant et al.
1995
; Dekker
and van Grondelle
2000
; van Gorkom and Schelvis
1993
) The answer is that the
first electron is released from the functional or chromophoric group bound to PSI
or PSII, which is the easiest way to do it upon excitation by light. Subsequent
electron releases occur in succession from the functional groups (for an analogy,
see CDOM and FDOM, chapter
“
Colored and Chromophoric Dissolved Organic
excitation is usually the first to take place, followed by the others. Therefore, Chl
dimers or Chl molecules (generally with emission wavelengths >675 nm) bound
to PSI or PSII are primarily responsible for excitation of electrons. In contrast,
proteins or aromatic amino acid residues (generally having shorter emission wave-
lengths: <370 nm) are not excited in presence of Chl molecules upon irradiation
(see also chapter
“
Fluorescent Dissolved Organic Matter in Natural Waters
”).
Third, why are PSI and PSII formed by a number of Chl molecules in their
structure? It is assumed here that Chl
a
(or dimer Chl
a
) molecules are extremely
photosensitive and can be excited by a small light intensity. Continuous H
2
O
2
gen-
eration in the presence of little light is enabled by the occurrence of high numbers
of Chl
a
molecules in PSI and PSII, which at the same time can contribute to the
continuous photosynthesis in organisms and plants under light conditions. The fac-
tors affecting the generation of H
2
O
2
(e.g. high or low light intensity, pH, nutrients
and so on) can affect photosynthesis and induce structural modifications in PSI
and PSII. For example, under intense light conditions there is an elevated produc-
tion of H
2
O
2
, the excess of which can be photolytically converted into HO
•
. The
