Environmental Engineering Reference
In-Depth Information
H
2
O
2
can degrade phytoplankton cells, thereby decreasing photosynthesis. The
synergistic effect of high contents of H
2
O
2
combined with elevated seawater tem-
perature (27-31 °C) can result in a 134 % increase in respiration rates of the coral
Galaxea fascicularis
, which can surpass the effect of either H
2
O
2
or high seawater
temperature alone (Higuchi et al.
2009
). A possible explanation is that an increase
in growth of plant species with increasing H
2
O
2
might enhance carbohydrate pro-
duction, and therefore enhance the activity throughout the food web.
4 Functions of Photosystems (I and II) in Organisms
During Photosynthesis
Photosynthesis is primarily initiated by the light-induced release of electrons
across a membrane, which is catalyzed by two multisubunits, special type of
membrane-bound pigment-protein complexes called photosynthetic reaction cen-
tres (RCs). They are photosystem I (PSI) and photosystem II (PSII) (Krauß
2003
;
Golbeck
1994
; Brettel
1997
; Li et al.
2006
; Rappaport and Diner
2008
; Müller
et al.
2010
; Nilsson Lill
2011
; Umena et al.
2011
; Renger and Holzwarth
2005
;
Fromme
2008
; Holzwarth
2008
). PSI of higher plants and algae (named PSI-200)
consists of the PSI core complex and the peripheral light-harvesting complex
LHCI. In cyanobacteria, it only consists of the PSI core (Schlodder et al.
2011
).
The PSI core complexes in cyanobacteria are organized preferentially as trimers,
whereas PSI in higher plants and algae is present only as a monomer (Boekema
et al.
1987
,
2001
; Shubin et al.
1993
; Kruip et al.
1994
; Jordan et al.
2001
; Amunts
et al.
2010
).
By studying the crystal structure of cyanobacterial PSI it has been shown that
it is composed of 128 cofactors including approximately 96-100 Chl molecules,
two phylloquinones, three [Fe
4
S
4
] clusters, 22 carotenoids, four lipids and a puta-
tive Ca
2
+
ion (Fig.
4
) (Krauß
2003
; Krauss et al.
1993
; Krauß et al.
1996
; Klukas
et al.
1999
; Jordan et al.
2001
; Ben-Shem et al.
2003
; Müller et al.
2010
; Webber
and Lubitz
2001
). The PSI antenna consists of 90 Chls, of which 79 are bound
to a heterodimeric core formed by subunits PsaA and PsaB, with 2
×
11 trans-
membrane
α
-helices (Krauß
2003
). The cofactors in the RC of PSI form two
quasi-symmetric branches (Fig.
4
), diverging from a Chl
a
∕Chl
a
pair (ec1A∕ec1B)
traditionally called P700 (Jordan et al.
2001
; Müller et al.
2010
). In each branch
there is a pair of Chl
a
molecules (ec2A∕ec3A or ec2B∕ec3B) and a phylloquinone
(PhQA or PhQB) and then the branches join again at the FX iron-sulfur (FeS)
cluster (Müller et al.
2010
). The carotenoids have a dual function in light harvest-
ing and photoprotection. The organic cofactors of the electron transfer chain are
bound to PsaA/PsaB and arranged in two branches of three Chl and one phyllo-
quinone molecule each, related by a pseudo-C2 axis (Krauß
2003
). These studies
show that the PSI reaction center or primary donor P700 in PSI is composed of six
chlorophyll (Chl)
a
cofactors: the P700 special pair Chls (analogous to the special
pair bacteriochlorophylls in purple bacterial reaction centers), two accessory Chls
