Biology Reference
In-Depth Information
also serves as the terminal energy emitter. Two other chromophorylated
proteins, ApcD and ApcF, also function as terminal emitters. Harvested light
energy is transferred from the terminal emitters to the chlorophylls of PSII
and PSI (
Mullineaux, 1992
;
Rakhimberdieva, Boichenko, et al., 2001
).
One type of cyanobacterial photoprotective mechanism, known as
State Transitions is triggered by light at the level of the phycobilisome and
involves regulation of the energy distribution between the two photosys-
tems under low light conditions; this is triggered by changes in the redox
state of the plastoquinone (PQ) pool (reviews:
van Thor, Mullineaux, et al.,
1998
;
Williams & Allen, 1987
;
Wollman, 2001a
). Exposure of cyanobacte-
ria to orange or green light, absorbed predominantly by phycobilisomes,
causes a reduction of the PQ pool and a relative decrease of the PSII fluo-
rescence yield is observed (State 2). Conversely, illumination with blue or
far red light, preferentially absorbed by PSI, induces the oxidation of the
PQ pool and a relative increase of the fluorescence yield is induced (State
1). Two theories have been proposed to explain the State Transition mech-
anism (for reviews, see
Biggins & Bruce, 1989
;
Mullineaux, 1999
;
van Thor,
Mullineaux, et al., 1998
). The first proposes that the phycobilisomes are
mobile elements which, by changing their association with PSII and PSI,
deliver energy preferentially to one or the other photosystem (
Allen, Sand-
ers, et al., 1985
;
Joshua & Mullineaux, 2004
;
Mullineaux, Tobin, et al., 1997
;
Sarcina, Tobin, et al., 2001
). The second theory proposes that the mobile
elements are the photosystems changing the 'spillover' of energy from PSII
to PSI chl
a
molecules (
Bruce & Biggins, 1985
;
Ley & Butler, 1980
;
Olive,
Mbina, et al., 1986
). More recently, McConnell et al. (2002) suggested that
changes in the redox state of the PQ pool induce independent changes
in the energy transfer between the phycobilisomes and the photosystems,
and between the photosystems themselves. Nothing is known about how
the reduction or oxidation of the PQ pool could induce the movement of
phycobilisomes or photosystems.
In contrast, the OCP-related NPQ mechanism is induced by strong blue
or white light intensities (
El Bissati, Delphin, et al., 2000
;
Rakhimberdieva,
Stadnichuk, et al., 2004
;
Wilson, Ajlani, et al., 2006
). Light activates a soluble
carotenoid protein, the Orange Carotenoid Protein (OCP), which interacts
with the phycobilisomes to increase thermal dissipation of absorbed energy,
resulting in a decrease of energy arriving at the reaction centres (
Wilson,
Ajlani, et al., 2006
;
Wilson, Punginelli, et al., 2008
). The increase of thermal
dissipation also causes a decrease of the yield of phycobilisome fluorescence
creating a nonphotochemical fluorescence quenching. This review focuses
