Biology Reference
In-Depth Information
1.3 Photosynthesis Under Excessive Light Conditions
In contrast to the conditions at the lower algal distribution limit, photosynthetic
organisms close to the water surface are exposed to high irradiances which may
even exceed their light energy requirement for photosynthetically provided anabo-
lism. Generally, these photosynthetic organisms do not suffer from energy shortage,
but instead they have to invest in proteins and cellular components which protect
them from potential damage by excess supply of energy (Wilhelm and Selmar
2011
). As consequence a reduction of photosynthetic activity, called photoin-
hibition, may occur (Powles
1984
;Krause
1988
;KrauseandWeis
1991
). As
defined by Franklin et al. (
2003
) photoinhibition is a generic outcome of the failure
of photoprotection to mitigate photoinactivation. This may result under high light
conditions in which damage of reaction center proteins exceeds photorepair in the
complex molecular structure of photosystem II (PSII). Photoacclimation is the
adjustment in structure and function of the photosynthetic apparatus that avoid or
rapidly repair damage and thus mitigate (chronic) photoinhibition. Photoprotection
(also called dynamic photoinhibition) refers to processes that decrease excitation
transfer to the reaction centers; these processes are mainly operative in the antenna
complexes. Loss of PS II reaction center function is called photoinactivation
(formerly called chronic photoinhibition), often accompanied by loss of reaction
center protein D
1
, which can be repaired under dim light conditions. Under exces-
sive light conditions a significant degradation of the reaction center protein (D
1
)of
PS II was found (Ohad et al.
1984
; Mattoo et al.
1984
). Nonfunctional photosyn-
thetic centers are able to protect active centers against continuous damaging effects
of excess absorbed light energy by harmless energy dissipation, mainly by heat
dissipation (Guenther and Melis
1990
;
ยจ
quist and Chow
1992
; Critchley and
Russell
1994
). Photodamage occurs if the rate of the D
1
-protein damage exceeds
the rate of its repair process, leading to a breakdown of the D
1
-protein pool (Aro
et al.
1993
). Photoprotection or dynamic photoinhibition is important for the
regulation of quantum yield of photosynthesis, namely by the xanthophyll cycle
in the chloroplasts of green and brown algae (Demmig-Adams and Adams
1992
).
Quantum yield of photosystem II (especially the in vivo fluorescence ratio
F
v
/
F
m
which represents photosynthetic efficiency after dark acclimation) is diminished
reversibly by increasing thermal energy dissipation under strong light conditions.
Carotenoids control this harmless dissipation of excessively absorbed light energy
by, e.g., de-epoxidation of violaxanthin to zeaxanthin (Ruban et al.
2007
) a process
catalyzed by a thylakoid-bound deepoxidase that is activated by a low lumenal
pH due to a high electron transport rate (Pfundel and Dilley
1993
). Zeaxanthin
may dissipate excessively absorbed light energy harmlessly as heat. The process
is reversed by the epoxidation of zeaxanthin to violaxanthin in dim light
(Xanthophyll-cycle). The low pH induces also protonation of the photosystem II
protein, PSBS, which induces a conformational change to specific chlorophyll- and
carotenoid-binding LHC proteins (Peers et al.
2009
). LHC may aggregate leading
to, e.g., PSBS-LHC-zeaxanthin complex in some green algae. As a consequence,
