Biology Reference
In-Depth Information
the exciton transfer from exited chlorophylls to other chlorophylls is disabled
and the LHCs are converted to heat dissipaters (Niyogi et al.
2005
). Dissipation
of excess excitation energy occurs by a charge transfer mechanism involving
a carotenoid radical cation (Ahn et al.
2008
) and/or by chlorophyll-to-carotenoid
energy transfer (Ruban et al.
2007
). The photosynthetic systems of algae share
many central functions with land plants. But recently, an ancient light-harvesting
protein (LHCSR) was described in
Chlamydomonas
which is involved in fast
regulation of algal photosynthesis (Peers et al.
2009
). LHCSRs are absent in
vascular land plants, but present in a variety of photosynthetic organisms, such as
diatoms that show an extremely high nonphotochemical energy-quenching capacity
included in the photoprotection mechanism (Eberhard et al.
2008
; Peers et al.
2009
). LHCSR transcripts accumulate under environmental conditions known to
induce photo-oxidative stress, including deprivation of carbon dioxide, sulfur,
or iron, as well as high light (Peers et al.
2009
). In algae LHCSR orthologues
are missing only in Rhodophytes (and cyanobacteria), which dissipate excess
light energy from phycobilisomes by a mechanism distinct from the typical,
above-described, energy-dissipating mechanism (energetic fluorescence-quenching
mechanism,
q
E
) (Wilson et al.
2006
).
The short-term acclimation of plants to high irradiances and its relation to
photosystem II photochemistry and fluorescence emission were reviewed in detail
by Dau (
1994a
,
b
). Moreover, a general overview of photoinhibition, its molecular
aspects and its mechanisms in the field are given by several articles in the topic
edited by Baker and Bowyer (
1994
), and the effects of a changing irradiance
environment, especially on marine macrophyte physiology, were also reviewed
(Franklin and Forster
1997
;Hader and Figueroa
1997
; Bischof et al.
2000a
;
Wiencke et al.
2007
).
In the marine habitat macrophytes are exposed to considerable diurnal changes
of the impinging photon fluence rates due to the position of the sun, clouding and,
especially, the tides (Hanelt and Nultsch
2002
). Therefore, at midday benthic
marine algae, which grow normally underwater at dim light conditions, can be
exposed to extremely high irradiances on sunny days during low tide in the
intertidal. As a consequence, light energy is excessively absorbed by the photo-
synthetic apparatus and, hence, the extent of its photodamage increases. One of the
damaging processes is the production of highly reactive oxygen species, which
attack target molecules such as the D
1
-protein, chlorophylls and unsaturated fatty
acids (Asada and Takahashi
1987
; see Chap.
6
by Bischof and Rautenberger). The
damage to the photosystem is counteracted by a repair process that involves
partial disassembly of inactive PSII, proteolytic degradation of the photodamaged
reaction center protein (D
1
) and cotranslational insertion of newly synthesized D
1
into PS II, also called D
1
repair cycle (Aro et al.
1993
; Barber and Andersson
1992
).
Permanent photodamage occurs when scavenging of oxygen radicals by superoxide
dismutase, hydrogen peroxidase or catalase is insufficient. Under light stress
conditions, the concentration of active oxygen is increased either by higher produc-
tion rates or by insufficient capacity of the oxygen-scavenging systems. The
reactive oxygen species seem to induce inhibition of
repair processes by
