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but they have the ability to regulate the amount of absorbed sunlight by changes
in leaf area, leaf angle, chloroplast movement and, on a molecular level, through
acclimatory adjustments in LHC antenna size. Excessively absorbed light can be
dissipated via several routes, including thermal dissipation. A number of other
reactions within the chloroplast can act as photochemical sinks for excess electrons,
and there are efficient antioxidant systems for the removal of reactive oxygen
species which are produced under high energy load of the system.
1.4 Effect of Ultraviolet Radiation on Zonation of Macroalgae
In most studies on marine macrophytes, there is common sense that the sensitivity
of photosynthesis to ultraviolet radiation (UVR) is a function of vertical zonation of
the species (Larkum and Wood 1993 ; Dring et al. 1996 ; Bischof et al. 1998a ; see
Chap. 20 by Bischof and Steinhoff). Moreover, Maegawa et al. ( 1993 ) regard solar
UVR as a major factor controlling the upper zonation limit of red macroalgae on the
shore. The potential of UV to inhibit photosynthesis of algae was first demonstrated
by Jones and Kok ( 1966 ). Results of Larkum and Wood ( 1993 ) indicated that
increasing UV levels of the solar radiation can cause similar effects comparable
to high PAR in all types of aquatic plants. In the field, high irradiances of PAR are
generally accompanied by higher UV-radiation. The mechanisms are, however,
likely to be different. UV radiation cannot be regarded as an “excessive energy
input” in a proper sense. Its maximal irradiance is much smaller than that of PAR,
and the UV wavebands do not contribute significant energy supply for photo-
synthetic chemistry. UV exhibits adverse effects on photosynthesis in a more direct
way, such as its waveband with high energy content is absorbed by aromatic and
sulfhydryl-containing biomolecules, causing a direct molecular damage (Vass
1997 ). The UV-B inhibition spectrum corresponds much more with the spectral
absorption by DNA and proteins than with photosynthetic pigments or the action
spectrum of photoinhibition (Jones and Kok 1966 ; Setlow 1974 ; Nultsch et al.
1987 ; Hanelt et al. 1992 ). Numerous studies have shown that recovery from
photoinhibition is delayed after exposure to additional UV-B irradiation (see the
review of H
ader and Figueroa 1997 ). In contrast, Flores-Moya et al. ( 1999 )
demonstrated that in the marine macroalga Dictyota dichotoma a delay of recovery
of photoinhibition is observed if the natural UV-B wavelength range is removed
from the solar spectrum, in specimens collected from a high UV environment. This
was later confirmed under simulated sunlight conditions with different aquatic
plants in New Zealand (Hanelt et al. 2006 ) or in field studies with natural sun
radiation (Hanelt and Roleda 2009 ). Positive effects of UV-B on growth and
abundance in phytoplankton are also reported by Thomson et al. ( 2008 ); some
taxa were most abundant in treatments of intermediate fluxes of UV-B radiation.
This suggests that moderate UV-B irradiances may enhance protection from PAR
and/or UV-A. In Synechocystis , UV-B radiation accompanied by low intensity
visible light enhanced synergistically protein-repair capacity, which provides
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