IMPACT OF UV RADIATION ON THE AQUATIC ENVIRONMENT - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 177

Macroalgae have to thrive with the prevailing irradiance regime of solar radiation
governed by the daily and seasonal changes as well as the tidal rhythm. Most
macroalgae are adapted to lower irradiances than terrestrial plants so that they face a
serious light stress when exposed to high irradiances during low tide and high solar
angles 9 , but they have developed strategies to adapt their photosynthetic apparatus to the
changing light conditions and to protect themselves against excessive radiation.
Macroalgae use the same mechanism of photoinhibition as higher plants to decrease
their photosynthetic activity during high light exposure by reversibly reducing the
photosynthetic electron transport chain 36 . The excess energy is thermally dissipated 37 .
However, even higher solar irradiances may cause photodamage which is not as readily
reversible as photoinhibition.
Different algal species have a clearly different behaviour upon irradiation 38-42 ;
especially their ability to cope with enhanced UV radiation varies widely among
species 43 . A number of ecologically important algae have been studied during the last
few years, found in the Atlantic, Pacific, Mediterranean, North Sea and Baltic Sea.
The phenomenon of photoinhibition can be studied by oxygen exchange
measurements. Oxygen sensors have been used in field incubators as well as in
microsensor studies. Oxygen gradient analysis has also been carried out in
cyanobacterial mats indicating that the photosynthetic rates were negligible near the
surface and maximal deeper in the mat 44 .
Alternatively, the photosynthetic efficiency can be determined by PAM (Pulse
Amplitude Modulated) fluorescence measurements developed by Schreiber et al. 45 .
PAM fluorescence determines the transient changes of chlorophyll fluorescence and can
be used to calculate the photochemical and non-photochemical quenching of the
photosynthetic apparatus 46 . This method reveals the regulatory processes and the
physiological status of the photosynthetic apparatus in vivo 37,47 . Often there is a strong
correlation between photosystem II chlorophyll fluorescence and oxygen evolution 48 but
it may not be a good indicator for growth and biomass production 49 .
There is a distinction between two different types of photoinhibition: dynamic
photoinhibition and chronic photoinhibition 50 . Dynamic photoinhibition is associated
with a decrease of the effective quantum yield and with an increase in non-
photochemical quenching (qN), related to the conversion of violaxanthin to zeaxanthin
which is a potent quencher for excess excitation energy both in algae and in higher
plants 51 . Simultaneously, an increment of pH-dependent processes or membrane
energization (qE) is found, which lowers the efficiency of PS II 50 .
Shade plants often show chronic photoinhibition which is characterized by slow
reversibility if at all. It is found in algae which are exposed to excessive irradiation. The
extent of chronic photoinhibition can be calculated by 1-qP. It has been known that
photosynthetic active radiation (PAR) induces photoinhibition, but recent measurements
indicate a strong role for solar ultraviolet radiation both in terrestrial and aquatic plants.
Especially the short wavelength range of UV-B affects photosynthesis in several species
of marine macroalgae even though its relative energy contribution is disproportionally
smaller than that of PAR in the solar spectrum, and many inhibitory effects of UV-B on
photosynthesis and chlorophyll fluorescence of several species of marine benthic algae
and phytoplankton have been documented 52-53 .