IMPACT OF UV RADIATION ON THE AQUATIC ENVIRONMENT - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 179

complicated pattern of the photosynthetic yield. Future research will have to show if this
behaviour is endogenously regulated.
The regulatory mechanisms to relieve light stress include modulation of antenna
size, thermal dissipation of excess excitation energy, involvement of antioxidant
enzymes and repair of photooxidative damage 63 . The photosynthetic apparatus is
protected by decreasing the electron transport chain (photoinhibition) and involvement
of the violaxanthin cycle 50 : at moderate irradiances zeaxanthin is converted to
antheraxanthin and finally to violaxanthin, while the reverse sequence is found during
exposure to excessive irradiation. Zeaxanthin is located in the vicinity of the excited
chlorophyll to facilitate thermal dissipation of excess excitation energy before the
chlorophyll can undergo intersystem crossing to the triplet state from which the energy
may be transferred to oxygen to produce the highly photooxidative singlet oxygen 63 .
The photoprotective role of the xanthophyll cycle has been investigated mostly in
microalgae 64 and to less extent in macroalgae, e.g. the green algae Ulva rotundata 65 and
Ulva lactuca 66 and the brown algae Dictyota dichotoma 67 and Lobophora variegata 38 .
Red algae do not have the xanthophyll cycle.
Passive protection against excessive visible or UV radiation relies on the
production of screening pigments such as carotenoids or UV-absorbing mycosporine-
like amino acids (MAAs). MAAs have been found in green, red and brown algae from
tropical, temperate and polar regions 68 . Wood 69 described that apical pieces of Euchema
striatum produced high concentrations of UV absorbing pigments under UV exposure.
The absorption maxima of MAA range throughout the UV-A and UV-B regions.
Macroalgae may contain several MAAs with different absorbance maxima. The
concentration of MAAs is correlated with depth distribution and UV exposure 70 . In
tropical algae, enhanced levels of carotenoids and UV-absorbing compounds were
found in tissues from the canopy compared to tissues from understory sites 71 . MAAs
may also have other biological functions including osmotic and antifreeze
mechanisms 68 .
6. Ecosystems
The Antarctic aquatic ecosystem
The Southern Ocean is characterized by large scale spatial and temporal
variability in productivity 72 . Therefore it is difficult to isolate UV-B specific effects
from other environmental effects 23 . At high latitudes, variability in solar elevation, cloud
cover, deep vertical mixing and the cover of ice and snow significantly complicate the
analysis of UV-B effects on phytoplankton. Recent estimates of the effect of 50 %
ozone reduction on total water column productivity agree with earlier findings showing
reductions between <5 % and 6 % 6,73 .
There is convincing evidence of UV-B damage to phytoplankton, but long-term
predictions are difficult because of acclimation and adaptation phenomena 74,75 , as well
as other factors 23 . Several models have been developed 23 to estimate ecosystem
productivity. Vertical mixing is a major factor modulating UV-B effects in
phytoplankton which needs to be taken into account in these models 23,73 . Container
studies clearly showed that photosynthesis of Antarctic phytoplankton is inhibited by