IMPACT OF UV RADIATION ON THE AQUATIC ENVIRONMENT - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 174

fertilizer 14 , but they are also indispensable for other agricultural and natural soils as well
as for marine habitats. Any reduction in cyanobacterial biomass productivity, e.g. by
enhanced solar UV-B, could be relevant on a global scale.
Solar UV-B affects growth, survival, pigmentation, motility, as well as the
enzymes of nitrogen metabolism and CO 2 fixation in cyanobacteria 15 . In some species
growth and survival decrease within a few hours of UV-B irradiation. These organisms
are major constituents in wet lands forming extended microbial mats. In order to survive
the extreme solar UV stress they must possess a high potential of adaptation to diverse
environmental factors.
The photosynthetic apparatus is strongly impaired by solar UV radiation. The
phycobiliproteins, which constitute a large share of the accessory pigments, are readily
cleaved 16-17 . The chromophoric groups, the phycobilins, are bleached with a faster
kinetics than chlorophyll a or the carotenoids. At lower doses, which do not bleach the
pigments, the energy transfer to the reaction center of photosystem II is impaired 16 . In
parallel to phycobiliprotein destruction, an increased synthesis of the same pigments is
induced under mild UV-B stress. Since they strongly absorb in the UV-B range and the
thylakoids, on which they are located, form a peripheral layer around the sensitive
centroplasma containing the DNA, one of the functions of phycobilins might be that of
an effective UV screen 17 . It has been calculated that these pigments can intercept more
than 99% of UV-B radiation before it penetrates to the genetic material.
Inhibition of photosynthesis by solar UV radiation has been demonstrated in a
number of marine and freshwater cyanobacteria. The key enzyme, RuBISCO (ribulose-
1,5-bisphosphate carboxylase/oxygenase), which is responsible for the CO 2
incorporation into organic material, was severely affected by UV-B treatment 16 .
Ammonium uptake was decreased by 10 % after exposure to solar radiation. The
nitrogen fixing enzyme nitrogenase is strongly affected by UV-B as well as the
ammonia-assimilating enzyme glutamine synthetase 18 , while nitrate reductase was
stimulated by UV-B.
During evolution cyanobacteria have developed a number of mitigating and
adaptive strategies to protect themselves from the deleterious effects of excessive
radiation, including the avoidance of high light areas, the synthesis of UV absorbing
pigments, DNA repair and recovery of the photosynthetic apparatus as well as the
production of chemical scavengers to detoxify the highly reactive oxidants produced
photochemically 19 . UV-absorbing pigments include scytonemins and mycosporine-like
amino acids (MAAs), as well as a number of chemically unidentified pigments 18 . The
protective mechanisms developed by cyanobacteria are discussed in more detail by
Sinha and Häder in this volume.
4. Phytoplankton
Solar UV-B is an important ecological stress factor that affects growth, survival
and distribution of phytoplankton, which are, as indicated above, the most important
biomass producer in aquatic ecosystems. Phytoplankton are a diverse group of normally
unicellular microalgae from many different classes which inhabit the euphotic zone of