In addition to DNA, which is a highly susceptible cellular target, the photosynthetic pigments are affected. The phycobiliproteins, especially, are readily bleached and cleaved (Sinha et al., 1995, 1996; Aráoz and Häder, 1997). Bleaching of these accessory pigments is far more efficient than that of chlorophyll a or carotenoids (Sinha et al., 1995). At lower doses the energy transfer to the reaction center of photosystem II is impaired (Sinha et al., 1996). Simultaneously with destruction, an increased synthesis of phycobiliproteins has been observed under mild UV-B stress. The fact that these pigments strongly absorb in the UV-B range and that they form a peripheral layer around the sensitive central part containing the DNA might indicate that phycobilins are effective screening pigments, as well (Aráoz and Häder, 1997). They are capable of intercepting more than 99% of UV-B radiation before it penetrates to the genetic material.
UV-B induced inhibition of photosynthetic activity has been demonstrated in a number of marine and freshwater cyanobacteria. Sinha et al. (1996) reported that, in addition to the bleaching of the photosynthetic pigments, RuBisCO (ribulose-1,5-bis-phosphate carboxylase/oxygenase) activity was severely affected by UV-B treatment. Ammonium uptake was reduced by 10% in cultures exposed to solar radiation.
The nitrogen fixing enzyme nitrogenase is inhibited by UV-B even after a few minutes of in-vivo exposure. A complete loss of activity was found within 35-55 minutes depending upon the species (Kumar et al., 1996b). The inactivation may possibly be due to the inhibition of ATP synthesis by UV-B. In contrast to the effect on nitrogenase, a stimulation of nitrate reductase by UV-B was found in all nitrogen fixing cyanobacterial strains studied so far (Sinha et al., 1995), while the ammonia-assimilating enzyme glutamine synthetase (GS) is inhibited.
Many cyanobacteria have developed a
number of adaptive strategies to reduce the negative effects of excessive
radiation, including the avoidance of brightly irradiated habitats, the
synthesis of UV screening pigments, and the production of chemical scavengers
that detoxify the highly reactive oxidants produced photochemically (Vincent
and Roy, 1993). Screening pigments include scytonemin and mycosporine-like
amino acids (MAAs), as well as a number of spectroscopically characterized
but chemically unidentified, water-soluble pigments (e.g., a brown-colored
pigment from Scytonema hofmanii and a pink extract from Nostoc
spongiaeforme) (Kumar et al., 1996b; Donkor and Häder, 1995).
Cyanobacteria such as Scytonema and Nostoc form filaments
that are embedded in a mucilaginous sheath. The screening pigment from
Scytonema hofmannii shows an absorption maximum at 314 nm and is
released into the medium during the late stationary phase of growth. These
organisms are more tolerant of UV-B irradiation than those that do not
contain such covering (Sinha et al., 1995). For example, other species
of Scytonema that do not produce this pigment are unable to survive
2 h of UV-B (2.5 Wm-2).
Karsten and Garcia-Pichel (1996) showed that screening pigments such as
scytonemins, carotenoids and mycosporine-like amino acids are incorporated
into the cytoplasm or the outer slime sheath, efficiently protecting the
organisms from solar short-wavelength radiation.