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Cyanobacteria are a group of prokaryotes that possess a higher plant-type oxygenic photosynthesis. In addition to being a key player in aquatic productivity, several of these organisms is capable of fixing atmospheric nitrogen either as free-living organisms or in symbiosis with many other species including protists, animals and plants (Sinha and Häder, 1997). They use the enzyme nitrogenase to reduce atmospheric nitrogen into ammonium ions (), which they make available for aquatic eukaryotic phytoplankton as well as higher plants (Kashyap et al., 1991; Sinha et al., 1996; Sinha and Häder, 1996; Kumar et al., 1996a). The agricultural potential of cyanobacteria has been recognized as a biological fertilizer for wet soils such as in rice paddies (Banerjee and Häder, 1996). Cyanobacteria are cosmopolitan and must possess a high potential of adaptation to diverse environmental factors. However UV-B is known to affect processes such as growth, survival, pigmentation, motility, as well as the enzymes of nitrogen metabolism and CO2 fixation (Donkor and Häder, 1996; Donkor and Häder, 1997). Depending on the species, growth and survival decrease within a few hours of UV-B irradiation. Cyanobacteria also make nitrogen in mid latitude agricultural systems, though not as massive as in paddy rice fields; therefore UV-B effects on these organisms could also be relevant on a global scale.

    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.

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