IMPACT OF UV RADIATION ON RICE-FIELD CYANOBACTERIA: ROLE OF PHOTOPROTECTIVE COMPOUNDS - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 212

supported by SDS-PAGE analysis of the phycobiliproteins which shows a loss in the
low molecular mass proteins between 16 and 22 kDa (phycobiliprotein D and E
subunits) and high molecular mass rod and rod-core linker polypeptides (molecular
masses between 24 and 45 kDa) as well as core membrane linker polypeptides
(molecular masses around 66 kDa). It seems probable that the supramolecular
organization of the phycobilisomes is disassembled largely due to the loss of linker
polypeptides after UV irradiation, as has been shown by electrophoretic
analyses 32,36-37,39 .
Effects on heterocyst differentiation and enzymes of nitrogen metabolism
Differentiation of vegetative cells into heterocysts has also been reported to be
severely affected by UV-B irradiation in a number of cyanobacteria 41 . Processes such as
heterocyst differentiation and nitrogen fixation are metabolically very expensive and
during extreme stress conditions may require cellular resources for their maintenance.
Since heterocysts are important primarily in supplying fixed nitrogen to the vegetative
cells, apparently vegetative cells growing in areas without external nitrogen sources
may be seriously affected. Most probably the C:N ratio is severely affected following
UV-B irradiation, which in turn affects the spacing pattern of heterocysts in a filament 41 .
In addition, major heterocyst polypeptides of around 26, 54 and 55 kDa have also been
shown to decrease following UV-B irradiation, suggesting that the multilayered thick
wall of heterocysts may be disrupted resulting in the inactivation of the nitrogen fixing
enzyme nitrogenase 41 . UV-B-induced membrane disruption, leading to changes in
membrane permeability and release of 14 C-labelled compounds, has been observed in a
number of cyanobacteria 32 . UV-B-induced inactivation of the nitrogen fixing enzyme
nitrogenase has been reported in many cyanobacteria 41-42 . The process of nitrogen
fixation has also been reported to be depressed during midday, when visible and UV
irradiance are higher 43 . Nitrogenase is a molybdoenzyme and requires ATP and
reductants for its activity. There is a possibility that the inhibition of the nitrogenase
activity might be due to reduced supply of reductants and ATP following UV treatment.
However, the loss of reductants and ATP does not occur immediately in cyanobacteria
since many N 2 fixing species are capable to drive nitrogenase activity at the cost of an
endogenous pool. It has been suggested that UV irradiation causes
inactivation/denaturation of the nitrogenase enzyme and this appears to be a novel
phenomenon 41-42 . Similarly, the primary ammonia assimilating enzyme, glutamine
synthetase (GS), is also affected by UV-B irradiation, whereas nitrate reductase (NR)
activity is stimulated 28,42 .
Effects on total protein profiles
Total protein profiles of several cyanobacteria as evidenced by SDS-PAGE show
a decrease in protein content with increasing UV-B exposure time, indicating that
cellular proteins are among the main targets of UV-B 28,39,41 , which is known to damage
proteins and enzymes, especially those rich in aromatic amino acids such as tryptophan,
tyrosine, phenylalanine and histidine, all of which show strong absorption in the UV
range from 270 to 290 nm 9,28,39,41-42 .