Environmental Engineering Reference
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was arrested within 120 - 180 min of UV-B irradiation, depending upon the species
type. Strains such as
Scytonema
sp. and
Nostoc commune
, filaments of which are
embedded in mucilaginous sheaths, have been reported to be more tolerant in
comparison to filaments which do not contain such coverings, such as
Anabaena
sp. and
Nostoc
sp.
28-31
. Several research groups have suggested that the cellular constituents
absorbing radiation in the range 280 - 315 nm are destroyed by UV-B radiation, which
may further affect the cellular membrane permeability and damage proteins, eventually
resulting in the death of the cell
9-10,28-32
.
Effects on pigmentation and phycobiliproteins
Photosynthetic organisms use various types of photoreceptor pigments to
efficiently harvest the full spectrum of the light energy. In cyanobacteria, light
harvesting is primarily carried out by a group of pigmented (accessory) proteins, known
as phycobiliproteins. which are the constituents of macromolecular complexes called
phycobilisomes (PBSs). There are three major classes of phycobiliproteins present in
the PBSs: phycoerythrin (PE, red colored, Ȝ
max
560 nm), phycocyanin (PC, blue colored,
Ȝ
max
620 nm) and allophycocyanin (APC, bluish green, Ȝ
max
650 nm), which are soluble
in aqueous media and constitute up to 50 % of the soluble proteins and about 24 % of
the dry weight of the cell
33-34
. The radiant energy captured by the phycobiliproteins is
transferred both within the PBSs as well as from the PBSs to the chlorophylls of the
thylakoid membrane with about 100 % efficiency
35
. Each phycobiliprotein is made up
of a heteromonomer of two different subunits, Į (12 - 20 kDa) and ȕ (15 - 22 kDa),
each of which contains one to four chromophores. The aggregation state of
phycobiliproteins suggest that the PBSs are composed of trimeric [(Įȕ
3
), single disc]
and hexameric [(Įȕ
6
), double disc] phycobiliprotein assemblages. In addition to the
pigmented proteins, the PBSs also contain about 15 % of proteins, mostly lacking in
chromophores, referred to as linker polypeptides, arbitrarily divided into two groups, A
(70 - 120 kDa) and B (25 - 70 kDa). These polypeptides play an important role in
stabilization of PBSs assembly and unidirectional energy flow within the PBSs and
from the PBSs to the photosynthetic reaction centers
33-35
.
The study of the effects of UV-B on pigmentation of various cyanobacteria has
revealed that all types of photosynthetic pigments such as chlorophyll
a
(Ȝ
max
at 437 and
672 nm) and the accessory light harvesting pigments (PC and PE) are affected by UV-B.
In some cases the accessory light harvesting pigment phycocyanin was bleached more
rapidly and drastically than any other pigment, such as the carotenoids (Ȝ
max
480 nm) or
chlorophyll
a
28,30
. This shows that UV radiation can photooxidize and thereby bleach all
types of photosynthetic pigments and may also cause a depression of Chl
a
and
carotenoids via reduced rate of biosynthesis. A decrease in the phycobiliprotein content
and the disassembly of phycobilisomal complex following UV-B irradiation have been
reported in a number of cyanobacteria, indicating impaired energy transfer from the
accessory pigments to the photosynthetic reaction centers
32,36-40
. Spectroscopic analyses
have shown a drastic decline in both absorption and fluorescence, as well as a shift of
the fluorescence peak towards shorter wavelengths, which are indicative of the
disassembly of phycobilisomes and hence the impaired energy transfer from the
phycobilisomes to the photosynthetic reaction centers
36-37
. This notion is further
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