Environmental Engineering Reference
In-Depth Information
phenolic subunits
78
. Purified scytonemin has an absorption maximum at 386 nm, but it
also absorbs significantly at 252, 278 and 300 nm
12,78-79
. Strong evidence for the role of
scytonemin as ultraviolet shielding compound has been presented in several
cyanobacterial isolated and collected materials from various harsh habitats, mostly
exposed to high light intensities
61,77,80
. It was also found that the well-known red and
green forms of scytonemin were related and could be interconverted by mild oxidation
and reduction
80
. Its role as a sunscreen was clearly demonstrated in a terrestrial
cyanobacterium
Chlorogloeopsis
sp.
77
. Scytonemin was also found to be responsible for
the screening of ultraviolet radiation in a number of cyanobacterial lichens, such as
Collema
,
Gonohymenia
and
Petulla
, all of which were collected from high light
intensity habitats
81
. However, intense illumination may not always be necessary for the
production of scytonemin, as the pigment has been reported to occur in a
cyanobacterium
Calothrix
deficient in Fe or Mg and exposed to low levels of
irradiation
82
.
In cyanobacterial cultures, as much as 5 % of the cellular dry weight may be
accumulated as scytonemin. Naturally occurring cyanobacteria may have an even higher
specific content
66
. The desiccation tolerant cyanobacterium
Lyngbya
sp., collected from
the bark of mango trees, has also been reported to contain scytonemin
79
. The correlation
between UV flux and scytonemin content in populations of shaded freshwater
Scytonema
and in colonies of
Rivularia
sp. were examined by Pentecost
83
. He found a
variable and even negative correlation between UV and scytonemin in
Scytonema
, but a
positive correlation in
Rivularia
. However, it was still probable that scytonemin
functioned as a radiation shield in
Scytonema
, even in shaded sites, when water relations
and cell division localization were taken into account. The correlation between UV
protection and presence of scytonemin has been established under solar irradiance in a
naturally occurring monospecific population of a cyanobacterium,
Calothrix
sp., and it
was shown that high scytonemin content is required for uninhibited photosynthesis
under high UV flux
84
. Studies indicate that the incident UV-A radiation entering the
cells may be reduced by around 90 % due to the presence of scytonemin in the
cyanobacterial sheaths
77,80,84
. Once synthesized, it remains highly stable and carries out
its screening activity without further metabolic investment from the cell. Rapid
photodegradation of scytonemin does not occur which is evidenced by its long
persistence in terrestrial cyanobacterial crusts or dried mats
77,79,84
. This strategy may be
invaluable to several scytonemin containing cyanobacteria inhabiting harsh habitats,
such as intertidal marine mats or terrestrial crusts, where they experience intermittent
physiological inactivity (e.g., desiccation). Under these metabolically inactive periods,
other ultraviolet protective mechanisms such as active repair or biosynthesis of damaged
cellular components would be ineffective
69,79,84
.
In addition to the above reports, another UV protective agent with absorption
maxima at 312 and 330 nm has been reported for the terrestrial cyanobacterium
Nostoc
commune
, a species that also produces scytonemin
85
. A brown
Nostoc
sp. that produces
three UV-absorbing compounds with absorption maxima at 256, 314 and 400 nm, has
been reported to be resistant to high light intensity and UV radiation
86
. The shielding
role of certain cyanobacterial pigments (a brown-colored pigment from
Scytonema
hofmanii
and a pink extract from
Nostoc spongiaeforme
) against UV-B induced damage
has been demonstrated
87
.
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