Environmental Engineering Reference
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part of the shikimate pathway
63-64
. A number of cyanobacteria isolated from freshwater,
marine or terrestrial habitats contain MAAs
12,61
. Presence of MAAs has also been
reported in Antarctic
51
as well as halophilic
65
cyanobacteria. The occurrence of high
concentrations of MAAs in the organisms exposed to high levels of solar radiation has
been described to provide protection as a UV-absorbing sunscreen
12,60
, but there is no
conclusive evidence for the exclusive role of MAAs as sunscreen, and it is possible that
they play more than one role in the cellular metabolism in all or some organisms
9,62,66
.
MAAs may act as antioxidants to prevent cellular damage resulting from UV-induced
production of active oxygen species
67
.
Studies with cyanobacteria have shown that MAAs prevent 3 out of 10 photons
from hitting cytoplasmic targets. Cells with high concentrations of MAAs are
approximately 25 % more resistant to ultraviolet radiation centred at 320 nm than those
with no or low concentrations
60
. This protection is unlikely to be effective for thin,
solitary trichomes, but may be especially important in some mat communities or large
phytoplankton. The MAAs in
Nostoc commune
have been reported to be extracellular
and linked to oligosaccharides in the sheath
68
. These glycosylated MAAs represent
perhaps the only known example of MAAs that are actively excreted and accumulated
extracellularly and therefore act as a true screen
69
. Experiments with rice paddy field
cyanobacteria,
Anabaena
sp.,
Nostoc commune
and
Scytonema
sp. have revealed the
existence of a circadian induction by UV-B radiation of shinorine, having an absorption
maximum at 334 nm
70-71
. The single-cell sunscreen effect of intracellular MAAs in
cyanobacteria is modest and only 10 - 30 % of incident photons were intercepted in a
fairly large-celled cyanobacterium
Gloeocapsa
sp.
60
, but the screening efficiency may
be substantially increased in colony- and mat-forming cyanobacteria
66
. There may be
physiological limitations to the accumulation of osmotically active compounds such as
MAAs within the cell, and it seems probable that the maximal specific content of MAAs
in the cell is regulated by osmotic mechanisms which is reflected by the fact that field
populations of halotolerant cyanobacteria contain unusually high concentration of
MAAs
65
. UV and osmotic stress have been reported to induce and regulate the synthesis
of MAAs in the cyanobacterium
Chlorogloeopsis
72-73
. The polychromatic action spectra
for the induction of shinorine in
Anabaena
sp. and
Nostoc commune
show a pronounced
peak at 290 nm
74-75
. In an action spectrum of MAA synthesis in another
cyanobacterium,
Chlorogloeopsis
, a distinct peak at 310 nm has been reported, and
reduced pterin has been proposed as a putative candidate for the induction of
shinorine
73
. It seems that the induction of MAAs in cyanobacteria is under photocontrol
(particularly UV-B).
Scytonemin
Scytonemin is a yellow-brown, lipid soluble dimeric pigment located in the
extracellular polysaccharide sheath of some cyanobacteria. The pigment was first
observed in some terrestrial cyanobacteria by Nägeli
76
and later termed “scytonemin”.
The occurrence of scytonemin is restricted to cyanobacteria, but it is widespread among
this diverse group, and more than 300 species with sheaths coloured with yellow to
brown pigments have been described. Scytonemin has an
in vivo
absorption maximum
at 370 nm
77
with a molecular mass of 544 Da and a structure based on indolic and
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