Environmental Engineering Reference
In-Depth Information
surface and interior of rocks, lakes, ponds, streams, moss beds, melt pools on glaciers
and ice shelves, and littoral marine sediments. In some of these habitats they constitute
the ecosystem dominant species in terms of total biomass and productivity.
Cyanobacteria are also a conspicuous element of mature microbial communities across
the surface of Antarctic rocks, as well as under and within translucent rocks
16
.
Cyanobacterial communities form crusts, films and spectacular mats up to several
centimeters in thickness, often intensely coloured with pigments. Mat-forming
cyanobacteria are especially widespread in high latitude ponds and streams. In these
environments the cyanobacteria often occupy shallow-water habitats that are exposed to
full sunlight. Even moderate levels of UV-B radiation can have a major physiological
impact on Antarctic cyanobacteria, but there are substantial differences between closely
related species in their ability to escape the damaging effects of this high-energy
waveband
51
. It has been concluded that the phototrophic organisms living in cold
environments may be especially prone to the damaging effects of UV-B radiation. These
findings are relevant to the perennially cold waters found in the north and south polar
zones, where stratospheric ozone depletion and the associated increase in ambient UV-B
radiation are proceeding most rapidly
6,52
. Thus, in natural habitats avoidance of UV-B
radiation seems to be of utmost importance for cyanobacterial growth and nitrogen
fixation.
5. Photoprotective compounds in cyanobacteria
Cyanobacteria may protect themselves from photodamage by adopting one or
more of the following strategies: (a) production of ultraviolet-absorbing/screening
substances such as mycosporine-like amino acids (MAAs) and scytonemin
11-13
;
(b) escape from ultraviolet radiation by migration into habitats having reduced bright
light exposure. Such strategies include phototactic, photokinetic and photophobic
responses
53-54
, vertical migration into deeper strata of mat communities
55
and sinking
and floating behaviour by a combination of gas vacuoles and ballast
56
; (c) production of
quenching agents such as carotenoids
15
or enzyme systems such as those containing
superoxide dismutase that react with and thereby neutralize the highly toxic reactive
oxygen species produced by UV radiation
16
; (d) availability of a number of repair
mechanisms such as photoreactivation and light-independent nucleotide excision repair
of DNA
18
and UV-A/blue light-guided repair of the photosynthetic apparatus
57
; and
(e) chromatic adaptation (variation in phycocyanin/phycoerythrin ratio), which allows
regulation of the balance of wavelengths of absorbed light
58
. Below we discuss only the
MAAs and scytonemin and their evolutionary history.
Mycosporine-like Amino Acids (MAAs)
MAAs are water-soluble substances characterized by a cyclohexenone or
cyclohexenimine chromophore conjugated with the nitrogen substituent of an amino
acid or its imino alcohol, having absorption maxima ranging from 310 to 360 nm and an
average molecular weight of around 300
11-13,59-61
. Accumulation in large quantities of
colorless UV-absorbing substances (now known as MAAs) in cyanobacterial cells were
reported in 1969 by Shibata
62
. Synthesis of MAAs probably takes place from the first
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