Environmental Engineering Reference
In-Depth Information
6. Evolutionary history of photoprotective compounds
The possibility that organic molecules might have acted as a UV screen in
aquatic environments of the early Earth was first considered by Sagan 88 . The nature and
evolutionary origin of the first specific photoprotective compounds on the Achaean
Earth is unknown, but according to one assumption early aromatic containing reaction
centres were some of the earliest UV screens that over the period altered from a
nonproductive dissipative UV screen to a light harvesting role in photosynthesis 89 .
MAAs play a vital role as osmotic regulators in some cyanobacteria 65 and such
alternative role may have given rise to the first UV-screening MAAs. MAAs evolution
as specific UV-protectants may represent an early innovation in dealing with Achaean
UV-B fluxes. Certain MAAs such as mycosporine-glycine specifically absorb in the
UV-B range of the spectrum. It has been postulated that later, as oxygen levels
increased, UV-A screening MAAs became more important since many of the effects of
UV-A are mediated through oxygen free radicals and thus the role of UV-A as a
damaging agent in the biosphere increased. In these compounds, the nitrogen atom
replacing the ketone function has a greater mesomeric effect on the benzene ring and the
absorbance is shifted towards the UV-A. Most probably, a mutation in the earliest UV-B
screening compounds resulted in a UV-A screen which became physiologically
advantageous. Since MAAs have been reported in several eukaryotic algae, it is likely
that they were passed to the eukaryotic algae by cyanobacteria in the plastidic line 13 . It
has been suggested that the protection against UV radiation provided by scytonemin
may have been an important ecophysiological factor in cyanobacterial evolutionary
history 90 . Scytonemin would have facilitated the ecological expansion of cyanobacterial
mat communities into exposed shallow-water and terrestrial habitats during the early to
middle Precambrian, despite the high levels of UV radiation impinging on the Earth's
surface at that time.
7. Conclusions
Increases in the level of UV-B radiation are likely to induce changes in
cyanobacterial community composition since there are great differences in susceptibility
of species to UV-induced damage. Species having the ability to accumulate
UV-screening substances or with more effective repair mechanisms will likely be
favoured. The ecological significance of photoprotective compounds in diverse
organisms as screening agents against UV-induced damage has yet to be fully
elucidated. The spatial distribution of MAAs within the cells is not well known.
However, photoprotective compounds may serve at least three different functions:
(a) protection of the cell from UV photodamage by playing the role of sunscreen 11-13 ,
(b) transfer radiant energy to the photosynthetic reaction centres, which is supported by
the fact that the emitted fluorescence spectrum of MAAs peaks at a wavelength near the
Soret band of chlorophyll absorption 70 , and (c) aid in osmotic regulation 65,72 .
Acknowledgement. The work outlined here was financially supported by the European
Union (DG XII, Environmental Program, ENV4-CT97-0580).
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