Environmental Engineering Reference
In-Depth Information
words, increasing temperatures
per se
may not topple the algae, but rather
gradually reduce their resilience to natural perturbations such that impacts
manifest either abruptly when the physiological threshold of existence
is fi nally exceeded or progressively as the cumulative effect of localized
failure to recover.
UV stress
In their natural environment, seaweed are exposed to excessive solar PAR
(photosynthetic active radiation, 400-700 nm) as well as to ultraviolet
radiation (UV-B, 280-315 nm and UV-A, 315-400 nm), especially in the upper
eulittoral and supralittoral zones (Hanelt 1998). UV radiation is usually
considered harmful at either organism or community levels (Häder et al.
2007), specially the UV-B range which is increasing at the Earth's surface as
a consequence of the decrease in stratospheric ozone concentration (Björn
et al. 1999, Bischof et al. 2006).
The increase in irradiance and light quality can promote photosynthesis,
but also inhibit many biological processes if it becomes excessive (Barber
and Andersson 1992), or if short wavelength radiation with high energy
content, such as UV-B radiation, is absorbed by biomolecules (Vass 1997).
Consequently, damage results in reduced photosynthesis and general
metabolic activity leading to a decrease in biomass production (Helbling
et al. 2003). Most of the seaweeds photoinhibition is due to PAR, as this
waveband has a high proportion of solar radiation energy reaching the
Earth's surface. However, in the upper meters of the water column, a
signifi cant percentage of photoinhibition is caused by UV-B, and to a lesser
extent by UV-A (Dring et al. 1996, Häder 1997).
Positive effects of UV-A have also been reported. UV-A enhances
carbon fi xation under reduced solar radiation (Barbieri et al. 2002), even
in absence of PAR, and allows photorepair of UV-B induced DNA damage
(Buma et al. 2003).
Biological effects of UV-B
The effects of UV-B exposure on biological systems range from molecular
to organism level, thereby affecting growth and reproduction, and
consequently the ecosystem structure and function (Bischof et al. 2006).
DNA is one of the most UV-sensitive molecules and UV-induced
damage occurs directly by the absorption of UV-B quanta by aromatic
residues, inhibiting replication or causing mutations, thereby affecting gene
expression. UV-B is also absorbed by aromatic residues present in certain
amino acids, and therefore affects proteins (Bischof et al. 2006). Lipids are
also affected, which may be destroyed by UV-B in the presence of oxygen,
