Environmental Engineering Reference
In-Depth Information
depth (Hoyer et al. 2001, 2003). Many taxa growing in the sublittoral are
not physiologically capable to produce MAAs, which explains their strong
sensitivity to solar radiation. Different portions of seaweed thallus do not
respond uniformly to the solar conditions. It was observed that young apical
or marginal zones, with growing cells, synthesize and accumulate MAAs
leading to cross sectional and longitudinal concentration gradients (Hoyer
et al. 2001). Older tissue regions exhibit thicker cell walls and a leathery
texture, and are therefore optically well protected. In contrast, higher
MAAs concentration in the most exposed outer cortex is essential guarantee
protection of the delicate meristematic cells (Bischof et al. 2006).
Other compounds that absorb UV radiation are phlorotannins, polymers
of phloroglucinol, which can be found in physodes and cell walls of brown
algae (Schoenwaelder and Clayton 1998, 1999). Phlorotannins have multiple
roles (Arnold and Targett 2002, 2003, Lüder and Clayton 2004, Amsler
and Fairhead 2006). Bischof et al. (2006) suggested four points to consider
phlorotannins as UV-protecting compounds: (1) high tissue concentration
which absorbs harmful radiation and prevents cell damages, observed in
outer cell layers in
Hormosira banksii
(Schoenwaelder 2002); (2) harmful
radiation induces its synthesis, performed in
Ascophyllum
nodosum
after
exposure of UV-B (Pavia and Brock 2000); (3) exudation in the surrounding
medium shielding harmful radiation, observed in
Macrocystis
integrifolia
(Swanson and Druehl 2002), in
Eisenia bicyclis
and
Ecklonia kurome
(Shibata et
al. 2006); and (4) an excess inclusion of phlorotaninns in cell walls shielding
harmful radiation. It was also demonstrated by Swanson and Druehl (2002)
that seawater containing phlorotannin exudates of
Macrocystis
increased
survivorship of germinating
Laminaria groenlandica
spores exposed to
UV-B. Added to these, Schoenwaelder et al. (2003) linked higher numbers
of physodes in
Fucus spiralis
embryos with a greater tolerance to elevated
levels of UV-A and UV-B.
DNA damage can be repaired photoenzymatically in the presence of
UV-A or blue light. This repair mechanism is known as “photoreactivation”
or “photoenzymatic repair” and it reverses the photodimer products
(cyclobutane-pyrimidine dimers; CPDs) (van de Poll et al. 2002). The
dimers are cytotoxic because they block DNA and RNA polymerase and
consequently inhibit genome replication and expression (Jordan 1996).
Experiments in
Palmaria palmata
and
Chondrus crispus
after the exposure
to artifi cial UV radiation evidenced no accumulation of DNA damage but
decrease in CPD concentration (van de Poll et al. 2002). The accumulation
of CPDs is accompanied by reduce growth rates in several phytoplankton
and macroalgal species (van de Poll et al. 2002, Buma et al. 2003).
