Biology Reference
In-Depth Information
with high toxicity might also be used to prevent from microbial assault. In fact,
iodocarbons such as CH
2
I
2
(diiodomethane) and CH
3
I (iodomethane) were emitted
after oligoguluronate treatment (Palmer et al.
2005
). Moreover, oxidative burst in
the apoplast is triggered by the recognition of oligoguluronates (K
upper et al.
2001
)
and bacterial lipopolysaccharides (Kupper et al.
2006
). Superoxide anions probably
produced by an oxidase with a flavoprotein subunit (i.e., NADPH oxidase) in this
process become subsequently converted to H
2
O
2
that can diffuse into the seawater.
There, H
2
O
2
may affect settling of microorganisms on the thallus surface. In the
apoplast where organic antioxidants such as ascorbate and glutathione are absent,
accumulated iodide is released under oxidative stress conditions to detoxify ROS.
Enzymatically, vIPO catalyzes the removal of H
2
O
2
by reoxidation of iodide.
Furthermore, due to fast, nonenzymatic reactions of iodide with O
3
,
1
O
2
, and
superoxide anions, these ROS can be scavenged efficiently, which makes iodide
the best antioxidant available in the apoplast of
L. digitata
(K
€
upper et al.
2008
). At
the end of the oxidative stress period, the iodide remaining in the surrounding
seawater is taken up again to the apoplasm where it
€
is re-associated with
biomolecules (K
upper et al.
2008
).
During low tide events when the thallus is uncovered by water (desiccation
stress) and simultaneously exposed to high photon fluxes (high light stress) and O
3
concentrations (ozone stress), iodide may have a significant impact on atmospheric
chemistry. Molecular iodine (I
2
) is released directly from the surface into the
atmosphere above the kelp beds by the abiotic reaction of O
3
with accumulated
iodide. Then, I
2
is photolytically (impact of high light) broken down to the iodine
radical (I
) on the thallus surface. The I
reacts with O
3
in the troposphere to IO
(iodine oxides) providing a precursor for aerosol particles (
€
3 nm) that may lead to
cloud condensation nuclei formation. Particle formation seems to be a result from
O
3
-scavenging reactivity of iodide on the thallus surface (K
>
upper et al.
2008
) rather
than by the iodine atom-releasing breakdown of iodocarbons as presumed earlier
(O'Dowd et al.
2002
). So, taken all these impacts together, the iodine metabolism in
kelps has not only a great impact on ROS scavenging in the apoplast and the
extracellular defence but also on atmospheric chemistry.
€
6.3 Antioxidative Strategies in Seaweeds on Seasonal
and Spatial Scales
For all seaweeds living in highly variable environments, it is essential to apply
protective and detoxifying mechanisms to scavenge and to minimize the adverse
effects of oxygen radicals, which are resultants from abiotic environmental stress
(Coll´n and Davison
1999a
,
b
; Rijstenbil et al.
2000
; Bischof et al.
2003
).
As already indicated above, macroalgae apply a multitude of enzymatic and
nonenzymatic tools to reduce the impact of ROS on their biological/cellular structures.
Studies on Arctic and cold-temperate macroalgae demonstrated the plasticity of the
