Biology Reference
In-Depth Information
definition of antioxidants also includes polyphenols, vitamin A and retinoids,
vitamin E (
-tocopherol), and various sulfur compounds. Hitherto, various phenolic
compounds, carotenoids, as well as mycosporine-like amino acids (MAAs)
(Karsten et al.
1998
; Aguilera et al.
2002b
; Schoenwaelder
2002
) have been
identified as potential antioxidants from Arctic macroalgae. However, currently
little information is available from field studies on the antioxidative potential of
Antarctic macroalgae. Interestingly, some MAAs which have received significant
research attention due to their UV-screening capacities have also been found to
reveal some antioxidative potential (de la Coba et al.
2009
), which presumably
makes them part of a general stress response in red macroalgae (see also Chap.
20
by Bischof and Steinhoff). Again, there is evidence that the cellular concentration
and composition of antioxidative compounds in seaweeds are highly species-
specific and dependent on the external environmental forcings (Coll´n and Davison
1999a
,
b
; Lohrmann et al.
2004
).
Although low surrounding temperatures can lead to adverse effects on the meta-
bolic functioning, they also may modulate the impact of high irradiances quite
substantially. It has been shown that the combination of high PAR at low water
temperature may confer adverse conditions to Antarctic macroalgae (Becker et al.
2010
). The reason behind is likely to be a temperature-dependent reduction in
photosynthetic secondary reactions (i.e., the Calvin-Benson cycle) resulting in a
reduced reoxidation of NADPH, and consequently in an elevated reduction state of
ferredoxin (Asada
1999
). However, while photosynthetic secondary reactions are
strongly temperature controlled, primary reactions are not. Thus, high irradiances of
PAR will pump electrons into the photosynthetic ETC and, at the high reduction state
of ferredoxin, a considerably higher share of electrons will be fuelled into the Mehler
reaction, yielding an increased concentration of O
2
. These environmental conditions
are frequently observed in shallowwaters of polar and cold-temperate regions (Coll
´
n
and Davison
1999b
,
2001
;Beckeretal.
2009
,
2010
). The impact of temperature on the
reactive oxygen metabolism was studied by Coll
´
n and Davison (
2001
) in the cold-
temperate intertidal brown alga
Fucus vesiculosus
. Here, particularly the increase in
the ROS-scavenging activity of SOD has been shown as a characteristic feature, which
was related to experimental temperature fluctuations. This, however, might also be
highly significant in terms of responses to seasonal changes in the abiotic environ-
ment, which are most pronounced in cold-temperate regions.
For completeness, it should be noted that apart from the chloroplasts, other
important ROS generation sites in plants are the inner mitochondrial membrane.
There, superoxide radicals might be generated, e.g., by the NADPH-dependent
electron transport involving the cytochrome P-450. In peroxisomes and
glyoxysomes, H
2
O
2
formed via enzymatic reactions (by glycolate oxidase and
acyl-CoA oxidase) is rapidly degraded by CAT to oxygen and water (see Dring
2005
). However, with respect to the quantitative significance toward ROS genera-
tion under abiotic environmental stress these processes seem to be of minor impor-
tance in plants and seaweeds. The generation of oxidative stress has been largely
attributed to the result of a stress-induced impairment/restriction of electron flow, as
it may be evidently the case under e.g., excess radiation, temperature stress, and
a
