Biology Reference
In-Depth Information
dehydroascorbate reductase (DHAR) as well as glutathione are involved in the
regenerating process often called ascorbate-glutathione (AsA-GSH) cycle. In
contrast to higher plants, seaweeds are additionally able to detoxify H
2
O
2
by
excretion out of the cell into their environment (Ross and Van Alstyne
2007
). It
has been shown for the chlorophyte
Ulva rigida
that H
2
O
2
excreted from the thallus
can reach concentrations up to 4.0
M in the surrounding seawater (Coll
´
netal
1995
; Coll
´
n and Peders
´
n
1996
). Catalase (CAT) is another enzyme that cleaves
H
2
O
2
to oxygen and water, but it seems that this enzyme exclusively detoxifies
H
2
O
2
produced by glycolate oxidase in peroxisomes during photorespiration (Gross
1993
). Coll´n and Davison (
2001
) concluded from their studies on the seaweed
Fucus vesiculosus
that CAT activity is regulated independently of the other
components of the reactive oxygen metabolism like SOD and GR and, thus, of
light. Because photorespiration is low at low temperatures, an increased CAT
activity is unlikely to be detected at suboptimal seawater temperatures. In
Ulva
rigidia
, no significant CAT activities have been detected when exposed to several
high light environments and only APX increased its activity (Coll´n et al.
1995
).
Peroxidase enzymes (PX) need the presence of a reducing substrate e.g., ascorbate
and glutathione which are regenerated in the AsA-GSH cycle. Another PX is
glutathione peroxidase (GPX), which has been intensively studied in mammalian
tissues and higher plants, but much less is still known for this enzyme in seaweeds
(Eshdat et al.
1997
; Contreras et al.
2009
).
Interestingly, the above-mentioned enzymes for oxidative stress management
(SOD and CAT) have hardly been reported for kelps (i.e., the representatives of the
brown algal order Laminariales). A single DNA sequence likely encoding for SOD
was isolated from the gametophyte of
Laminaria digitata
(Crepineau et al.
2000
;
see Bartsch et al.
2008
), but the further analysis of 1985 gene transcripts did not
reveal the expression of any of these enzymes in
L. digitata
(Roeder et al.
2005
).
Instead, it is apparent that the stress-induced expression of bromoperoxidases is
involved in oxidative stress management. These enzymes are typically catalyzing
the oxidation of halide ions to hypohalous acid, in the required presence of H
2
O
2
.It
was shown that haloperoxidases in
Laminaria
are mainly located near the cuticle in
the external cortex region of the thallus (Almeida et al.
2001
) and around the
mucilaginous channels specifically in
L. hyperborea
.In
Saccharina latissima
and
L. digitata
bromoperoxidases were only found being active in the blade (Jordan
et al.
1991
; Mehrtens and Laturnus
1997
). Oxidative stress in kelps was shown to
increase halomethane production (Palmer et al.
2005
) and the inhibition of photo-
synthetic electron transport. Consequently, elevated H
2
O
2
production via the
Mehler reaction reduced the halogenation process (Goodwin et al.
1997
).
Haloperoxidase is thus considered as another key enzyme for oxidative stress
management in kelps, and in this respect further studies on other ecologically
important species of seaweeds are urgently needed (see Bartsch et al.
2008
).
Next to ROS-scavenging enzymes there is a suite of a large number of different
nonenzymatic intermediate metabolites present which confer antioxidative
properties and thus may additionally or alternatively be employed to ROS scaveng-
ing in macroalgae, like e.g., ascorbate, glutathione, carotenoids, etc. Today's
m
