Biology Reference
In-Depth Information
6.2 Stress-Induced ROS Production and Its Effects
on Seaweed Performance
Being photoautotrophic organisms conducting oxygenic photosynthesis, macroalgae
display the same major generation sites for oxygen radicals as identified for higher
plants. Any malfunction of photosynthesis triggered by an unfavorable environment
will finally be reflected by an increase in the level of internally generated ROS. As
photosynthetic primary reactions are directly related to the generation of oxygen
radicals, hampered dissipation of excessive PAR has been identified as an important
inducer of ROS formation in plants (Asada and Takahashi
1987
; Asada
1999
;
Bischof et al.
2002
,
2003
; see also Chap.
1
by Hanelt and Figueroa). Excessively
absorbed light energy may result in an over-excitation of chlorophyll
a
molecules
present in the photosynthetic reaction centers, which may then pass into the so-called
triplet state. The critical characteristic of this process is that triplet chlorophyll (
3
Chl)
may subsequently promote the formation of
1
O
2
, which is highly reactive with any
kind of cellular component (see Ledford and Niyogi
2005
). Higher plants as well as
algae try to avoid formation of
3
Chl by dissipation of excess radiation energy by the
xanthophyll cycle, in which energy is dissipated harmlessly as heat by the intercon-
version of specialized xanthophylls located in the antennae (see Jahns et al.
2009
). In
addition,
3
Chl may be deactivated by carotenoid associated to the photosynthetic
reaction centers (
-carotene and lutein).
A second and quantitatively by far the most important source of ROS formation
in photosynthesis is the formation of O
2
in the process of Mehler reaction or
photoreduction of oxygen: At a high reduction state of ferredoxin (e.g., under high
light conditions, or in case electrons are not drained off at sufficient pace under
varying stress conditions), electrons coming from PSI might be transferred to O
2
rather than to ferredoxin, which generates O
2
(Asada
1999
). This ROS may
subsequently reduce metal ions like Fe
3+
and Cu
2+
univalently. The O
2
, however,
is quenched via the enzyme SOD, yielding O
2
and H
2
O
2
. The latter, highly reactive
compound may also either react with the formerly reduced metal ions (“Fenton
reaction”: Fe
2+
and Cu
+
) or with another O
2
(“Haber-Weiss reaction”). In both
cases, the extremely reactive OH is formed (see Dring
2005
). This aggressive
compound initiates free radical cascades and may denature proteins, nucleic acids,
and peroxidise lipids, potentially resulting in biochemical dysfunctions and struc-
tural damages such as membrane leakage. As plants do not have specific protective
agents against OH, the only means of protection is to scavenge both the O
2
and
H
2
O
2
as fast as possible and to suppress metal ion reduction (Asada
1999
).
Moreover, H
2
O
2
may also harm multiple biological components, and therefore, it
has to be detoxified as well. This is achieved by the enzyme ascorbate peroxidase
(APX). In order to minimize destructive effects by H
2
O
2
via free diffusion across
biological membranes, APX is located closely to SOD in the neighborhood of PSI
as well. Its cofactor, ascorbate, is regenerated by the vitamin's two oxidized forms:
monodehydroascorbate (MDHA) and dehydroascorbate (DHA). The enzymes
glutathione reductase (GR), monodehydroascorbate reductase (MDHAR), and
b
