Biology Reference
In-Depth Information
nutrient limitation. The induction of ROS formation by mechanical stress, however,
has been a matter of debate and has now been more attributed as an oxidative burst
response, similar to the responses upon wounding of the thallus (Coll ´ n and
Peders ´ n 1994 ; Ross et al. 2005 ; see Dring ( 2005 ) and Potin ( 2008 ) for review).
There is a multitude of adverse effects that ROS may confer to biological
components. One prominent target of ROS is biological membranes, i.e., the lipids,
which are differentially sensitive depending on their respective saturation state.
ROS-induced peroxidation of membrane lipids results in the formation of
aldehydes, e.g., malondialdehyde as readily shown for a variety of marine
macroalgae under excessive radiation conditions, both in the PAR and ultraviolet
(UV) range (Bischof et al. 2002 , 2003 , 2006 ; Dummermuth et al. 2003 ). Due to
their complex structures, proteins are also highly susceptible to the interaction with
ROS. A multitude of ROS impacts on proteins have been described, interfering on
all levels, the primary, secondary, and tertiary structure of proteins, e.g., by specific
interaction with the respective amino acids involved based on their differential
substitutes (see Dring 2005 ). For example, in enzymes ROS may impact on active
centers, and in particular ROS-induced impairments of iron-sulfur centers have
been described to result in inactivation of enzyme function (see Dring 2005 ).
An important, visual result of oxidative stress is “photobleaching”: the loss of
photosynthetic pigments like chlorophyll due to their photo-oxidation. This phe-
nomenon has been frequently observed in seaweeds exposed to high PAR (Bischof
et al. 2002 , 2006 ). In example, loss in pigmentation in response to steep microscale
gradients of solar exposure has been studied within green macroalgal mats (Vergara
et al. 1998 ; Bischof et al. 2002 ). Supported by high nutrient loads, the green
macroalga Ulva rotundata may form thick mats consisting of multiple thallus
layers. Within these assemblages a steep gradient of irradiance persists, which
results in different physiological effects. Whereas the pigments in the top layers
(canopy) are directly exposed to solar radiation and become visibly photobleached,
subcanopy algae suffer from light limitation. Under the high and prolonged radia-
tion conditions, the canopy algae may become completely bleached within 2 days
of exposure. The drastic loss in chlorophyll is accompanied by an increase in
malondialdehyde levels, which is an indicator for light stress-induced lipid peroxi-
dation (Bischof et al. 2002 , 2003 ). High production of malondialdehyde was also
observed in the top layers of mats of another green seaweed, Chaetomorpha linum ,
however, with not as pronounced changes in chlorophyll concentration as in the
latter species (Bischof et al. 2006 ).
6.2.1 Seaweed Exposure to Heavy Metal Stress and Reactive
Oxygen Metabolism
Very recent studies have highlighted the particular significance of heavy metal
exposure for the induction of oxidative stress in seaweeds. In natural seawater,
Search WWH ::




Custom Search