Biology Reference
In-Depth Information
concentrations of heavy metals are usually very low. Because copper (Cu
2+
)
concentration ranges between 0.05 and 3 g L
1
(Contreras et al.
2005
,
2009
), it is
a micronutrient, which is used as cofactors for enzymes (e.g., oxidases, SODs) and
ETC components (e.g., plastocyanin). However, due to industrial activities such as
copper mining, Cu
2+
concentrations may rise significantly when highly
contaminated wastewaters are drained off into coastal areas. Ecologically, such
an impact reduces species richness as well as changes the structures of coastal
communities. Consequently, only opportunistic macroalgae, characterized as
highly copper tolerant, are able to grow under these extreme conditions. For
example, copper-impacted coastal areas with 25 g L
1
and higher in northern
Chile are inhabited by only two macroalgal species (
Ulva compressa
,
Scytosiphon
lomentaria
), although
Lessonia nigrescens
dominates
related, but pristine,
ecosystems (Contreras et al.
2009
).
Macroalgae, independently of their heavy metal-tolerance, are able to accumu-
late copper in their tissues as a function of its concentration in seawater
(Ratkevicius et al.
2003
; Andrade et al.
2006
). Tissues of
U. compressa
form
copper-impacted sites; an approx. 40-fold higher copper content was measured
than in conspecifics from pristine habitats (Ratkevicius et al.
2003
). These free
copper ions (Cu
+
,Cu
2+
), which are not bound in protein complexes in the cell, are
able to increase ROS production substantially by their involvement in the
Haber-Weiss and the Fenton reaction (Pinto et al.
2003
). The highly reactive
hydroxyl radical (OH), which is a product of both reactions, changes and
inactivates biological macromolecules by oxidation and peroxidation. Therefore,
macroalgae exposed to copper excess showed increase in lipid peroxidation,
indicating cell membrane damage. If oxidative stress is not sufficiently detoxified
by the cellular ROS-scavenging system, it may consequently lead to cell death.
However, the antioxidant defence system is also activated by copper excess. A
general response of macroalgae seems to be the stimulation of those enzymes that are
involved in scavenging of lipid and fatty acid peroxidation. In copper-tolerant
macroalgae (
U. compressa
,
Ulva fasciata
,
S. lomentaria
), activities of APX and
peroxiredoxin (PRX) detoxifying H
2
O
2
and fatty acid hydroperoxides increased
significantly as a result of copper excess. This response is attributable to enhanced
gene expression providing new enzymes rather than modulation of already existing
enzymes, which represents an efficient acclimation mechanism to cope with copper
stress (Contreras-Porcia et al.
2011
). In
U. fasciata
, ROS acts as a signal for
upregulation of genes encoding for antioxidant defence enzymes (FeSOD, APX,
GR) because their transcript levels increased by exogenously applied H
2
O
2
concentrations above 0.2 mM (Wu et al.
2009a
,
b
). Moreover, induction of transcrip-
tion of antioxidant enzyme genes by copper led to an increase in activities of MnSOD,
FeSOD, APX, CAT, and GR to cope with copper stress successfully. However, at high
copper concentrations (above 20
MCuSO
4
), copper might be responsible for a
selective inhibition of APX and CAT, which resulted in an insufficient ROS detoxifi-
cation with enhanced H
2
O
2
accumulation and lipid peroxidation (Wu and Lee
2008
).
However, one must take into account that these mechanisms of copper acclimation are
species-specific rather than a general response.
m
