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of the recent samples, with an annual decrease of 2.7% but there were no obvious correla-
tions between hair cortisol and hair POP concentrations (Bechshøft et al. 2012). Thus, corti-
sol in polar bear hair appears to be a relatively unspecific biomarker of their contamination
by persistent organic pollutants (POPs) but as a relevant biomarker of general stress.
4.2.2 Oxidative Stress and Lipid Peroxidation
According to Sies (1991), oxidative stress may be defined as “a disturbance in the prooxida-
tive-antioxidant balance in favor of the former, leading to potential damage.” The presence
of free radicals and reactive species of oxygen (ROS) in biological systems and their mode
of action are well established in biology and medicine (Halliwell and Gutteridge 2007),
and have been recently reviewed in aquatic ecosystems (Abele et al. 2012). Oxidative stress
is induced by a wide range of environmental factors including UV stress, oxygen short-
age, pathogen invasion, presence of symbionts, cyanobacterial toxins such as microcystin,
contaminants such as transition metal ions (Fe, Cu, Cr, Hg, As), pesticides (insecticides,
herbicides, fungicides), oil, and related contaminants (Blokhina et al. 2003; Lushchak 2011;
Abele et al. 2012). For emerging contaminants, oxidative stress is recognized as a main
effect of nanoparticles on biota (Moore 2006; Klaine et al. 2008; Canesi et al. 2011). In addi-
tion, natural factors such as temperature and salinity may enhance the production of ROS
(Lushchak 2011). Consulting Google Scholar in February 2012 with search terms “aquatic”
and “oxidative stress” yielded 20,800 occurrences, whereas the search terms “marine” and
“oxidative stress” showed 39,600 occurrences. Rapid browsing of this mass of data shows
at least that nearly all taxa are affected.
Cellular responses to oxidative stress include adaptation, damage, repair, senescence,
and death (Halliwell and Gutteridge 2007). Oxidative stress gives rise to antioxidant
defenses that provide a number of biomarkers of defense (Chapters 2 and 3), but when
defenses are overwhelmed, oxidative damage is observed, providing biomarkers of dam-
age. ROS induce modification of lipids, proteins, and nucleic acids. Assessing lipid and
protein oxidation is classically used in environmental studies (Chapter 2; Lushchak 2011).
Malondialdehyde (MDA), an oxidative by-product of lipid peroxidation, is commonly used
as a biomarker of oxidative damage. It is classically detected through spectrophotometric
detection of the thiobarbituric acid-MDA derivative, but this has been criticized for its
lack of specificity (Chapter 2). More accurate methods (high-performance liquid chroma-
tography or gas chromatography coupled to UV-Vis, fluorescence, and mass spectrometry
detectors) have been recently reviewed (Miyamoto et al. in Abele et al. 2012). Another pos-
sibility lies in the direct analysis of various radical species. Evaluation of oxidative DNA
damage in aquatic organisms has also been well developed (Abele et al. 2012), using sev-
eral damage parameters (Chapter 13).
Because environmental conditions (oxygen level, UV intensity, temperature, salinity,
diet) are recognized as inducers of oxidative stress in aquatic organisms (Blokhina et al.
2003; Lushchak 2011; Miyamoto et al. in Abele et al. 2012), particular attention must be paid
to natural fluctuations that can interfere with contamination effects, acting as confound-
ing factors in the interpretation of biomarkers of oxidative damage (Chapter 2). Seasonal
and reproductive cycles, which are often accompanied by changes in membrane lipid com-
position, uptake of fatty acids for energy supply, or changes in antioxidant defenses, are
known sources of natural changes in MDA levels (Miyamoto et al. in Abele et al. 2012).
Organ-specific and age effects must also be taken into account to avoid misinterpretation,
as exemplified in the case of mercury-induced peroxidative damage in bivalves (Ahmad
et al. 2011).
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