Frontiers in Marine Science (Jun 2014)
Using shark biomarkers as tools for biomonitoring the health of atlantic waters
Abstract
Introduction The natural environment is continuously overloaded with foreign contaminants and is the ultimate sink for many of these substances, as is most of the aquatic environment (Stegeman and Hahn, 1994; Van der Oost, et al., 2003). Changes caused by pollutants in the ecosystem have an earlier impact at a lower level of biological organization, usually consisting of the production of specific enzymes that act in the detoxification of foreign compounds, allowing the development of early-warning tools for potential effects at later response levels (Bayne et al., 1985; Liska, 1998). PCBs, PFCs and metals are some of the pollutants that can be found in the oceans, all of them having negative repercussions to the living organisms present in the water (Van der Oost, et al., 2003). When an organism acts to eliminate or neutralize these compounds, or the metabolites deriving from them, the detoxification process involving enzymes such as glutathione S-transferase (GST) causes the production of reactive oxygen species (ROS), leading to an increase in oxidative stress (Lackner, 1995; Van der Oost, et al., 2003; Di Giulio and Meyer, 2008). To cope with this oxidative stress, organisms host a series of enzymatic and non-enzymatic antioxidants such as superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), glutathione reductase (GR) or reduced glutathione (GSH), that act to prevent oxidative damage in macromolecules such as membrane lipids and DNA. Pollutants can also have a neurotoxicity effect on the organisms or disturb their energetic metabolism, which can also be assessed with biomarker analysis such as the activity of cholinesterases (ChE), and lactate dehydrogenase (LDH) and isocitrate dehydrogenase (IDH), respectively. Fish biomarkers, such as the latter ones, may thus be very useful tools for environmental risk assessment, providing a mean to monitoring the environmental quality of aquatic ecosystems (Van der Oost, et al., 2003). Sharks are at the top of the food chain, thus playing a very important role in the oceans. Being apex predators, these animals are more susceptible to environmental contamination via bioaccumulation and biomagnification through the food web. A good example of the aforementioned is the fact that sharks tend to have higher levels of mercury in their bodies than most fish species, particularly monomethylmercury, which is the most toxic form of the metal and accounts for more than 95% of organic mercury in fish muscle tissues (Bloom, 1992; Porcella, 1994; Hueter et al., 1995). According to Bonfil (1994) and Stevens (2009), blue sharks (Prionace glauca) are one of the most abundant and heavily caught fishes in the world, with an estimated 20 million individuals caught annually as target or by-catch. This research project aimed to develop and apply biomarkers in sharks, relating the biochemical responses with tissue contaminant body burden, as tools to assess the physiological state of sharks and biomonitor oceanic deep waters. Materials and methods Blue shark (Prionace glauca) individuals were captured aboard a commercial fishing boat. Muscle, brain, liver, bile, spleen and blood from each shark were collected as soon as the animals were captured, and all samples were immediately kept on ice, until they were stored in the lab at −80 °C. The samples were homogenized and processed according to the different parameters to be tested. The biomarkers measured in blood, brain, liver and muscle are described in figure 1. In sum, the biomarkers analysed are involved in detoxification (GST), oxidative stress defence (SOD, CAT, GPx, GR, TG), oxidative damage (Lipid peroxidation – LPO – and DNA damage), neurotoxicity (ChE) and energy metabolism (LDH and IDH). Fluorimetric and spectrophotometric techniques were used to quantify all biomarkers using a microplate reader. Chemical analysis were also performed, targeting contaminants on blood, liver and muscle tissues. Results and discussion Sampling and homogenization/processing protocols were optimized and can now be readily and consistently followed in further work with this species. Activities of antioxidant enzymes were different in the distinct tissues analysed and in general were lower in plasma than on the other tissues. The biochemical parameters analysed showed different antioxidant responses in organisms with different levels of contamination. The results obtained in this first biomarkers screening are promising as it allowed for a better understanding on how blue sharks deal with the intake and accumulation of different xenobiotics and which are the most suitable tissues for specific biomarker testing and more efficient biomonitoring. Lastly, this approach presents a potential to be adapted to other species which are also on top of food chains, providing an even more robust insight on the oceanic deep waters health status.
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