Potamanautes warreni biomarker assays to monitor silver nanomaterial contaminants in aquatic environments
There has been extensive growth in nanoscale technology in the last few decades to such a degree that nanomaterials (NMs) have become a constituent in a wide range of manufactured commercial and domestic products. This surge has resulted in uncertainties regarding their environmental impact, due to the significant increases in the amount of NMs released into the environment (Dowling et al., 2004) through intentional and unintentional releases. Like many other toxins, the aquatic environment is particularly vulnerable as it acts as a sink for nanoparticles (NPs) (Scown et al., 2010). The escalating growth of NMs has not advanced without efforts to understand its properties. Despite the dramatic advances in both the production and application of NMs, very little is known regarding their interaction with and effects on environmental and human health. Given the lack in scientific knowledge, particularly under various environmental conditions, it is often difficult to accurately assess the potential exposure pathways to ecological receptors. Of all NMs, silver nanoparticles (AgNPs) are the most widely used NPs, present in several consumer products mainly because of their anti-bacterial properties. It is estimated that the annual production exceeds 1000 tons/year (Piccinno et al., 2012). The increase uses of AgNPs in consumer products (e.g. textiles, cosmetics and personal hygiene), household appliances (e.g. washing machines and vacuum cleaners) and medical equipment have led to their increase release into the environment, thereby posing an environmental risk and human health concern. Silver NPs are known to induce the production of Reactive Oxygen Species (ROS) (Ahamed et al., 2010; Levard et al., 2012; Piao et al., 2011). Also since AgNPs are oxidized to ionic Ag (Ag+), it is still unclear whether the effects of ROS can be attributed to Ag+ release or to the AgNP itself (Fabrega et al., 2009; Miao et al., 2009). The behaviour of AgNPs is collectively influenced by inherent (nanoparticle size, shape, surface area, surface charge, crystal structure, coating, solubility/dissolution) and environmental factors (temperature, pH, ionic strength, salinity, organic matter). Climate change predictions indicate that the frequency, intensity and duration of extreme natural events (such as temperature elevations) will increase in the future (IPCC, 2001; IPCC, 2007). Global warming and climate change could increase atmospheric temperatures by 2.4 – 6.4 °C (IPCC, 2001; IPCC, 2007). The main feature associated with global climate change is the anticipation of wetter winters (i.e. increased flood events) and drier, warmer summers (i.e. extreme temperatures). These changes are likely to affect the inputs of contaminants into the environment as well as affect their behaviour, fate and transport, and toxicity in aquatic environments. It is known that the current temperature predictions in climate change scenarios could directly affect aquatic ecosystem communities (Carpenter et al., 1992), since temperature is also regarded as an important abiotic factor influencing growth and production of primary producers (i.e. algae, macrophytes etc.), and may also affect species distribution. For example, Liu et al. (2010) reported higher dissolution rates of AgNPs with increased temperature. Similarly, sudden hydrographic activity like high flood conditions may cause resuspension and redistribution of sediments. Few studies have linked the foreseeable climate change with contaminant release and ecosystem impacts. Similarly, few studies have analyzed the behaviour of NMs in the environment considering these predicted changes in mean temperatures. This thesis focuses on the effects of AgNPs on oxidative stress responses in the Cape River crab Potamonautes perlatus. The present work was undertaken to interpret the biological effects of AgNPs (<100 nm) on P. perlatus, as well as to assess its effects under different environmental conditions. To understand the uptake, accumulation and biological effects of AgNPs, freshwater microcosms were produced to mimic a typical aquatic environment and temperature manipulated microcosms to which a commercially-available AgNP powder was added. Nanoparticles were characterized in the dry state and in suspension under different environmental conditions. Dissolution of total Ag was measured by inductively coupled plasma mass spectrometry (ICP-OES). Nanoparticle toxicity was assessed by measuring mortality and biomarkers of oxidative stress (CYP450, SOD, CAT, GST) evaluated in crab tissues. The overall results demonstrated that: (1) AgNPs may be transformed in both size and state under variable environmental conditions. The formation of smaller aggregates at higher temperatures suggests higher toxicity, (2) the release of free metal ions from NPs and NPs aggregates contribute to a higher toxicity towards aquatic organisms, (3) oxidative stress is a significant mechanism of AgNP toxicity and consequently enzymatic activation/ inhibition with increasing AgNP concentration and temperatures, (4) oxidative stress responses to AgNPs particles were significantly modulated by temperature stress in P. perlatus, (5) mortality was observed from day 2 with maximum mortality achieved at day 7, (6) enzymes involved in detoxification, i.e. CYP450, has functional significance in the haemocytes, (7) P. perlatus has proved to be a significant target for AgNP exposure and, furthermore, has proved to be a suitable species to assess the ecotoxicity of AgNP in the aquatic environment, (8) antioxidant enzymes activities (are valuable tools to assess the oxidative status of crab tissues co-exposed to AgNPs and temperature. Furthermore, the results obtained in this study contributed to the understanding of the behaviour, bioavailability, uptake and toxicity of AgNPs under variable temperatures. Recommendations for future work are given in Chapter 6.
Nanomaterials, Silver nanoparticles, Aquatic environment
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