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Earthworms: An Environmental Indicator Species

Use of nanoparticles in a wide range of consumer and industrial applications is already widespread and expected to increase exponentially during the next decade.   While the properties of the corresponding bulk materials may be familiar and well understood, nanoparticles have unique physiochemical properties and little is currently understood about their long term impact on health and the environment.  With their small size, and high surface area compared to overall volume, nanoparticles are both highly reactive and easily absorbed by cells.

Studies have demonstrated exposure to nanoparticles raise levels of oxidative stress in a variety of micro and macro organisms.  This increase in oxidative stress is thought to be key to the increased toxicity observed in nanoparticles preparations as compared to their bulk material counterparts.

Copper (Cu), copper oxide (CuO), and zinc oxide (ZnO) are all common nanoparticle ingredients used in consumer goods, such as sunscreen and medicine, as well as industrial applications, such as gas sensors and photovoltaic cells. Their presence as soil contaminates has already been confirmed in some locations (Nowack & Bucheli). Because earthworms swallow soil and organic residues from the ground surface, they are easily exposed to nanoparticles and other soil contaminants in their environment and serve as an important sentinel species for the health of soil ecosystems.  To further understand the real world implications of nanoparticle contamination, a recent paper by Mwaanga et al. examined the toxicity of Cu, CuO and ZnO nanoparticles on earthworms living in both urban soil they collected, and in a standard artificial soil mix often used for earthworm studies.   Earthworms were exposed to varying levels of the target nanoparticles in each soil type over a period of 14 days, after which samples were assessed for markers of oxidative stress including; superoxide dismutase activity (K028-H), hydrogen peroxide (K034-H) and glutathione (K006-H). Samples were also assessed for metal ion concentration (Flame atomic absorption spectrophotometry) and overall protein concentration (K041-H) for normalization.

Results demonstrated a correlation between the amount of nanoparticle contamination in the soil and the metal ion levels in the worm tissue.  However, the ability of the Cu, CuO and ZnO nanoparticles to cause oxidative stress in earthworms varied depending upon the soil type in which the worms lived.   The artificial soil mix had much higher levels of organic matter, which seemed have a huge influence on the amount of oxidative stress caused by nanoparticle exposure.  Effects of nanoparticle exposure on SOD activity, glutathione levels and hydrogen peroxide levels all being significantly less in the artificial soil mix as compared to the same nanoparticle concentration in the urban soil samples.  This study shows that there is much that we still do not understand about how nanoparticle contamination effects ecosystems, but it also highlights the role of experimental design.   A study that only examined the nanoparticle effects on earthworms living in the standard, laboratory friendly, artificial soil mix would produce results that in fact do not mirror the real ecological impact on worms living the natural environment.   Ecology is multifaceted and complex, and it is important always to consider what artificial constraints that may be introduced when designing an experiment before drawing conclusions from the results.

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