Researchers have found that nanosized copper particles affect fish gills through a different and as-yet-unknown mechanism than dissolved copper ions do. The results, published in ES&T ("Exposure to Copper Nanoparticles Causes Gill Injury and Acute Lethality in Zebrafish"), suggest that copper nanoparticles are more lethal than those composed of carbon or titanium dioxide (TiO2). Researchers not connected with the work caution that the study is just a first step in assessing the effects of copper and other metals at nano scales.
Copper is known to be toxic to fish. Even trace amounts of the metal can damage a salmon's sense of "smell" or destroy a trout's liver. But this damage generally comes with exposure to dissolved copper ions.
In the new ES&T work, David Barber and colleagues at the University of Florida conducted a shorter but similar experiment. They exposed zebrafish to copper nanoparticles and monitored what happened to the particles over 2 days. They tracked whether the copper went into solution or remained as discrete particles in tap water, which usually contains natural organic matter and ions typically found in natural waters. Nanoparticles tend to agglomerate under such conditions, unless they are specially coated, but the team found that about one third of the nanoparticles remained suspended and undissolved for 48 hours.
After determining the 2-day lethal concentrations, Barber and colleagues exposed zebrafish to nonlethal levels of copper nanoparticles. When the fish were exposed to 100 µg/L levels of nanoparticles, they showed different gene expression patterns and morphological changes in their gills, compared with those exposed to soluble copper, in some cases at comparable levels.
"The issue of dosimetry has to be addressed," Barber says. The mass of metals in each case was not necessarily the mass present in suspension 2, 3, or even 24 hours later, he notes. The team concludes that copper nanoparticles "are acutely lethal to zebrafish" at much lower concentrations than those illustrated by experiments with TiO2 particles in daphnia or with carbon-based nanoparticles, such as fullerenes, in fathead minnows.
But some researchers note that the experimental levels used by Barber's team are higher than those likely to occur in the environment. Agglomeration in natural waters could capture copper and other nanoparticles, so wild fish probably will not face such high concentrations, says Alistair Boxall of York University (UK). Nonetheless, the team used relatively unpurified water, which makes the experiment closer to real water exposures than most, he comments.
Copper nanomaterials are currently used mostly in microelectronics, says Boxall, and "the release to the environment is therefore likely to be minimal." But if such nanoparticles become more widely used in personal-care products, for example, they may end up in water bodies or passing through wastewater treatment plants that cannot strain them out. Nanoparticles that sorb to treated biosolids later applied to agricultural fields could then get into the environment.
"Any study at this stage is important because so little is known," says Marisol Sepulveda, an aquatic ecotoxicologist at Purdue University. Sepulveda says that relatively few studies have assessed what happens to nanoparticles over time or have characterized the size distribution of the nanoparticles as Barber and colleagues did. The experiment suggests that long-term effects are likely from exposures to copper nanoparticles, she says. In addition, chronic exposures could impact more internal organs and "longer-term exposures might affect more than just gills."
However, Mary Ellen Lane, a biochemist and cell biologist at Rice University, observes that the physical and genetic responses to copper nanoparticles indicate that zebrafish can mount a "sustained response to the damage." In this 2-day experiment, the nanocopper "may be somehow less harmful" than the dissolved copper, Lane concludes, "but what's pretty clear is that the stuff is bad – that's indisputable."