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ISIS Report 03/09/12

Nanoparticles Bioaccumulate and Harm Soybean Crops

New research suggests nanoparticles are a threat to agricultural production as well as health Dr. Mae-Wan Ho

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Common nanoparticles threat to major food crop and human health

The potential health hazards of nanoparticles have been known for close to a decade ([1] Nanotoxicity: A New Discipline, SiS 28). But regulation on manufacture and environment release is still lagging far behind industrial developments [2] (Nanotoxicity in Regulatory Vacuum, SiS 46). A wide variety of nanoparticles is flooding the market; and the concomitant build-up of nanoparticles discharged into the environment may well have profound effects also on ecology and agriculture.

A study published online in PNAS Early Edition reports that the soybean plant is susceptible to some of the most commonly manufactured nanoparticles [4].

The research team led by Patricia Holden at the University of Texas El Pasos grew soybean plants in soil amended with nanoparticles currently manufactured at high volumes for numerous industrial applications: cerium oxide (CeO2) as catalyst and additive and zinc oxide (ZnO) widely used in sunscreens. They found that nano-CeO2 diminished plant growth and yield, and shut down nitrogen fixation in the root nodules of the legume at high concentrations. For nano-ZnO, the metal was taken up and distributed throughout the plant tissues, potentially giving an overdose of Zn to people and animals eating the soybean.

This is bad news. Both nanoparticles have been found toxic to cells. Nano-CeO2 induced apoptosis (programmed cell death) and autophagy (self-ingestion!) in human peripheral blood cells at relatively low doses [5]; while human skin cells exposed to ZnO suffered oxidative stress and DNA damage after 6 hours [6]. Oxidative stress was evident even at low concentrations of 0.008 to 0.8 mg L-1. Oxidative stress is now implicated in cancer development (see [7] Cancer an Epigenetic Disease and other articles in the series, SiS 54).

How nanoparticles enter the soil

Scientists have been concerned for some time over the rapid expansion of manufactured nanoparticles, as they can build up in soils and enter our food supply. The nanoparticles can enter soils through the atmosphere; nano-CeO2 as fuel additive is released in the exhaust with the combustion of diesel fuel [8].  Nanoparticles can also enter the soil in the ‘biosolids’ from conventional wastewater treatment plants. This is a major route, as half of biosolids in the US are spread on land [9]. The US Environmental Protection Agency requires pretreatment of industrial waste to limit metal discharge into publicly owned wastewater treatment plants [10]. But manufactured nanoparticles are neither monitored nor regulated, and are known to have a high affinity for activated sludge bacteria [11].

Soybean is an major commodity crop and highly exposed to nanoparticles

The researchers decided to look at soybean because it is the fifth largest crop in the world, and second largest in the US [12]. It is also highly exposed to nanoparticles.  Soybean is farmed intensely with fossil-fuel powered equipment, resulting in large deposition of nanoparticles from the exhaust. It is amended with wastewater treatment biosolids as a matter of routine. Previous studies have already shown that soybean plants bioaccumulated pharmaceuticals [13] and metals [14] from biosolid amended soils.

Soybean plants were grown to the seed production stage in soil amended with nano-CeO2 at 0, 0.1, 0.5 or 1 g.kg-1, or nano-ZnO at 0, 0.05, 0.1, or 0.5 g.kg-1 [4]; these concentrations were previously found to affect hydroponically-grown plants and microorganisms, but the effects on soil-grown plants were unknown.

Effects on plant growth and development

The plants grown in soils amended with ZnO appeared normal although the mean leaf count in the high nano-ZnO treatment was significantly lower than controls. The number of pods also varied with concentrations, with significantly more pods at high versus low concentrations.  There were significant differences in water content, with stems from high nano-ZnO treatment and leaves and pods from all nano-ZnO treatments drier than controls.  The dry weight of above ground biomass did not differ significantly from controls.

Plants grown in CeO2 amended soil, on the other hand, had reduced leaf count at all concentrations compared to controls, with the most impact at low CeO2 concentrations. Furthermore, plants harvested from the lowest CeO2 concentration were significantly shorter than controls. All nano-CeO2 treated plants yielded less biomass compared with controls and the difference was significant for the high level treatment.

Below ground roots from median and high treatments of both ZnO and CeO2 were significantly drier than controls. Dry root biomass was increased in the high nano-ZnO treatment compared with controls.

The number of root nodules was similar across treatments and not significantly different from controls. But the nodules were drier for medium and high nano-ZnO treatments, while the dry nodule biomass was significantly greater for the high nano-ZnO plants compared with controls.

The nitrogen-fixing potential per nodule was similar across all ZnO treatments and controls, and not significantly different from low nano-CeO2 plants. However, it decreased by more than 80 % compared to controls in the medium and high nano-CeO2 plants. The effect was similar to that of high cadmium (a known toxic metal) treatment previously reported.

In summary, both above and below ground biomass was more abundant, but drier, in plants grown with nano-ZnO, and the difference was significant for high nano-ZnO. However, for nano-CeO2, plant growth was stunted both above and below ground at all concentrations. While low amounts of nano-CeO2 did not significantly alter N fixation in root nodules, this was strongly inhibited at medium and high concentrations of nano-CeO2.

Accumulation of Ce and Zn

The dried plant tissues were assayed for Ce or Zn. The results showed that both metals entered and accumulated in the plant tissues. Ce was mobilized from the soil and accumulated in the roots. The nano-CeO2 particles were also present in root nodules. The concentrations of Ce in different plant tissues are presented in Table 1.

As can be seen, Ce concentrations in the roots and nodules from medium and high nano-CeO2 treatments are greatly increased compared with controls, up to 711 times for roots and 165 times in nodules. But the metal does not translocate substantially above the ground.

Table 1   Concentration of Ce in plant tissues at harvest

*the actual units are mg Ce.kg-1

The accumulation of Zn is quite different (Table 2). It occurs both below and above ground. For high nano-ZnO treatment, Zn accumulates nearly 4 times and 2 times controls in roots and nodules, and more than 6 times controls in stem, 4 times in leaf and 2-3 times in pod. Such high Zn accumulations could have long-term impacts on plant and human health.

Table 2   Concentration of Zn in plant tissues at harvest

In conclusion

The study shows that two manufactured nanoparticles currently produced in large volumes are likely to impact significantly on the production of a major global food and feed commodity, the soybean. In the case of nano-ZnO, food quality is affected due to bioaccumulation, and in the case of CeO2, soil fertility is compromised. The authors highlight the importance of managing waste streams to control the exposure of agricultural soils to manufactured nanoparticles.

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