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
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Common nanoparticles threat to major food crop and human
The potential health hazards of nanoparticles have been
known for close to a decade ( Nanotoxicity: A New Discipline,
SiS 28). But regulation on manufacture and environment release is still
lagging far behind industrial developments  (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 .
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 ; while human skin cells
exposed to ZnO suffered oxidative stress and DNA damage after 6 hours . 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  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 . 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 . The US Environmental Protection
Agency requires pretreatment of industrial waste to limit metal discharge into
publicly owned wastewater treatment plants . But manufactured nanoparticles
are neither monitored nor regulated, and are known to have a high affinity for
activated sludge bacteria .
Soybean is an major commodity crop and highly exposed to
The researchers decided to look at soybean because it is the
fifth largest crop in the world, and second largest in the US . 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  and metals  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 ;
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
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
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
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
Table 1 Concentration of Ce in plant tissues at harvest
*the actual units are mg
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
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.