posted on 2014-07-15, 00:00authored byBo Meng, Xinbin Feng, Guangle Qiu, Christopher
W. N. Anderson, Jianxu Wang, Lei Zhao
Cultivation
of paddy rice for human consumption is a dominant agricultural
activity throughout Asia. High levels of mercury (Hg) in rice grain
pose a potential threat to human health, although the extent of risk
is dependent on the chemical speciation of Hg inside the grain. We
have investigated the speciation and localization of Hg in three fractions
of rice grain (hull, bran, and white rice) collected from a Hg-contaminated
region in China. On a mass basis, the majority of inorganic mercury
(IHg) in a rice grain is found in hull and bran. However, the majority
of the more toxic species methyl mercury (MeHg) is found in edible
white rice. Our data show that during grain processing, most of the
IHg (∼78%) is eliminated, but the majority of the MeHg remains
in the food product (∼80%). Synchrotron radiation microscopic
X-ray fluorescence (SR-μXRF) mapping shows strong localization
of Hg at the surface of brown rice grains, corresponding to the pericarp
and aleurone layer. We infer that this Hg is predominantly IHg absorbed
from the atmosphere. Based on X-ray absorption near-edge spectroscopy
(XANES) data we propose that IHg in bran is primarily bound to cysteine,
and is associated with phytochelatins. Consequently, IHg is largely
immobile and restricted to the outer layers of rice grain. MeHg in
bran is primarily bound to cysteine and is associated with proteins.
However, this MeHg–cysteine association behaves like a mobile
nutrient and is actively transported to the endosperm during seed
ripening. Concentration of MeHg–cysteine in white rice has
implications for public health. There is growing evidence for Hg contamination
of rice throughout Asia due to point and diffuse sources of Hg pollution.
The magnitude of the associated risk must be quantified through better
understanding of the localization and speciation of mercury in rice.
Our work makes an effort to contribute to this understanding.