Heavy metals

Heavy Metal Adaptation
Luis Rafael Herrera-Estrella, Laboratorio Nacional de Geno´mica para la Biodiversidad,
CINVESTAV Campus Guanajuato, Irapuato, Guanajuato, Mexico
Angel Arturo Guevara-Garcı´a, Universidad Nacional Auto´noma de Me´xico, Cuernavaca,
Morelos, Mexico
Based in part on the previous version of this Encyclopedia of Life Sciences
(ELS) article, Heavy Metal Adaptation by LuisRafael Herrera-Estrella,
Angel Arturo Guevara-Garcı´a and Jose´ Lo´pez-Bucio.
Fifty-three chemical elements fall into the category of heavy metal (HM) defined as the
group of elements with densities higher than 5 gcm23. In an ecological sense, any metal
or metalloid that causes environmental pollution or that cannot be biologically
degraded and is therefore bioaccumulated, should be consideredas an HM. HMs are
natural components of the Earth’s crust, but as a consequence of human activities, in
many ecosystems the soil concentration of several HMs has reached toxic levels. The
adaptative responses of living organisms to HM contamined environments are efficient
processes that include genetical, physiological, anatomical and ecological traits highly
conserved in nature. These traitsgive certain species the ability to survive in toxic
concentrations of HM elements. Actually, using multidisciplinary strategies including
genomic, genetic, molecular, biochemical, physiological, ecological and agronomic
approaches, substantial progress is being made to elucidate the mechanism involved in
HM adaptation.
Introduction
Heavy metals have been defined as the group of elementsthat have densities higher than 5 g cm23, and 53 chemical
elements fall in this category. However, according physical,
chemical or toxic properties, several definitions have been
proposed to describe a heavy metal. Interestingly, the concept
can include elements lighter than carbon, but excludes
some of the heaviest metals. In an ecological scenario, a
heavy metal could be considered as anymetal or metalloid
element that causes environmental pollution, that does not
have any vital function and is toxic at low concentrations
(such as lead and mercury), that has a vital function but is
harmful to the organisms in high concentrations (such as
copper and molybdenum) or that cannot be biologically
degraded and tends to bioaccumulate. Wherever heavy
metals are defined, livingspecies must actively respond to
protect themselves from metal poisoning in contaminated
sites. See also: Plant Macro- and Micronutrient Minerals
Heavy metals are natural components of the Earth’s
crust mainly localized as disperse components in rock formations.
Few ecosystems present natural heavy metal toxicity,
such as aluminium, iron and manganese toxicity in
acid soils. However, in aconsiderable number of places,
heavy metal concentrations in the soil increase to toxic
levels through agriculture, manufacturing and mining or
industrial waste disposal practices. Recent findings suggest
that the adaptive response of living organisms to contaminated
environments can be a rapid and efficient process.
Insights into the mechanisms involved in heavy metal adaptation
are beginning tobe obtained. See also: Adaptation
and Natural Selection: Overview
Differences in Ecology
Soil properties and the adaptive response
The distribution of living organisms in ecosystems affected
by heavy metal toxicity depends not only on the kind,
combination and concentration of metals, but also on environmental
factors such as soil chemistry, heterogeneity
and, in particular, nutrientstatus (principally nitrogen,
phosphorus and potassium).
Gradients of soil conditions often determine areas of
increasing stress or disturbance that impose a strong selective
pressure on the colonizing species. During evolution,
adaptations have developed in a very wide range of
organisms from all major taxonomic groups. This is
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Article Contents
. Introduction
. Differences...