Speciation versus bioavailability of heavy metal in soils
Bacterial traits such as, the releasing of chelating substances,
acidification of the microenvironment and influencing changes
in redox potential affect heavy metals bioavailability in soils
(Lasat, 2002). Despite of the fact that microbial physiology
exposed to high concentration of heavy metals is negatively
affected, microbes essentially require various heavy metals as
essential micronutrients for normal growth and development
(Ahemad, 2012). Among metals, some are essential for most
redox reactions and are fundamental to normal cellular
functions (Table 2). The interaction of bacteria with metal
species, whether for basic metabolic requirements or to protect
from their toxic effects, depends upon the metal speciation,
i.e., bioavailable forms (Table 3).
Bacteria directly affect metal bioavailability by changing
heavy metal speciation in the rhizosphere. In addition, they
protect the plants from the phytotoxicity of excessive metals
by changing the speciation from bioavailable to the
non-bioavailable forms in soils (Jing et al., 2007).