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Transformation
of
Other
Elements |
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Transformation
of
Other
Elements
in
Soil -
Other
important
elements
that
undergo
transformation
through
the
agency
of
microorganisms
include
potassium,
iron,
manganese,
selenium,
mercury,
zinc
and
copper.
There
is
evidence
supporting
both
direct
and
indirect
biological
alteration
of
these
metals
leading
to
their
availability,
solubility
and
oxidation
reduction,
although
many
details
are
lacking.
Potassium
is
a
major
cation
that
is
essential
for
plant,
microbial
and
animal
growth.
Plants
obtain
their
potassium
requirement
from
the
soil
solution.
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Although
a
major
plant
nutrient,
little
is
known
about
its
transformation.
Most
of
the
potassium
is
bound
to
various
cellular
Structures
and
is
non
exchangeable.
In
crop
residues,
the
metal
is
not
strongly
bound
and
the
microbial
action
is
not
very
critical
in
its
mobilization.
Moreover,
this
element
exists
in
a
monovalent
state
in
biological
systems
and
therefore
does
not
go
through
the
oxidation
reduction
that
the
other
metals
undergo.
The
soil
microflora
however,
appears
to
have
an
effect
on
its
solubility
mostly
through
the
production
of
various
organic
and
inorganic
acids,
alteration
of
pH
etc.Iron
is
required
as
an
essential
element
for
cellular
growth.
It
is
abundant
in
the
soil
and
readily
undergoes
transformations
through
microbial
activity.
Although
abundant,
it
sometimes
becomes
a
limiting
element
for
plant
growth.
Certain
bacteria
are
able
to
oxidize
ferrous
iron
to
the
ferric
state
while
many
heterotrophs
can
attack
soluble
organic
iron
salts
with
the
precipitation
of
iron.
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Some
bacteria
and
fungi
produce
acids
in
their
environment
which
may
bring
iron
into
solution.
Iron
may
also
be
precipitated
by
non
iron
oxidizing
bacteria
by
creation
of
alkaline
conditions
while
solubilization
may
occur
through
acid
formation
or
due
to
the
creation
of
reducing
conditions.
Pyrites,
a
typical
iron
disulphide
is
converted
to
ferric
iron
mainly
by
Thiobacillus
ferrooxidans.These
bacteria
are
able
to
grow
and
derive
energy
by
the
oxidation
of
ferrous
iron
and
also
by
the
oxidation
of
sulphur.
Many
fungi
are
also
known
to
solubilize
iron
present
in
minerals
mainly
due
to
the
acids
they
produce
in
the
growth
medium
containing
a
utilizable
carbon
source.
In
waterlogged
conditions,
reduction
of
iron
occurs
mainly
through
the
agency
of
microorganisms.
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Among
the
bacteria
which
reduce
ferric
to
ferrous
iron
are
species
of
Bacillus
Clostridium,
Klebsiella,
Pseudomonas
and
Serratia. Manganese
is
an
essential
micronutrient
for
the
growth
of
plants,
animals
and
microorganisms
and
occurs
in
the
soil
in
the
tetravalent
(Mn+4)
or
divalent
(Mn+2)
form
while
only
the
latter
is
utilized
by
the
plants
and
microorganisms.
A
considerable
portion
of
manganese
like
iron
may
also
be
bound
in
organic
complexes.
Manganese
can
also
undergo
autooxidation
depending
upon
the
pH.
The
active
organisms
in
manganese
oxidation
include
species
of
Arthrobacter,
Bacillus,
Corynebacterium,
Klebsiella,
Pseudomonas
and
the
fungi
such
as
Cladosporium,
Curvularia
and
Fusarium.
It
appears
that
the
microbial
contribution
to
manganese
oxidation
is
mostly
by
the
production
of
acids
and
by
changing
the
pH
of
the
soil.Reduction
of
manganese
by
bacteria
is
an
indirect
process.
For
example
by
decreasing
the
pH,
lowering
of
the
O-R
potential
and
removal
of
oxygen
as
a
result
of
microbial
activity
the
level
of
exchangable
manganese
in
the
soil
has
been
found
to
increase.
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In
some
organisms,
MnO2
serves
as
an
electron
acceptor
in
respiration
(RH2+MnOa--+Mn(OH)2+R).
Immobilization
of
manganese
is
not
much
of
importance
since
microbial
cells
do
not
contain
more
than
0.05
per
cent
of
the
element
and
this
would
not
affect
the
process
of
oxidation
or
reduction.
There
is
however,
no
doubt
that
there
is
a
manganese
cycle
in
the
soil
involving
the
divalent
and
tetravalent
and
other
oxidation
states
of
the
element.
In
recent
years,
mercury
transformation
through
microorganisms
has
attained
importance
because
of
the
increased
use
of
mercurials
as
pesticides
and
the
toxicity
of
this
metal
to
both
microbial
and
higher
forms
of
life.
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Mercury
is
present
in
the
soil
at
a
small
concentration
(less
than
1
ppm)
and
most
of
the
soil
mercury
comes
from
the
mercurial
pesticides
used
in
agricultural
practices.
Metallic
mercury
(Hg)
is
volatilized
from
the
soil
and
this
appears
to
be
largely due
to
the
activity
of
microorganisms.
Such
volatilization
does
not occur
in
soils
that
have
been
sterilized.
Metallic
mercury
is
liberated
from
cultures
of
several
microorganisms
incubated
in
media
containing
small
amounts
of
phenyl
mercuric
acetate,
ethyl
mercuric
chloride
or
mercuric
chloride
Liberated
mercury
can,
also
be
converted
into
monomethyl
or
dimethyl
compounds
and
a
variety
of
organisms
such
as
the
Bacillus,
Clostridium,
Pseudomonas,
Aspergillus,
Neurospora,
Scopulariopsis
and
yeast
have
been
known
to
bring
about
such
methylation.Selenium,
is
not
an
essential
element
for
plant
growth
but
sometimes,
because
of
its
presence
in
excess
can
cause
toxicity.
Selenium
added
to
the
soil
as
selenite
is
converted
into
selenium
by
the
reduction
brought
about
by
a
number
of
bacteria,
lactinomycetes
and
fungi.
Like
mercury,
it
is
also
subject
to
methylation
and
this
is
brought
about
by
fungi
such
as
Aspergillus,
Candida,
or
Fusarium
and
bacteria
such
as
Corynebacterium.
Conversions
include,
mineralization
of
organic
molecules
containing
selenium,
assimilation
into
proteins,
oxidation
and
reduction
of
selenite
and
selenium
and
methylation.
Zinc
and
copper
are
minor
elements
essential
for
both
plant,
microbial
and
animal
growth.
However,
since
microorganisms
contain
very
small
amounts
of
these
metals,
microbial
assimilation
perhaps
has
no
relevance.
The
microbes
however,
increase
the
solubility
of
these,
elements
by
changing
soil
conditions,
by
the
production
of
organic
acids,
reduction
in
pH
etc.
For
example,
Thiobacillus
ferrooxidans
is
capable
of
bringing
about
the
oxidation
of
cuprous
to
cupric
ions.
Soluble
copper
may
also
be
precipitated
by
organisms
that
produce
H2S
under
anaerobic
conditions
thereby
making
it
less
available
to
plant
growth.
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