This document discusses microbial transformations of sulphur, iron, and manganese in soil. It describes how various bacteria and fungi play key roles in mineralizing, immobilizing, oxidizing, and reducing these elements. Specific microbes like Thiobacillus, Desulfovibrio, and Leptothrix are involved in important processes like oxidizing elemental sulphur and iron, and reducing sulphates and ferric iron under different environmental conditions. The cycling and availability of these elements in soil largely depends on microbial activity.

1. AN ASSIGNMENT ON MICROBIAL TRANSFORMATIONS OF
SULPHUR ,IRON AND MANGANESE
GUIDED BY
Dr. PRIYADARSHANI SADAWARTI
KHAMBALKAR
Dept.Of Soil Science And
Agril.Chemistry
SUBMITTED BY
OMPRAKASH PARIHAR
M.Sc (Ag), Agronomy
Rajmata Vijayaraje Scindia Krishi Vishwa Vidyalaya, Gwalior
COLLEGE OF AGRICULTURE, GWALIOR (M.P.)

2. SULPHUR
• Sulphur is the most abundant and widely
distributed element in the nature and found
both in free as well as combined states.
Sulphur, like nitrogen is an essential element
for all living systems.
• Plant residues contain sulphur in the form of
proteins, amino acids and vitamins.
Mineralization of these compounds releases
sulphates.

3. • The C: S ratio of 50: 1 is critical. Decomposition
of plant materials with wider C: S ratio causes
immobilization of sulphur. In anaerobic soils,
sulphates are reduced to hydrogen sulphide by
the action of Desulfovibro spp of bacteria.
Hydrogen sulphide is toxic to crops.
• Elemental sulphur when applied to the soil is
converted to sulphate by Thiobacillus spp.
Earth's crust contains about 0.06 per cent of
sulphur. It is present as sulphides, sulphates and
in organic combination with carbon and
nitrogen.

5. • The original source of sulphur in soil is
sulphides of metals in plutonic rocks. During
weathering process, these sulphides are
oxidised to sulphates. These sulphates are
present as insoluble sulphate salts in arid and
semiarid regions.
• The nature of applied sulphur is in the form
of sulphates of Ca, Mg, K, Na or NH4 in soil
solution in arable soils. These sulphates may
be adsorbed on 1: 1 clay or hydrous oxides of
Fe and AI. It may be absorbed by plants and
microorganisms.

6. • Sulphates are reduced to sulphides in
waterlogged soils and form H2S, FeS etc.
Elemental sulphur is oxidized to sulphates by
microorganisms in aerated soils.
• In the soil, sulphur is in the organic form
(sulphur containing amino acids-cystine,
methionine, proteins, polypeptides, biotin,
thiamine etc) which is metabolized by soil
microorganisms to make it available in an
inorganic form (sulphur, sulphates, sulphite,
thiosulphale, etc) for plant nutrition. Of the total
sulphur present is soil only 10-15% is in the
inorganic form (sulphate) and about 75-90 % is
in organic form.

7. • Cycling of sulphur is similar to that of nitrogen.
Transformation / cycling of sulphur between organic
and elemental states and between oxidized and
reduced states is brought about by various
microorganisms, specially bacteria- Thus “the
conversion of organically bound sulphur to the
inorganic state by microorganisms is termed as
mineralization of sulphur". The sulphur / sulphate,
thus released are either absorbed by the plants or
escapes to the atmosphere in the form of oxides.
• Various transformations of the sulphur in soil results
mainly due to microbial activity, although some
chemical transformations are also possible (eg.
oxidation of iron sulphide) the major types of
transformations involved in the cycling of sulphur are:
1. Mineralization 2. Immobilization 3. Oxidation and
4. Reduction

8. • 1. Mineralization: The breakdown /
decomposition of large organic sulphur
compounds to smaller units and their
conversion into inorganic compounds
(sulphates) by the microorganisms. The rate
of sulphur mineralization is about 1.0 to 10.0
percent / year.
• 2. Immobilization: Microbial conversion of
inorganic sulphur compounds to organic
sulphur compounds.

9. • 3. Oxidation: Oxidation of elemental sulphur and
inorganic sulphur compounds (such as H2S, sulphite
and thiosulphale) to sulphate (SO4) is brought about
by chemoautotrophic and photosynthetic bacteria.
• When plant and animal proteins are degraded, the
sulphur is released from the amino acids and
accumulates in the soil which is then oxidized to
sulphates in the presence of oxygen and under
anaerobic condition (water logged soils) organic
sulphur is decomposed to produce hydrogen sulphide
(H2S). H2S can also accumulate during the reduction of
sulphates under anaerobic conditions which can be
further oxidized to sulphates under aerobic
conditions,

10. • Ionization
a) 2 S + 3O2 + 2 H2 O ——–> 2H2SO4 ————–> 2H (+) + SO4
(Aerobic)
Light
b) CO2 + 2H2S————–> (CH2 O) + H2 O + 2 S
Light
OR H2 + S + 2 CO2 + H2 O ———> H2 SO4 + 2 (CH2 O)
(anaerobic)
• The members of genus Thiobacillus (obligate
chemolithotrophic, non photosynthetic) eg, T.
ferrooxidans and T. thiooxidans are the main organisms
involved in the oxidation of elemental sulphur to
sulphates. These are aerobic, non-filamentous,
chemosynthetic autotrophs. Other
than Thiobacillus, heterotrophic bacteria (Bacillus,
Pseudomonas, and Arthrobacter) and fungi (Aspergillus,
Penicillium) and some actinomycetes are also reported to
oxidize sulphur compounds. Green and purple bacteria
(Photolithotrophs) of genera Chlorbium, Chromatium.

11. • Rhodopseudomonas are also reported to oxidize
sulphur in aquatic environment.
• Sulphuric acid produced during oxidation of sulphur
and H: S is of great significance in reducing the PH of
alkaline soils and in controlling potato scab and rot
diseases caused by Streptomyces bacteria. The
formation of sulphate / Sulphuric acid is beneficial in
agriculture in different ways : i) as it is the anion of
strong mineral acid (H2 SO4) can render alkali soils fit
for cultivation by correcting soil PH. ii) solubilize
inorganic salts containing plant nutrients and thereby
increase the level of soluble phosphate, potassium,
calcium, magnesium etc. for plant nutrition.

12. • 4. Reduction of Sulphate: Sulphate in the soil is assimilated
by plants and microorganisms and incorporated into
proteins. This is known as "assimilatory sulphate
reduction". Sulphate can be reduced to hydrogen sulphide
(H2S) by sulphate reducing bacteria
(eg. Desulfovibrio and Desulfatomaculum) and may
diminish the availability of sulphur for plant nutrition. This
is “dissimilatory sulphate reduction” which is not at all
desirable from soil fertility and agricultural productivity
view point.
• Dissimilatory sulphate-reduction is favored by the alkaline
and anaerobic conditions of soil and sulphates are reduced
to hydrogen sulphide. For example, calcium sulphate is
attacked under anaerobic condition by the members of the
genus Desulfovibrio and Desulfatomaculum to release H2S.

13. • CaSO4 + 4H2 ———–> Ca (OH)2 + H2 S + H2O.
• Hydrogen sulphide produced by the reduction of
sulphate and sulphur containing amino acids
decomposition is further oxidized by some species of
green and purple phototrophic bacteria (eg.
Chlorobium, Chromatium) to release elemental
sulphur.
Light
CO2 + 2H2 + H2S ———–> (CH2O) + H2O + 2
S.
Enzyme Carbohydrate
Sulphur
The predominant sulphate-reducing bacterial genera
in soil are Desulfovibrio,

14. IRON
• Iron exists in nature either as ferrous (Fe++) or
ferric (Fe+++) ions. Ferrous iron is oxidized
spontaneously to ferric state, forming highly
insoluble ferric hydroxide.
• Plants as well as microorganisms require traces
of iron, manganese copper, zinc, molybdenum,
calcium boron, cobalt etc. Iron is always
abundant in terrestrial habitats, and it is oftenly
in an unavailable form for utilization by plants
and leads to the serious deficiency in] plants.

15. Soil microorganisms play important role in the
transformations of iron in all number of
distinctly different ways such as:
• Certain bacteria oxidize ferrous iron to ferric state
which precipitate as ferric hydroxide around cells
• Many heterotrophic species attack on in soluble
organic iron salts and convert into inorganic salts
• Oxidation-reduction potential decreases with
microbial growth and which leads to the formation of
more soluble ferrous iron from highly insoluble ferric
ions
• Number of bacteria and fungi produce acids such as
carbonic, nitric, Sulphuric and organic acids which
brings iron into solution

16. • Under anaerobic conditions, the sulfides formed
from sulphate and organic sulphur compounds
remove the iron from solution as ferrous sulfide
• As microbes liberate organic acids and other
carbonaceous products of metabolism which
results in the formation of soluble organic iron
complex. Thus, iron may be precipitated in
nature and immobilized by iron oxidizing
bacteria under alkaline soil reaction and on the
other hand solubilization of iron may occur
through acid] formation.

17. • Some bacteria are capable of reducing ferric iron
to ferrous which lowers the oxidation-reduction
potential of the environment (eg. Bacillus,
Clostridium, Klebsiella etc).
However, some chemoautotrophic iron and
sulphur bacteria such as Thiobacillus
ferroxldans and Ferrobacitlus ferrooxidans can
oxidize ferrous iron to ferric hydroxide which
accumulates around the cells.

19. • Most of the aerobic microorganisms live in an
environment where iron exists in the oxidized,
insoluble ferric hydroxide form.
• They produce iron-binding compounds in order to
take up ferric iron.
• The iron-binding or chelating compounds / ligands
produced by microorganisms are
called "Siderophores". Bacterial siderophores may
act as virulence factors in pathogenic bacteria and
thus, bacteria that secrete siderophores are more
virulent than non- siderophores producers

20. • Therefore, siderophore producing bacteria can be used as
biocontrol agents eg. Fluorescent pseudomonads used to
control Pythium, causing damping-off diseases in seedlings.
Recently Vascular – Arbusecular – Mycorrhiza (VAM) has
been reported to increase uptake of iron.
• esulfatomaculum and Desulfomonas. (All obligate
anaerobes). Amongst these species Desulfovibrio
desulfuricans are most ubiquitous, non-spore forming,
obligate anaerobes that reduce sulphates at rapid rate in
waterlogged soils. While species of Desulfatomaculum are
spore forming, thermophilic obligate anaerobes that reduce
sulphates in dry land soils.
All sulphate-reducing bacteria excrete an enzyme
called “desulfurases” or "bisulphate Reductase". Rate of
sulphate reduction in nature is enhanced by increasing water
levels , high organic matter content and increased
temperature.

22. (i) Oxidation of ferrous to ferric compounds
• This reaction often proceeds non-biologically.
However, when the pH of the medium is below 5,
oxidation proceeds only in the presence of certain
types of autotrophic bacteria. These types of iron
bacteria belong to the Thiobacillaceae {Thiobacillus
ferrooxidans, Ferrobacillus ferrooxidans).
• They are very resistant to high concentrations of H+
and heavy metals like Fe++ and Cu++. They are found
in media rich in ferrous salts (e.g. acid mine water)
where masses of brown ferric hydroxide are formed.
A second type of iron bacterium belongs to the genus
Gallionella.

23. • It is also a bacterium of slowly running water; it
grows at neutral reaction and is thought to
contribute to the oxidation of ferrous
compounds under conditions of restricted
oxygen supply (Kucera and Wolfe 1957).
• A third type of bacteria implicated in the
accumulation of large amounts of ferric
hydroxide are the sheath-forming iron bacteria
of the Chlamydobacteriaceae (Leptothrix
ochracea, L. discophora etc.).

24. • These organisms require a pH of about 6-7 for growth.
Under these conditions ferrous iron is readily oxidized
non-biologically. Accumulation of large masses of
flocculent ferric hydroxide is accomplished by the
pronounced tendency of the bacteria to deposit this
material on their sheaths (Mulder 1964; Mulder and
van Veen 1963).
• Although these three types of iron bacteria are
common in ironcontaining waters, evidence is
available that also in soil microbial oxidation of
ferrous iron may occur. This was clearly shown in
experiments with acid soil of pH 3 by Gleen (1950).
• However, it may be expected that in neutral and
alkaline soils ferrous iron is readily oxidized by a non-
biological reaction.

25. (ii) Reduction of ferric to ferrous iron
• This is undoubtedly a microbiological process
which occurs when the soil is kept under
anaerobic conditions (e.g. flooding).
Microorganisms of various types may be
assumed to contribute to the reduction process
(Alexander 1961).

26. (iii) Formation of ferrous sulphide
• Production of H>S from sulphate by the anaerobic
sulphate-reducing Desulfovibrio desulfuricans
precipitates Fe++ as FeS. This compound may also
be obtained when metallic iron is present in an
anaerobic sulphatecontaining medium
(corrosion).

27. Manganese
• Manganese (Mn) is an essential plant mineral nutrient,
playing a key role in several physiological processes,
particularly photosynthesis. Manganese deficiency is a
widespread problem, most often occurring in sandy soils,
organic soils with a pH above 6 and heavily weathered,
tropical soils. It is typically worsened by cool and wet
conditions (Alloway 2008). Numerous crop species have
been reported to show high susceptibility to Mn deficiency
in soils, or a very positive response to Mn fertilization,
including cereal crops , legumes, stone fruit, palm crops,
citrus, potatoes, sugar beets and canola, among others.
The impact of Mn deficiencies on these crops includes
reduced dry matter production and yield, weaker structural
resistance against pathogens and a reduced tolerance to
drought and heat stress.

28. (i) Biological manganese oxidation by specific
microorganisms
The organisms involved in this type of manganese
transformation include bacteria, fungi and yeasts.
They are able to oxidize manganous compounds to
manganic oxides (presumably MnO2.,) at pH values as
low as 6 and sometimes below 6. Organisms of this
type have been isolated as iron bacteria of the
Leptothrix group from slowly running waters by
several water microbiologists, including the present
authors (Mulder 1964, Mulder and van Veen 1963).

29. • Beijerinck (1913) isolated manganese-oxidizing
bacteria and fungi from soil, and von Wolzogen Kiihr
(1927) obtained manganese-oxidizing bacteria from
the filter beds of water works. Manganese oxidation
by a mixture of two bacteria (a Corynebacterium sp.
and a Chromobacterium sp.) has been recorded by
Bromfield and Skerman (1950) and by Bromfield
(1956). The former authors also studied three
manganese-oxidizing fungi obtained from a
manganese-deficient Australian soil. Zavarzin (1962/
63) obtained manganese oxidation by a mixture of
two Pseudomonas strains. Furthermore he described
a manganese-oxidizing bacterium (Metallogenium)
requiring the presence of a living fungus for growth
and manganese oxidation (1964). Tyler and Marshall
(1967a, b) isolated a manganese-oxidizing
Hyphomicrobium from running lake water.

31. (ii) Role of microorganisms in
rendering soil manganese unavailable
• There is no agreement among research workers as
to the effect of microorganisms in rendering soil
manganese unavailable to plants. Microbiologists
usually believe that microbial formation of
manganic oxides is the main cause of the decreased
availability of the manganese when added in the
bivalent form to neutral or slightly alkaline soils
containing a certain amount of organic matter.
• Some authors are of the opinion that fixation of
Mn++ by soil organic matter is the main reason for
its unavailability.

32. • To demonstrate that biological transformation of Mn+ +
plays an important part in rendering added manganese
insoluble, samples of a neutral sandy soil supplied with
114 mg Mn in the form of MnSO± per 100 g soil were
exposed to toluene-chloroform or to water vapour.
Watersoluble as well as versenate-acetate-soluble
manganese (Beckwith 1955) were estimated after
different periods of time. In addition
manganeseoxidizing bacteria and fungi were
determined. After a 4 months' incubation period all the
added manganese of the untreated soil had become
insoluble as contrasted with 12 per cent in the toluol-
chloroform-treated soil. After 2 months' incubation
these values were 70 and 13% and after 3 weeks, 26-5
and 11-4%, respectively. Manganese-oxidizing fungi had
almost entirely disappeared by the toluene-chloroform
treatment, while the manganese-oxidizing bacteria also
showed a serious drop in number.

33. (iii) Reduction of manganic to manganous
compounds
• Reduction of MnO2 in soil is a biological process.
Several microorganisms which decompose organic
matter can apparently use MnO2 instead of oxygen as
H acceptor, reducing it to Mn + + (Man and Quastel
1946).
• MnO2. can furthermore be reduced to Mn++ by
certain compounds formed by microbial processes,
e.g. H2S. The biological reduction of manganic oxides
is strongly favoured by a drop in soil pH to a value
below 5.5. Roots of living plants may promote the
solubilization of MnO2 by excreting organic acids or
other compounds stimulating bacterial activity
(rhizosphere effect).

34. Thanks