C H A P T E R

26
Thiobacillus
Murugan Kumar, Mohammad Tarique Zeyad,
Prassan Choudhary, Surinder Paul, Hillol Chakdar,
Mahendra Vikram Singh Rajawat
ICAR-National Bureau of Agriculturally Important Microorganisms (ICAR-NBAIM), Mau,
Uttar Pradesh, India

1. Introduction

Sulfur-oxidizing microorganisms are generally autotrophic, but a few heterotrophic

bacteria like Alcaligens, Paracoccus, Pseudomonas, Xanthobacter, and Bacillus have shown oxida-
tion of reduced inorganic sulfur compounds exhibiting chemolithotrophic growth pattern.
Thiobacillus and its related genera like Acidithiobacillus, Halothiobacillus, and Thermithiobacillus
are the important group of sulfur oxidizers which exhibit three types of metabolism like
obligate chemolithotrophic, heterotrophic, and mixotrophic mode of growth. There are two
mechanisms that are involved in the oxidation of sulfur by this group of microorganisms;
i) Aerobic oxidation, in which sulfur is oxidized into sulfite by using O2 as terminal electron
acceptor and ii) anaerobic oxidation, in which hydrogen-sulfide-ferric ion oxidoreductase
oxidize the elemental sulfur with Fe3þ acting as terminal electron acceptor to generate sulfate
(Zhang et al., 2018). These sulfur-oxidizing bacteria find their application in agriculture and
industries owing to their ability of oxidize reduced form of sulfur. The acidity resulted due to
oxidation of sulfur when managed well can help crop plants for uptake of various other nu-
trients like P, Zn, Fe, Mn, and Cu. At the industrial level, controlled sulfur oxidation by these
group of organisms make them an important candidate for bioleaching, biomachining, bio-
flotation and in various other applications where removal of sulfur from complex substances
can lead to economic and environmental benefits.

2. Taxonomy and reorganization of the genus Thiobacillus

The genus Thiobacillus comprises of a wide range of Gram-negative, nonspore-forming, rod

shaped, colorless chemolithoautotrophic sulfur bacteria, which obtain their energy by

Beneficial Microbes in Agro-Ecology

https://doi.org/10.1016/B978-0-12-823414-3.00026-5 545 © 2020 Elsevier Inc. All rights reserved.
546 26. Thiobacillus

oxidizing inorganic sulfur compounds belonging to b-1 subgroup of the class Betaproteobacte-
ria (Kelly and Harrison, 1989a,b; Kuenen et al., 1992; Kelly et al., 2007). These are ubiquitously
found in different natural environments and comprises of halophiles, thermophiles, extreme
acidophiles, or facultative anaerobes, and some are tolerant to high levels of toxic metals
(Hutchinson et al., 1966, 1967; Rawlings, 2002; Wood and Kelly, 1985, 1991). Some of these
species were heterotrophic and mixotrophic, whereas some were even lacking autotrophic
capability (Kelly and Harrison, 1989a,b). Thus, genus Thiobacillus is very heterogeneous.
The genus Thiobacillus initially comprised of two species of obligately chemolithoauto-
trophic sulfur-oxidizing bacteria: the aerobic Thiobacillus thioparus, and the facultative
denitrifier, Thiobacillus denitrificans (Beijerinck, 1904). Later, many other additional species
were described, based on colony morphology differentiation and a limited number of
physiological characteristics including variation in the sulfur substrates utilized for energy
production and the product formed after oxidation (Boden et al., 2012).
The first comprehensive attempt for classification of the species was based on physiolog-
ical and cultural properties of the species (Hutchinson et al., 1965, 1966, 1967, 1969). The
chemotaxonomic characteristics viz. ubiquinone, fatty acid, and DNA base composition
were also utilized for the identification and the classification of thiobacilli (Katayama-
Fujimura et al., 1982). 5S rRNA gene sequence based phylogenetic analysis has also been
attempted for the genus Thiobacillus (Lane et al., 1985). The taxonomy of genus has
become more difficult as some of the morphologically distinct colorless sulfur bacteria viz.
Thiomicrospira showed physiological similarity with well-established heterotrophic
eubacterial genera such as Paracoccus, Xanthobacter, and Aquaspirillum (Friedrich and
Mitrenga, 1981; Wiegel, 1992; Katayama et al., 1995). In the first edition of Bergey’s Manual
of Systematic Bacteriology (1998), at least 32 species were named, but many of them were
never validated or were subsequently lost from culture. As studies based on modern
taxonomic methods began to reveal that some of the species are only superficially related,
virtually every paper describing a new Thiobacillus species in the last decade has mentioned
the need to reorganize the genus (Kelly and Wood, 2000). This reorganization resulted into
dramatic changes in the distribution of the colorless sulfur bacteria, particularly in the status
of species, which were earlier placed in the genus Thiobacillus (Table 26.1).
Advances in diagnostic methods and 16S rDNA-based classification leads to the reorgani-
zation of the genus and many species were assigned to other existing or new genera,
including Acidiphilium, Acidithiobacillus, Halothiobacillus, Paracoccus, Starkeya, Thermithiobacil-
lus, and Thiomonas (Kelly and Wood, 1998, 2000; Kelly et al., 2000, 2005, 2007; Hiraishi and
Imhoff, 2005; Katayama et al., 2006; Battaglia-Brunet et al., 2011). Only three validly named
species and one putative species were recognized in the second edition of Bergey’s Manual of
Systematic Bacteriology (Kelly et al., 2005). Recently, based on 16S rDNA sequence, only six
distinct species viz. T. thioparus, T. denitrificans, T. aquaesulis, T. thiophilus, “T. plumbophilus,”
and “T. sajanensis” are accepted (Boden et al., 2012).

3. Thiobacillus and its interactions with the physical environment

Genus Thiobacillus have been subject to reclassification based on myriad characteristics like
molecular phylogenetics, ecology, and physiology (Kelly and Wood, 2000; Zhang et al., 2019).
Species of the genus Thiobacillus generally occupy extreme ecological niches especially

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 3. Thiobacillus and its interactions with the physical environment 547
TABLE 26.1 Summary of the reorganization of the sulfur bacteria, particularly the status of species
previously placed in the genus Thiobacillus.
Sub-class Current name Synonyms

a Acidiphilium acidophilum Thiobacillus acidophilus, Thiobacillus organoparus

a Paracoccus pantotrophus Thiosphaera pantotropha

a Paracoccus versutus Thiobacillus versutus, Thiobacillus rapidicrescens, Thiobacillus A2

a Starkeya novella Thiobacillus novellus
b Thiobacillus aquaesulis e
b Thiobacillus denitrificans e
T
b Thiobacillus thioparus Thiobacillus thiocyanoxidans, Bacterium thioparum
b Thiomonas cuprina Thiobacillus cuprinus
T
b Thiomonas intermedia Thiobacillus intermedius
b Thiomonas perometabolis Thiobacillus perometabolis, Thiobacillus rubellus
b Thiomonas thermosulfata Thiobacillus thermosulfatus, Thiomonas thiosulfata
b Unknown Thiobacillus plumbophilus
g Acidithiobacillus albertensis Thiobacillus albertis
g Acidithiobacillus caldus Thiobacillus caldus

g Acidithiobacillus ferrooxidans Thiobacillus ferrooxidans, Ferrobacillus ferrooxidans

T
g Acidithiobacillus thiooxidans Thiobacillus thioxidans, Thiobacillus concretivorans, Thiobacillus kabobis,
Thiobacillus thermitanus, Thiobacillus lobatus, Thiobacillus cretanus,
Thiobacillus umbonatus
g Halothiobacillus halophilus Thiobacillus halophilus

g Halothiobacillus hydrothermalis e
g Halothiobacillus kellyi e
T
g Halothiobacillus neapolitanus Thiobacillus neapolitanus, Thiobacillus X
T
g Thermithiobacillus tepidarius Thiobacillus tepidarius
g Thioalcalivibrio denitrificans e
g Thioalcalivibrio nitratus e

g Thioalcalivibrio versutus e
g hioalcalimicrobium aerophilum e
g Thioalcalimicrobium sibericum e
g Thiomicrospira chilensis e
g Thiomicrospira crunogena e

(Continued)

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548 26. Thiobacillus

TABLE 26.1 Summary of the reorganization of the sulfur bacteria, particularly the status of species
previously placed in the genus Thiobacillus.dcont’d
Sub-class Current name Synonyms

g Thiomicrospira frisia e
g Thiomicrospira kuenenii e
T
g Thiomicrospira pelophila e
g Thiomicrospira thyasirae Thiobacillus thyasiris

Adapted from Robertson, L.A., Kuenen, J.G., 2006. The genus Thiobacillus. In: Dworkin, M., Falkow, S., Rosenberg, E., Schleifer, K.H.,
Stackebrandt, E. (Eds.), The Prokaryotes. Springer, New York, NY.

acidic environment and cling themselves to sulfur/iron-rich environment with heavy-metal

interactions, which make them survive in nutrient-deficient conditions. Hydrothermal vents,
activated sludge disposal sites, and sources of sulfur gases, such as sediments or anaerobic
soils releasing H2S can become potential sites of occurrence of Thiobacillus (Donati et al.,
2003; Li et al., 2012). They drive their metabolic activities using sulfur, sulfides, polythionates,
thiocyanate, and thiosulphate as energy sources. Strains of Acidithiobacillus (At.) ferrooxidans
have been reported to grow by coupling the oxidation of hydrogen to the reduction of
elemental sulfur (Ohmura et al., 2002; Yu et al., 2015).With increase in various molecular
biology techniques, identification of many species is possible which are responsible for
mineral dissolution (Romo et al., 2013).
Thiobacillus plays an integral role and has wide applications in the field of wastewater
treatment, agriculture, biohydrometallurgy, and maintenance of geomicrobiological cycle.
The geochemistry studies of cave waters have been known to contain a wealthy, sulfur-
based lithoautotrophic microbial ecosystem (Johnson, 2012; Jones et al., 2008; Macalady
et al., 2008). Studies have been focusing on the microbial biofilms being formed in cave
waters from sulfidic springs (Engel et al., 2004). Similarly hydrothermal microbial ecosystems
are known to harbor sulfur utilizing Thiobacillus spp. (Teske and Reysenbach, 2015).
Thiobacillus ferrooxidans has been known to reduce acetylene to ethylene in the absence of
nitrogen sources indicating the possession of nitrogenase enzyme by this microbe
(Mackintosh, 1978). The role of Thiobacillus and its coexistence with Leptospirillum spp. in
the bioleaching ecosystems is of utmost significance to the microbiologists (Benner et al.,
2000; Escobar and Godoy, 1999; Korehi et al., 2013). Pyrite bioleaching was reported by
Gregory J. Nelson with 10 laboratories participating in the study involving T. ferrooxidans
13661 and L. ferrooxidans (Olson, 1991). Pyritic mine ceilings found to be dripping with
interstitial water, generally red in color, are populated by primarily Acidithiobacillus ferrivor-
ans and L. ferrooxidans (Johnson, 2012). In 1971, Beebe et al. reported an extracellular lipid,
which facilitates the oxidation of sulfur in Thiobacillus spp. (Beebe and Umbreit, 1971).
Chromium IV reduction has been reported employing Thiobacillus ferrooxidans and
Thiobacillus thiooxidans by the production of sulfite and thiosulfate. Acid mine drainage
systems, deep-sea sediments, and oil-contaminated soils are also sources of importance if
the evolution and ecology of Thiobacillus species are to be understood (Huang et al., 2016;
Jiao et al., 2017).

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 4. Agricultural importance 549
Thiobacillus thioparus THI115 has been reported to degrade carbonyl sulfide (COS), which
is a well-known stratospheric sulfate aerosol (Ogawa et al., 2017). Since Thiobacilli do not find
themselves compatible with media containing organic matter especially low molecular
weight organic acids, it was a matter of investigation how it could decontaminate sludge
by itself, although much organic wastes are deposited over those sites. Fournier et al. worked
on the synergistic mechanisms of T. ferrooxidans with other acidophilic microorganisms
and demonstrated a yeast Rhodotorula rubra, which helped in the ferrous iron oxidation
and acidification by T. ferrooxidans, providing insight into the microbial assemblage property
of Thiobacillus with heterotrophic microbes (Fournier et al., 1998). Thiobacillus have also been
reported to increase the availability of rock phosphate in soil (Jazaeri et al., 2016). Ale Agha
et al. (2018) necessitates the use of sulfur fertilizers along the inoculation of Thiobacillus for
better oxidation of sulfur in irrigated and rainfed agricultural soil (Beheshti Ale Agha
et al., 2018). Importantly, iron oxidation by T. ferrooxidans is more susceptible to inhibition
by chloride, phosphate, azide, cyanide, and nitrate at low concentrations (below 0.1 M).
On the other hand, its sulfur oxidation activity is more sensitive to high osmotic pressure
(Harahuc et al., 2000). With low pH and temperature tolerance Thiobacillus species have
survived extreme ecological nook and corners of the world. It is a matter of focused research
how these extremophiles can find its application in matters concerning environmental
pollution and toxicity on a global scale.

4. Agricultural importance

Thiobacillus and its related genera are believed to play an important role in crop production
through S-oxidation, P-solubilization, and solubilization of other nutrients like Zn, Fe, etc.
They are used in the reclamation of alkaline soil, making nutrients available to crop plants
which were hitherto unavailable in alkaline soils.

4.1 Thiobacillus and sulfur nutrition to crop plants

Sulfur is the fourth major plant nutrients after nitrogen, phosphorus and potash and the
form in which plants take up sulfur is SO2 4 . Sulfur enters the soil system through microbial
activities like mineralization, immobilization, and redox reactions. Microbes play an impor-
tant role in releasing S from sulfide minerals to soil ecosystem. All reduced forms of S
must be oxidized first to sulfate form as plants can take up S only in this form. Though mi-
crobes from all three domains are involved in S-oxidation, a major role is played by bacteria
and within the domain bacteria, Thiobacillus, and related genera are important sulfur
oxidizers. Members of these genera obtain their energy/electrons from reduced sulfur
compounds and carbon from either carbon-dioxide or organic carbon compounds (Prasad
and Shivay, 2018). Although early reports have shown that Thiobacillus and its related genera
are not present in soil in significant amounts (Chapman, 1990; Lawrence and Germida, 2011),
a study conducted in 2010 have shown that the population of Thiobacillus spp. increases with
the addition of elemental sulfur and carry out sulfur oxidation for about two weeks
(Yang et al., 2010). Thus, it is an important group of S oxidizers present in soil. Several studies

I. Bacteria
550 26. Thiobacillus

have shown that inoculation of thiobacilli increases sulfur nutrition to crop plants, yield, and
quality. Kapoor and Mishra (1989) observed that sulfur was rapidly oxidized in a field soil
of pH 8.0 and the rate of oxidation could be enhanced by inoculation with T. thiooxidans.
Halothiobacillus sp. with the ability to oxidize thiosulphate, was found to increase maize
growth parameters and uptake of nutrients like sulfur, phosphorus, potassium, calcium,
and manganese (Rangasamy et al., 2014). Thiobacillus spp. is generally applied along with
elemental sulfur as a substrate for sulfur oxidation. Inoculation of Thiobacillus at the rate of
104 cells per gram of soil along with application of elemental sulfur at various levels increased
the yield, nutrient availability in the soil, and nutrient uptake by plants in calcareous soils
(Besharati, 2017). Inoculation of Thiobacillus with sulfur significantly increased fruit weight,
fruit yield, seed weight, seed yield, and oil content in the medicinal pumpkin under deficit
irrigation conditions (Masoodi and Hakimi., 2017).

4.2 Thiobacillus and P nutrition in crop plants

Phosphorus is the second-most important plant nutrient after nitrogen which plays
an important role in physiology of crop plants and hence in the plant growth, yield, and
quality of the produce. Most phosphorus in the soil are in the form unavailable to crop
plants and phosphorus nutrients are the most vulnerable to fixation in the soil. Hence, a
significant amount of phosphorus added to crop plants were fixed in the soil and are
unavailable for plant uptake. When plants are fed with adequate amount of all other
nutrients, inadequate amount of phosphorus can lead to poor growth and poor root
development, and hence affect the yield and quality (Chen et al., 2008). The use of
phosphorus-solubilizing bacteria and phosphorus mobilizers like mycorrhizal inoculation
are the two major strategies of phosphorus nutrient management through microorganisms.
Another common strategy is to use sulfur-oxidizing bacteria in combination with a reduced
form of sulfur (usually elemental sulfur) with or without a complex phosphorus source like
rock phosphate. A sulfur and iron oxidizing bacterium Acidithiobacillus sp. B2 isolated from
waste waters of copper sulfide mine was shown to leach 34.5% of inorganic phosphorus
from rock phosphate. Hence this strain can be an important component of
P-solubilization in soil and for the production of phosphatic fertilizers from insoluble source
(Avdalovic et al., 2015). A sulfur-oxidizing bacteria Thiobacillus thiooxidans, when inoculated
in soil along with application of elemental sulfur, rock phosphate, and vermicompost
was shown to increase water-soluble phosphate content in the soil (Aria et al., 2010).
Acidithiobacillus sp. was inoculated along with application of sulfur in presence of rock
phosphate and other biofertilizers. The results of the experiment have shown that such
application can be an effective alternate to P and K for growing sugarcane with soils low
in P and K (Stamford et al., 2006). In another study, combined application of sulfur, rock
phosphate, and Thiobacillus sp. significantly increase soil extractable phosphorus and
phosphorus uptake by plants (Jazaeri et al., 2016). Application of sulfur-oxidizing
bacterium, Thiobacillus sp., along with a phosphorus solubilizing bacterium, Bacillus
sp., was shown to increase nutrient uptake, yield, and oil content in canola (Salimpour
et al., 2012).

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 5. Industrial importance 551

4.3 Inoculation of Thiobacillus for zinc and iron nutrition to crop plants
Canola, when inoculated with nitrocara biofertilizer and sulfur-oxidizing bacterium
Thiobacillus sp., along with the application of Zn and Fe micronutrient as foliar spray was
shown to accumulate increased zinc and iron in grains. This application also increased the
grain yield and oil content in canola (Jashni et al., 2017). Thiobacillus and sulfur application
with zinc had promising effect on increased yield of green beans. These studies were
confirmed by spray of zinc with sulfur and Thiobacillus on green beans. In this application
sulfur first reduce the soil pH of arid regions, and thereby availability of NPK and other
soil nutrients increased. Secondly, application of zinc makes suitable quantities of this
element available to plants without polluting the soil. Finally, maximum increase in food
yield occurred due to sulfur with Thiobacillus in comparison to other treatment (Motamed
et al., 2018). Inoculation with Thiobacillus thiooxidans along with the spray of iron and zinc
improved the yield in maize as well as grain quality (Hagh et al., 2016). Inoculation of Thio-
bacillus sp. along with the application of ground rubber was shown to enhance Fe and Zn
content in wheat plants (Asadollahzadeh et al., 2019).

4.4 Thiobacillus and soil reclamation

Members of Thiobacillus and related genera are found to involve in reclamation of
alkaline, sodic and calcareous soils and soils with heavy-metal contamination. Reclamation
of calcareous soil in Iran was carried out with the application of elemental sulfur, vermi-
compost along with the inoculation of sulfur-oxidizing bacterium Thiobacillus sp. It was
shown that these amendments resulted in considerable improvement in available P content.
To assess the impact of reclamation, a black seed crop is taken up, and it was shown that in
the treatment receiving these amendments uptake of P and N increased along with the yield
and oil content (Seyyedi et al., 2015). An effort was made to alleviate soil salinity by
applying sulfur and sulfur-oxidizing bacterium Acidithiobacillus sp. together. The results
indicate that sulfur inoculated with Acidithiobacillus sp. reduced soil pH, especially when
applied without gypsum (Stamford et al., 2015). In sodic soils of Brazil application of sulfur
along with inoculation of Acidithiobacillus sp. have shown better results in reclamation as
compared to treatment receiving gypsum or sulfur individually (Stamford et al., 2007).
Acidithiobacillus ferroxidans has been utilized in biosynthesis of schwertmannite a
soil amendment for immobilization of arsenic in contaminated soil. Two types of schwert-
mannite (SCH and A-SCH) were prepared in the study and both were found to immobilize
As in contaminated soil effectively (Chai et al., 2016).

5. Industrial importance
The major potential industrial application of Thiobacillus and related genera are
bioleaching, bioflotation, biomachining, and biosynthesis.

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552 26. Thiobacillus

5.1 Bioleaching
Solubilization of metal ions from insoluble solids employing S/Fe oxidizing bacteria is
called bioleaching. Bioleaching can also be achieved using the byproducts of these bacterial
metabolism. Acidithiobacillus ferroxidans, A. thiooxidans, A. caldus, A. ferridurans,
A. ferrivorans, Thiobacillus thioparus, and Halothiobacillus halophilus are the important bacterial
species that has the potential to be involved in bioleaching (Kutschke et al. 2015; Vainshtein
et al., 2015; Sajjad et al., 2019). Among them, A. ferroxidans is the most significant bacteria
which can employ, contact, noncontact, and cooperative mechanisms of bioleaching. Contact
mechanism is the one where bacterial cells adhere on the surface of the ores (Fe3þ ion is the
mediator here) and the resulting ferric complex decompose the metal ores. In noncontact
mechanisms Fe3þ ions produced due to the metabolism of bioleaching bacteria exhibiting
planktonic growth. Fe3þ ions in turn oxidize metal ores and are reduced to Fe2þ ions and
the cycle continues. In cooperative mechanism a combination of contact and noncontact
mechanisms exerts over the metal ores. Here the metal ores are dissolved by the attached
bacterial cells as well as by the Fe3þ ions produced due to the metabolism of planktonic
bioleaching bacteria. Various metal ions like, copper, zinc, arsenic, cobalt, uranium and nickel
can be removed and separated out from their respective ores employing A. ferroxidans. Dong
et al. (2013) have shown the potential of Acidithiobacillus ferroxidans in extracting copper from
different ores. They have shown copper extraction potential of 18%, 54%, 67%, 85%, and 95%
from porphyry chalcopyrite, covellite, massive sulfide copper, bornite, and djurleite
respectively. It was found that zinc recovery from sphalerite is higher in a mixed culture
of A. ferroxidans and A. thiooxidans as compared to the bioleaching by either of the pure
cultures. This is because a complementation occurs between the two bacteria. A. ferroxidans
oxidize Fe2þ to Fe3þ, and in turn maintain a high redox potential; at the same time A.
thiooxidans oxidize the layers elemental sulfur formed during the dissolution of zinc. The
elemental sulfur layer that are formed in the absence of any oxidation due to A. thiooxidans
block the continuous dissolution of zinc (Le-xian et al., 2006). Acidithiobacillus ferroxidans
have been used in presence of Fe2þ ions to extract nickel from heazelwoodlite and pentlandite
with a recovery of 35% and 40% respectively (Giaveno and Donati, 2001; Watling, 2008). A
moderately acidophilic thiobacilli, Halothiobacillus halophilus was shown to extract nickel
from a canadian sulfide ore. Supplementing the process with formate at 0.3% has increased
the nickel recovery from 13.5 to 1008 mg per liter over a period of 34 days (Vainshtein et al.,
2015). Another important aspect of bioleaching is the removal of metal contaminants from
sewage sludge, contaminated soil and sediments. It has been shown that A. ferroxidans
was utilized in the removal of metal ions like arsenic, manganese, copper and aluminum
(Lombardi and Garcia, 2002) so that the treated sewage sludge can be used as organic matter
in agricultural fields.

5.2 Bioflotation
The process of removing gangue material from ores and minerals is called mineral
beneficiation, in which froth flotation is a vital process. Toxic, nondegradable chemicals
like xanthates, petroleum oils, cyanides, and many more are used for this process in

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 6. Conclusion and future perspectives 553
conventional beneficiation. Stricter environmental regulations demand the use of alternative
methods for beneficiation of minerals. Microorganisms as a whole and their metabolites
like polysaccharides, proteins and other surfactants has got the potential to be used in
flotation and flocculation (Behera and Mulaba-Bafubiandi, 2017). This process of using
microorganisms and/or their metabolites for beneficiation of minerals is called bioflotation.
Zinc ore, sphalerite and lead ore, galena have shown enhanced flotation when treated with
A. thiooxidans. Adhesion of bacterial cells or their metabolites (extracellular polymeric
substances, proteins) is believed to be the mechanism for bioflotation. Adhesion of bacterial
cells is proved by the change in isoelectric point toward a higher pH (Natarajan 2003). Acid-
ithiobacillus ferroxidans has been used in bioflotation of pyrite, chalcopyrite, and sarcheshmeh,
a copper ore of Iran (Hosseini et al., 2005; Behera et al., 2012).

5.3 Biomachining
Removal of metal from a work-piece employing microorganisms is called biomachining.
Biomachining using microorganisms offers various advantages like high precision and
quality, less heat and stress during the process and avoidance of any damage/distortion to
the workpiece (Zhang et al., 2018). Like bioleaching, many initial studies on biomachining
have been carried out employing A. ferroxidans. Xenofontos et al. (2015) isolated six different
strains of A. ferroxidans and found that the strain A. ferroxidans B1 was the most effective in
machining copper with a removal of 23.2 mg pure copper per cm2 after 45 h. The mechanism
of biomachining operated by the strain is indirect bioleaching due to Fe3þ ions regenerated
through oxidation.

5.4 Removal of sulfur from solids and gases

Removal of sulfur is required in coal industries, petroleum industries, rubber industries,
and many more. Acidithiobacillus ferroxidans have been utilized to remove sulfur from coal.
Just as in the case of bioleaching here also the removal of sulfur is by either contact or non-
contact mechanisms (Hong et al., 2013). Treatment of waste rubber with A. ferroxidans have
been found to decrease the sulfur content. Sulfur compounds present in waste rubber are
gradually oxidized to sulfates, so that the rubber substrates are separated out (Wang et al.,
2011). Thiobacillus sp. RA101 immobilized in Ca-alginate beads was found to remove sulfur
from synthetic spent sulfide caustic in a petroleum refinery (Potumarthi et al., 2008). Acidithio-
bacillus ferroxidans and A. thiooxidans have been found useful in removal of sulfur compounds
from gases too. Desulfurization efficiency of A. ferroxidans has been found to be in the range
of 91%e99% in various studies (Zhang et al., 2018).

6. Conclusion and future perspectives

The use of Thiobacillus and related genera for agriculture in providing nutrients to crop
plants gained importance after the addition of sulfur from the environment were reduced

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554 26. Thiobacillus

due to stricter environmental regulations. Although several studies have highlighted the
importance of this genera isolated from extreme environments there are no concrete evidence
for the population build up in the rhizosphere where the conditions are altogether different
from their isolation source. Future research needs to take up this perspective of colonization
potential of these microorganisms in rhizosphere of different crops.

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