A common feature of the members
of these three genera is their ability to gain energy from the oxidation
of reduced sulfur compounds (see Chapter 16). Nearly all of them are able
to grow autotrophically, and about one-half can also grow organotrophically.
Other inorganic sources of energy for some members of this group include
hydrogen, ferrous iron, and other reduced metal ions. The ability to use
organic compounds for carbon or energy varies among these bacteria. Many
strains are highly specialized, and these obligate chemolithotrophs are
only able to use carbon dioxide as their carbon source, and compounds such
as sulfide or thiosulfate for energy. They can sometimes assimilate a small
number of organic compounds, but only to a limited extent and in a restricted
pattern (see Chapter 14). In contrast, the facultative chemolithotrophs
are physiologically versatile and can grow autotrophically, heterotrophically,
or mixotrophically (in the latter, inorganic and organic compounds are
used simultaneously for energy and carbon). When grown as batch cultures
on mixtures of these substrates, the versatile species often show diauxie,
and the autotrophic pathways (e.g., the Calvin cycle for carbon dioxide
fixation) are frequently repressed. At least one strain is a chemolithoheterotroph
which can derive energy from the oxidation of reduced sulfur compounds,
but cannot grow autotrophically because it lacks the Calvin cycle.
All species in the three genera
are respiratory, being able to use oxygen as their terminal electron acceptor.
A few can denitrify, and several appear to only be able to reduce nitrate
to nitrite.
Habitats
In nature, the distribution of
reduced inorganic sulfur compounds is only one of the factors governing
the presence and activity of the colorless sulfur bacteria, and a complex
of chemical, physical, and microbial interactions is almost
as important a controlling force.
One crucial factor is the requirement for the simultaneous presence of
an electron donor and electron acceptor (i.e., oxygen or nitrogen oxides).
Sulfide is one of the most important natural substrates, and many of the
colorless sulfur bacteria grow in the narrow zones and gradients where
sulfide and oxygen coexist (Nelson and Jannasch, 1983). Such gradients
can be found, for example, in stratified lakes and at the interface between
aerobic water and an anaerobic sediment, but may be even more common in
microniches in environments that contain anaerobic pockets (e.g., sediments
and wet soils). Similarly, other gradients can influence the growth and
selection of sulfur oxidizers. Thus, it has been shown that Thiobacillus
species respond differently to varying redox potentials (Sokolova and Karavaiko,
1968; Timmer ten Hoor, 1977). The occurrence of acidophilic bacteria in
apparently neutral environments indicates the existence of pH gradients
associated with acidic microniches.
Sulfur oxidizers have been enriched
and isolated from natural sites chosen by the obvious presence of inorganic
sulfur or iron compounds (e.g., sediments from rivers, canals, estuaries
and tidal flats, acid sulfate soil, (hot) acid springs, mine drainage effluent,
hydrothermal vents, and wastewater treatment systems). The thiobacilli,
especially the specialized chemolithotrophs, have long been viewed as one
of the most important groups of organisms contributing to the transformation
of sulfur in soils and sediments, although only a few systematic studies
on their occurrence and distribution have been carried out. However, there
is now evidence that, depending on the particular environmental conditions,
the mixotrophic and heterotrophic sulfur oxidizers may be at least equally
important in the overall conversion of reduced sulfur compounds to a more
oxidized state (Guitonneau and Keiling, 1932; Kuenen, 1975; Mason and Kelly,
1988; Robertson et al., 1989a). In general, sulfur and iron oxidizers are
ubiquitous in soil, sediments, and freshwater and marine environments.
They grow over a wide range of pH values and temperatures (see Tables 1
and 2). Thus, it is relatively easy to find them in nature. An obvious
factor that limits their growth is the availability of reduced inorganic
sulfur compounds, a limitation that is especially important for the obligate
autotrophs that completely depend on these compounds for growth. Despite
their ability to incorporate some organic compounds into cell material,
the contribution of the latter group to the breakdown of organic compounds
in nature is probably negligible. Gottschal and Kuenen (1980) showed that
the versatile species are generally favored in freshwater environments
when the turnovers of available inorganic and organic substrates are approximately
equivalent. When the inorganic substrates are dominant, obligate autotrophs
should be favored and, similarly, heterotrophs will be favored when organic
compounds make up the bulk of the available substrate (Fig. 1). However,
this does not necessarily hold true for marine situations (Kuenen et al.,
1985). Further work remains to be done to discover what other selective
pressures are operative in the marine milieu.
Because many of the colorless
sulfur bacteria produce sulfuric acid or ferric iron, they are often associated
with the oxidative corrosion of concrete and pipes, and have been implicated
in the corrosion of buildings and ancient monuments. Acid and metal pollution
can also be a result of the activities of thiobacilli in mine wastes (Tuovinen
and Kelly, 1972). On the more positive side, the production of acid can
be used, for example, in leaching processes for the extraction of metals
from poor ores that are unsuitable for extraction by conventional metallurgical
methods. In addition, the pyrite-oxidizing capacity of T. ferrooxidans
and related organisms has also been successfully exploited for the desulfurization
of coal (Bos et al., 1988; Bos and Kuenen, 1990).
In soils, thiobacilli may sometimes
be responsible for the solubilization of sulfur compounds, thus making
sulfur available (as sulfate) for assimilation by other microorganisms
and plants. The sulfur budget and the role of microorganisms in the sulfur
cycle of cultivated soils is, of course, important for assessing fertility.
In Australia, for example, thiobacilli are scarce in sulfur-deficient areas.
Under suitable climatic conditions (e.g., in the tropics) rock phosphate
pelleted with sulfur and seeded with thiobacilli has been shown to be a
useful, slow-release source of phosphate and sulfate for soil fertilization
(Swaby, 1975).
Marine thiobacilli have been
isolated from oceans, hydrothermal vents, and coastal and tidal areas,
and classified by reference to nonmarine cultures. Characteristically,
some (but not all) marine forms have a requirement for (sodium) chloride
levels comparable with the salinity of their natural environment. It is
difficult to explain their presence in the open oceans - perhaps they are
able to scavenge volatile sulfur compounds released from bottom sediments
during the process of decomposition. At the hydrothermal vents, they are
believed to be important producers of organic compounds at the start of
the geochemically based food chain (Jannasch, 1985). However, as they are,
on the whole, dependent on oxygen generated by photosynthesis for electron
acceptors, they cannot, therefore, be considered "primary producers" in
the strict sense of the term.
Physiology
By definition, all of the species
described in this chapter can derive energy for growth from the oxidation
of reduced sulfur compounds if provided with a terminal electron acceptor,
but the conditions required (e.g., pH) may vary somewhat (Jackson et al.,
1968). Moreover, the ability of all of the colorless sulfur bacteria to
oxidize certain sulfur compounds has not been rigorously tested. Until
this is done, the differentiation of the various species, especially within
the genus Thiobacillus, on the basis of the oxidation of different sulfur
compounds should be treated with caution. Some of the reactions can be
summarized as follows:
H2S + 202
----> H2SO4
2H2S + 02
----> 2S° + 2H20
2S° + 3O2 + 2H20
----> 2H2SO4
Na2S203
+ 202 + H20 ----> Na2SO4
+ H2SO4
4Na2S203
+ 02 + 2H20 ----> 2Na2S406
+ 4NaOH
2Na2S406
+ 7O2 + 6H20 ----> 2Na2SO4
+ 6H2SO4
2KSCN + 4O2 + 4H20
----> (NH4)2SO4 + K2S04
+ 2CO2
5H2S + 8KNO3
----> 4K2SO4 + H2SO4
+ 4N2 + 4H20
5S° + 6KNO3 +
2H20 ----> 3K2SO4 + 2H2SO4
+ 3N2 |
Moreover, some Thiobacillus species
can derive energy from ferrous iron oxidation as follows:
2FeS2 + 2H20
+ 7O2 ----> 2FeSO4
+ 2H2SO4
4FeSO4 + 02
+ 2H2SO4 ----> 2Fe2(SO4)3
+ 2H20 |
The complete oxidation of reduced
sulfur compounds to sulfate is common to all of the obligate and facultatively
autotrophic sulfur oxidizers, and the ability of a culture to lower its
pH while growing on thiosulfate is one of the criteria that has been used
for the detection of these bacteria. The pH often rises in cultures of
heterotrophs able to partially oxidize inorganic sulfur compounds because
of the formation of polythionates (e.g., tetrathionate from thiosulfate).
It should be noted, however, that a lack of acid production is not a decisive
criterion for the differentiation between the chemolithotrophic and chemoorganotrophic
sulfur oxidizers because under unfavorable cultural conditions, even the
chemolithotrophs accumulate intermediates in significant amounts. For example,
it has been observed that T. ferrooxidans, when growing on thiosulfate,
trithionate, or sulfide, produces sulfur which is not only detectable outside
the cells, but also as a finely dispersed precipitate outside the cell
membrane (Fig. 2; Hazeu et al., 1988). T. neapolitanus and Thiobacillus
strain O both produce sulfur during growth on reduced sulfur compounds
under various environmental stresses (e.g., oxygen tension), but do not
appear to accumulate sulfur internally (Stefess and Kuenen, 1989).
The
Genus Thiobacillus
All members of this genus are
Gram-negative rods, some of which are motile. As mentioned above, classification
is still, generally, based on morphological and physiological features.
Inevitably, some strains have been studied more thoroughly then others.
Detailed taxonomic descriptions can be found in Kelly and Harrison (1989).
However, a few key points are summarized in Tables I and 2. For convenience
these species have been separated into three groups: the neutrophilic specialists,
the versatile species, and the acidophiles.
THIOBACILLUS
FERROOXIDANS; SYNONYM FERROBACILLUS FERROOXIDANS; TYPE STRAIN ATCC
23270. T. ferrooxidans is an obligate autotroph which is able to
derive energy from the oxidation of ferrous iron as well as reduced sulfur
compounds. During growth on thiosulfate trithionate, tetrathionate, and
sulfide, T. ferrooxidans has been observed to accumulate fine sulfur
deposits (Fig.2) which are predominantly associated with the cell wall
(Hazeu et al., l 988). Early reports of facultatively autotrophic T.
ferrooxidans strains are now believed to have been due to the presence
of facultatively autotrophic (see T. acidophilus) or heterotrophic
contaminants, some of which have been difficult to remove as their presence
can stimulate the growth of the autotroph (Harrison, 1984). The ability
to oxidize Fe2+ is the key characteristic that is generally employed in
isolation procedures. Initial enrichments with reduced sulfur compounds
frequently produce mixtures of various acidophilic thiobacilli, whereas
enrichment on Fe2+ can result in almost pure cultures of T. ferrooxidans. |