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Assignment-1
Desulfurococcus mobilis
1. INTRODUCTION
Desulfurococcus mobilis is a type of Archae. Its higher order taxa: Archae
(Domain), Crenarchaeota (Phylum), Thermoprotei (Class), Desulfurococcales (Order),
Desulfurococcaceae (Family. Along with Desufurococcus mobilis, there are
Desulfurococcus amylolyticus, Desulfurococcus fermentans and Desulfurococcus
mucosus. D.mobilis is crenarchaeotes (Greek crene, spring or fount, and archaois,
ancient or primitive) and was thought to resemble the ancestor of the Archaea in
accordance to its simplicity in structure. This hyperthermophiles are called so as, the
optimum temperature it resist is higher compared to other microorganisms as so the
habitat distribution is widespread within geothermal area.
2. HABITAT
The existence of hyperthermophilic microorganisms in the deep terrestrial
subsurface has been noted for a long time, but only in the past decade has there been an
increasing interest towards exploring subterranean microbial life in deep-surface
environments. Till then, they found most common and accessible thermal habitats are hot
springs, sulfatara, and geothermally heated soils that usually contains elemental sulfur.
They are scattered all over the world. Examples are sulfur-rich hot springs in
Yellowstone National Park, Wyoming, and the waters surrounding areas of submarine
volcanic activity. The subsurface environment can be divided into non volcanic areas that
are cold and volcanic areas that are hot. Sometimes the thermal energy is visible on the
surface as hot springs and sulfatara fields, and sometimes it is visible in the form of
volcanic eruptions. The temperature at non volcanic areas increases by 25°C for each
kilometer of depth; therefore, D.mobilis should thrive in the deep subsurface.
One example experimental lab done for determining the growth of D.mobilis
confirms the habitat for this therma loving Archae. Despite the use of various media and
enrichment conditions, growth was observed only in media with source borehole water.
The cultures were dominated by the thermophilic archeon Desulfurococcus mobilis and a
few bacterial sequences closely related to Aquificales clone SRI-48. This may suggest that
subsurface growth conditions are needed for growing the non cultivated subterranean
cells detected in samples.
Geothermal reservoirs are a novel, volcanic, high-temperature environment for
hyperthermophilic microorganisms .Iceland is one of the world's largest subsurface hot
spots; its crust is very permeable, and therefore there are ideal paths through
interconnected subsurface conduits for microorganisms that are able to survive over
geological time in subsurface environments. The extent of subsurface dissemination
depends on dispersal barriers, such as seawater and temperature, or hydrological barriers.
The results highlight a new indigenous microbial biosphere and indicate that
hyperthermophilic microorganisms disseminate in the subsurface within volcanic zones
through subsurface conduits, as has already been observed with meteoric groundwater
and on the surface in cold aquifers. Since the discovery of hydrothermal activity on
oceanic spreading centers in the Eastern Pacific in 1977 and 1979, hot springs have been
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Desulfurococcus mobilis
found at numerous locations on ridge systems. The Mid-Atlantic ridge is one of these
systems, and Iceland, with its volcanic activity, is the only place where it emerges from
the sea. The Eyjafjordur region, in northern Iceland, is one of several localities in Iceland
with known submarine geothermal activity. The geothermal activity is in basaltic lava 6
to 12 million years of age.
Our findings indicate that this freshwater hydrothermal vent system, located on
the seafloor at a depth of 65 m, is a new kind of submarine geothermal vent. Sampling
large quantities of hydrothermal vent fluid from submarine hot springs has been very
difficult until now. The location of the vent site places it within submarine hot springs,
and its large precipitate cone structures resemble chimney structures in the deep-sea
hydrothermal vents. However, the chemical composition of the cones and the discharging
fluid was different from that of any previously described submarine hydrothermal vent
system. The Mg-rich clays have formed as hot alkaline SiO2-rich; geothermal fluids have
mixed with cold Mg-rich seawater. Anhydrite is the dominant chimney-building mineral
in deep-sea hydrothermal vents but was not present in the Eyjafjordur vent. The cones
were composed mainly of clay minerals that are rare in most seafloor hydrothermal
chimneys and have never before been found as the dominant mineral phase. However,
these minerals are commonly encountered as precipitates in geothermal wells in Iceland.
The chemical and microbial analyses indicated that the fluid originates from the
surrounding terrestrial groundwater that has been filtrated through the crust below the
Eyjafjordur fjord. The apparent 14C age of the vent fluid is 11,000 years, similar to that
of onshore geothermal water in the region. The stable isotopic ratios (δ D and δ 18O)
suggest that the water has recharge, from the high inland mountains located about 100 km
south of Eyjafjordur.
The origin of the microbial community in the submarine hot springs is at present
unclear. Nevertheless, it is likely that the thermophilic marine species have disseminated
in the sea to the vent fields. Spore-forming terrestrial thermophiles could also have
disseminated by air or runoff water from land to the vent field at the sea bottom.
However, it is more likely that terrestrial anaerobes, such as D. mobilis, have penetrated
though dikes and faults at least 1.8 km from land to the vent site. This hypothesis is
further supported by the apparently complete absence of halotolerant Thermus species in
this system.
Desulfurococcus species can be found in many hyperthermophilic environments,
ranging from underwater thermal vents (also known as "black smokers") to freshwater
hot springs in Iceland and the United States. Like most thermophiles, their exact
ecological role is not known for certain.
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Desulfurococcus mobilis
3. CHARACTERISTICS
3.1 Morphology
As in the name, Desulfurococcus mobilis is having cocci-shaped, cocci refers to its
spherical rounded structure. Its sizes are ranging from 0.5-0.15µm in diameter thus
enormous variation in cell size and volume appears to be a rather primitive feature and is
in line with the 16S rRNA phylogeny of the corresponding hyperthermophiles. Cell
volume varies by more than four orders of magnitude. The small sizes of D.mobilis
confer great advantages to pass narrow pores of soils and rocks in order to colonize hot
subterranean environments.
As comparing with other species in the family of Crenarchaeote, D.mobilis is
morphologically different and metabolically less versatile. The intracytoplasmic is
generally absent. Archeal lipids have ether linkages between glycerol and their
hydrophobic side chains as a site of energy biosynthesis. They are lack of fatty acids and
composed of repeating unit of the hydrocarbon molecule, isoprene. Archeal lipids for
D.mobilis form monolayer, consists of byphythanylglycerol tetraether (caldarchaeol). It is
halosensitive and oxygen sensitive, indicating a very deep origin.
3.2 Structural cell wall
Gram Negative Archae, D.mobilis lacks the outer membrane and complex
peptidoglycan network or sacculus of gram-negative bacteria. Instead they usually have a
surface layer of protein or glycoprotein subunits. The chemistry of archael cell walls is
quite different from that of bacteria. None have the muramic acid and D-amino acids
characteristic of bacterial peptidoglycan. Not surprisingly, all Archae resist attack by
lysozyme and B-lactam antibiotics such as penicillin. D.mobilis has a layer of protein or
glycoprotein outside their plasma membrane but since the layer is not thick enough as
Gram Positive Archae, D.mobilis will be decolorized by alcohol, just like Gram-Negative
bacteria did.
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D.mobilis cells are surrounded by an interesting protein subunit envelope. This
unusual filamentous structure is tetragonally arrayed in a lattice form and covers the
entire cell. The lattice is formed by cross-like subunits of proteins. Distortions in this
protein lattice make for relatively large gaps in the cell surface. When glycerol is added
to this envelope, the latticing partially dissolves.
3D image of latticed protein on Desulfurococcus' cell envelope.
Subunit proteins of latticed cell envelope.
Cell wall: Protein, glycoprotein, pseudomurein, wall-less, paracrystalline surface
layer. Protein synthesis: 70s ribosomes, insensitive to chloramphenicol and
cycloheximide, dipthamide present in elongation factor
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3.3 Genetic Material
An Archaebacteria generally have small circular chromosomes and plasmids to
contain its genetic material. Since an Archae is frequently unicellular, cell differentiation
is irregular. It contains multi genes with few operons and has none of nuclear membrane
thus simple compared to Eucarya. An intron has been found in the 23S ribosomal RNA
gene of Desulfurococcus mobilis, making it the first known instance of an intron within
the ribosomal RNA of a prokaryote. An intron is the segments of non coding DNA. RNA
polymerase, the remarkable enzyme responsible for gene transcription found in Archaea
is quite complex.
3.4 Effects of temperature
Temperature has a profound effect on microorganisms. Organisms exhibit distinct
key temperatures (minimal, maximal, and optimal growth temperatures).
Desulfurococcus mobilis is a type of hyperthermophiles, from the Greek word; hyper
(over), therme (heat) and philus (loving). Thus, the maximum temperature for
hyperthermophiles is 95ºC whilst its minimum temperature is 70°C. But, for the optimum
growth, it ranges around 85-88°C. The temperature optimum of the enzyme was
determined in an experiment by treating the pre-23S rRNA of Desulfurococcus mobilis
over the range 20−80°C; maximum cleavage occurred at 65−70°C. A most important
factor influencing the effect of temperature of growth is the temperature sensitivity of
enzyme-catalyzed reactions. As the temperature ranging from 85-88°C, metabolism as a
whole is mostly active, and D.mobilis grows faster due to increasing the rates of its
enzyme reactions. Further increase exceeding the optimum range will result in enzyme
and transport carrier denaturation. The lipid monolayer simply melts and disintegrates.
Temperature is particularly stressful because microorganisms are isothermal.
D.mobilis has few adaptations in its structure in order to live at high temperature. Their
cell inventory is adapted so that they can resist to heat or cold denaturation. Few
questions were raised upon their suitability living in high temperatures;
1. How can protein adapt and function to such different conditions and still be
made of 20 amino acids?
2. How they can maintain their structural characteristics (integrity, conformational
flexibility) in such a wide range of temperatures?
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First and foremost, reverse DNA gyrase and positive coils archael histones, Sac7d
increase the DNA stability. Lipid containing a monolayer (byphythanylglycerol
tetraether), the hydrocarbon chain linked structure confer the lipid stability. The solidity
of monomers contributed when ATP and NAD+ have half-life <30 min at 120ºC. For
protein stability, D.mobilis’ thermostable protein structures are considered as follows;
1. Small-solvent exposed-surface area.
2. Increased packing-density (reduced cavities)
3. Decreased length of surface loops.
4. Optimization of hydrophobic interaction.
5. Extended ion pair networks.
6. Hydrogen bond between charge-neutral residues
3.5 The need for motility
D.mobilis is monotrichious(mono,one and trichinous,hair) motile, possessing a
single mono polar bundle of flagella. It is called polar since it is located at an end of the
Archaea cell. Flagella consists of filament (extend from the cell surface to the tip), basal
body (embedded in the cell) and the hook (short and curved segment).
Prokaryotic flagella operate differently from eukaryotic flagella. The filament is
in the shape of a rigid helix, and the bacterium moves when this helix rotates.
Considerable evidence shows that flagella act just like propellers on a boat. The direction
of flagellar rotation determines the nature of bacterial movement. Monotrichious
flagellum on D.mobilis rotates counterclockwise (when viewed from outside the cell)
during normal forward movement, whereas the D.mobilis itself rotates slowly clockwise.
The rotating helical flagellar filament pushes the cell forward in a run with the flagellum
trailing behind. Monotrichious bacteria stop and tumble randomly by reversing the
direction of flagellar rotation.
This single flagellum, the long filamentous appendages is an effective swimming
device that gives advantage for D.mobilis to propel towards a favorable environment or
away from the adverse one. D.mobilis, a kind of hyperthermophiles do not always swim
aimlessly, but moving as response of attraction to temperature. The movement is called
thermotaxis. In the case of geothermal Archaea, motility is needed when the temperature
nearly increase to maximum or decrease to minimum. Without a flagellum as an asset of
motility in this harsh condition, D.mobilis cell membrane will be disrupted and protein
will be disintegrated due to enzyme denaturation.
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3.6 Dependence of pH
pH is a measure of the hydrogen ion activity of a solution and is defined as the
negative logarithm of the hydrogen ion concentration.
pH = - log [H+]
. A neutrophiles will loves the pH ranging from 0.5-5.5 whilst an acidophiles loves the
pH ranging from 5.5 -8.0. D.mobilis can either be neutrophilic or acidophilic
microorganisms since its optimum temperature is 5.5. Each enzymes in D.mobilis
functions most rapidly at this specific optimum pH. In case, when the pH deviates too
greatly from an enzyme’s optimum, activity slows and the enzyme may be damaged.
Microorganisms can usually adjust to changes in environmental pH by maintaining an
internal pH that is near neutrality. To maintain this, few mechanisms has been proposed;
plasma membrane impermeability of protons and internal buffering. Peptides and amino
acids in complex media have a strong buffering effect for pH homeostasis, the
maintaining of pH.
4. NUTRITION:
Nutrition is defined as the process by which organisms obtain energy in the form
of food for growth, maintenance and repair. D.mobilis is a type of heterotrophs which use
reduced, preformed organic molecules usually from other organisms as carbon sources.
This chemoorganotrophic heterotrophs need source of energy and electrons for its cell
nourishment. Thus, D.mobilis obtains energy from the oxidation of inorganic or inorganic
compounds (chemotroph) and use reduced organic compounds as its electron sources
(organotroph). Since this species is named Desulfurococcus mobilis, it is somehow
related to sulfur. Sulfur is needed for the synthesis of certain amino acids and other
molecules, and D.mobilis meets their sulfur needs by assimilatory sulfate reduction.
Organic compound + S°  H2S + CO2
It is noted here that the electron donor is the organic compound whilst the electron
acceptor is Sº metabolizers, D.mobilis. Sº is reduced to H2S anaerobically. They grow
organotrophically by respiration or fermentation with sulfur as the electron acceptor.
Besides that, three other organic compounds are required by each and every
microorganism which are; amino acids (protein synthesis), purines and pyrimdines
(nucleic acid synthesis), and vitamins (enzyme cofactors). 11 amino acids were found to
be necessary for metabolism during cultivation.
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Desulfurococcus mobilis
In cultivation of Dessulfuroccoccus in laboratory for research, the above details
have to considered. At last, a culture medium is prepared to grow, transport, and store
microorganisms. Below are contents of a complex medium for cultivation of D.mobilis in
the lab:
Add:
Yeast extract (Difco)
Resazurin
Sulfur, powdered
Na2S x 9 H2O
Amount (per liter)
1.0 g
1.0 mg
5.0 g
0.5 g
pH is adjusted to be 5.5 with 10 N H2SO4. Sterilize separately sodium sulfide
(autoclave stock solution under N2) and sulfur (steaming for three hours each of three
successive days). Prepare the medium anaerobically (100% nitrogen).
Growth was not significant in any of the enrichments, except in medium prepared
with reservoir water, complex nutrients, and elemental sulfur as above. As an
organotroph, D.mobilis utilize complex substrates such as yeast extract, casein and
tryptone as its carbon sources.
5. REPRODUCTION
6. IMPORTANCE
Crenarchaeotes may constitute a significant fraction of the oceanic picoplankton. It
contains none of pathogens.
7. CONCLUSION
1. A number of thermophiles and hyperthermophiles within the domains of Bacteria and
Archaea have been isolated from continental and deep-sea oil reservoirs and from other
deep wells where the temperature does not exceed 113°C . In volcanic areas, however,
heat from the mantle is transported to the surface by conduction (heat flow), by volcanic
eruptions, and by water in geothermal systems. Such energy has been harnessed as
geothermal energy and is used in Iceland for such applications as heating and electricity
production. The temperature of the source water is generally 50 to 130°C in wells that are
typically drilled to a depth of 1,500 to 2,000 m. The bottoms of the boreholes could
therefore be regarded as subterranean hot springs, the boreholes could be regarded as
windows into the springs, and pipelines could be regarded as extensions, allowing for
access to the subterranean springs. These environments have been minimally
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investigated, while terrestrial surface hot springs are one of the most extensively studied
natural environments for thermophiles.
2. Two strains belonging to Desulfurococcus mobilis and to the Thermus/Deinococcus
group were isolated from borehole RG-39. The results indicate that subsurface volcanic
zones are an environment that provides a rich subsurface for novel thermophiles.
3.Here, we report the diversity of microbes associated within the geothermal fluid of
boreholes that reach about 2,000 m below the surface, with reservoir temperatures being
below 100°C. Culture-dependent and culture-independent molecular phylogenetic
surveys showed that some of the community members are shared with terrestrial thermal
springs but that some of the diversity appears to be unique to this subsurface biosphere.
We present here the existence of an indigenous thermophilic subterranean community in
the subsurface in Iceland, and we suggest that non endemic thermophiles may be
disseminated in the subsurface.
1.We have shown that two deep subsurface geothermal fields in Iceland with
temperatures below 100°C harbor a diverse community of unknown thermophiles. In situ
whole-cell hybridization with fluorescently labeled 16S rRNA-based oligonucleotide
probes indicated that the archaeal and bacterial cells were still intact. From enrichment
cultures it was confirmed that cells were thermophilic, growing at high temperatures.
Direct 16S rDNA analyses showed that all sequences could be affiliated with
thermophilic divisions and that almost all OTUs were new and have not yet been
obtained in pure cultures.
The maximum temperature measured in the system was about 72°C, but the
reservoir temperature is unknown. However, the silica geothermometer indicated
reservoir temperatures of about 80°C. Moreover, the highest growth temperature of the
isolates was approximately 92°C; this also indicates a higher reservoir temperature than
72°C. The presence of apparently purely terrestrial thermophiles in the submarine hot
hydrothermal fluid confirms the freshwater origin of the fluid and that the seawater does
not penetrate the principal geothermal channels below the seafloor. However, small
chimneys located on the giant cones have porous structures and are made of two
mineralogical zones. Ambient aerobic seawater could possibly penetrate these zones; mix
with the hot fluid, and form small niches for thermophilic microorganisms that are
influenced by different environmental factors. The origin and the characterization of the
strains isolated from small chimneys suggest habitats of various temperatures, pHs, and
salinities. The most halotolerant strains were isolated from the outer zone of the chimney.
They grew aerobically at pH 9.0 and in the presence of 4% (wt/vol) NaCl, but not at pH
10.0 or at 76°C. This suggests that cold seawater mixes into the vent fluid in the outer
zone with increasing salinity and decreasing temperature. Strains isolated from the inner
part had broader growth ranges. None of them grew at 4% NaCl, many grew at 3% NaCl
(77%) and at 76°C (70%), but only a few grew at pH 10.0 (27%). This indicates
decreasing seawater mixing in the inner part of the chimney. All 12 aerobic strains
isolated from concentrated fluid grew at pH 10.0, but only 1 grew above 2.0% NaCl.
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