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DESULFORUDIS AUDAXVIATOR

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CONTENTS

 * 1 Classification
 * 2 Description and Significance
 * 3 Genome Structure
   * 3.1 Horizontal Gene Transfer
   * 3.2 Related Bacterial Species
 * 4 Metabolism
   * 4.1 Energy
   * 4.2 Nitrogen
   * 4.3 Carbon
 * 5 Ecology
 * 6 References


CLASSIFICATION


D. audaxviator

D. audaxviator

Kingdom: Bacteria

Phylum: Firmicutes

Class: Clostridia

Order: Clostridiaceae

Family: Peptococcaceae

Genus: Candidatus Desulforudis

Species: Candidatus Desulforudis audaxviator


The name “audaxviator” was inspired by a quote from the book Journey to the
Center of the Earth, by Jules Verne. The Latin phrase “audax viator” translates
to “bold traveler”, a reference to the bacterium’s subterranean habitat.[4][8]


DESCRIPTION AND SIGNIFICANCE

D. audaxviator was first discovered by NAI’s Indiana-Princeton-Tennessee
Astrobiology Initiative’s (IPTAI) team members in 2005/2006.[8] It was found in
a South African gold mine, at a depth that was previously thought to be
uninhabited by life.[3] D. audaxviator was found at level 104, 2.8km deep in the
Mponeng gold mine.[4] The groundwater in which they were found exceeded 60° C,
had a pH of 9.3, was anoxic, and was severely nutrient deficient.[4][7]

D. audaxviator is a rod shaped Gram-positive bacterium. They are motile,
sporulating, sulfate reducing chemoautotrophs, and are categorized as
thermophiles and obligate anaerobes.[4][6] These bacteria are the only known
resident of their environment, and therefore live independently of other
organisms.[4][8] This discovery revealed that it is possible for a single genome
to encode all proteins necessary for the biological portion of a basic
ecosystem.[4][8] D. audaxviator comprises the only known ecosystem relies on
radioactivity instead of sunlight, or chemical energy from the earth (see
section on metabolism).[7][8]

D. audaxviator and other deep subterranean microorganisms have been heavily
researched for several reasons. D. audaxviator’s simplistic environment coupled
with its complete life-supporting genome makes it a good representation of an
organism that could be capable of life on an alien planet.[7][8] This bacterium
performs all of the processes needed to sustain life if given a source of
radioactivity, sulfate, water, ammonia and carbon dioxide, all of which are
thought to be present on rocky planets such as Mars.[8] D. audaxviator could
also be a model of prehistoric life on Earth, providing valuable insight to the
origins of life. [3][7]


GENOME STRUCTURE

D. audaxviator’s genome is 2.35 megabase pairs (Mbp) in size, containing 2157
protein encoding genes.[4] This is larger than the expected <2000 genes found in
other free-living organisms.[4]

Other details of the genome[4]:

- 86.8% of the genome codes for protein

- The G+C content of the genome is 60.9%

- The average length of an open reading frame is 910 base pairs


HORIZONTAL GENE TRANSFER

Certain genes in the D. audaxviator genome are considered to be products of
archaeal horizontal gene transfer.[4] Some of these genes and gene products
include an extra copy of an archaeal-type sulfate adenylyltransferase (to
convert ATP and sulfate to diphosphate and adenylyl sulfate), a H⁺ translocating
pyrophosphatase (to convert pyrophosphate into two phosphate ions), a
nitrogenase (to fix N₂ gas), CRISPR-associated genes (useful in viral defense),
a 2nd carbon monoxide dehydrogenase system (to convert CO into CO₂), and genes
for formation of gas vesicles.[4]

The archaeal genes present in the D. audaxviator genome could imply that it or
an ancestor was once in contact with an archaeal species.[10] For example, D.
audaxviator possesses a carbon dioxide fixing pathway that is similar to that of
archaea.[3] As evidence that this could have happened, Takai et al. (2001)
researched life in other mine shafts, revealing that archaea can live at extreme
depths. Samples were taken from the East and West Driefontein gold mines, the
Kloof mines, and the Beatrix Mine. All three of these mines are located in the
Witswatersrand basin, the same basin as the Mponeng gold mine. Archaeal species
were found in fissure water 0.87km to 3.08km deep. These species had high C+G
content (57-60%) like D. audaxviator, although most were determined to be
methanogens.[9]


RELATED BACTERIAL SPECIES

Similar bacteria have been found in other environments and locations. Alawi et
al. (2011) found a related bacterium (94% genome similarity) in wells 3.5km deep
under a geothermal plant in South Germany.[1] A different study by Balci et al.
(2012) revealed another D. audaxviator-like bacterium (94% genome similarity) to
be living in Lake Maslak sediments in Istanbul, Turkey. This bacterium, although
not living at a great depth, is also a sulfate reducing bacterium.[2] Another
bacterium (91% genome similarity) was detected by Itävaara et al. (2011), it was
found between 1.2 and 1.5km deep in the Outokumpu borehole, drilled in Eastern
Finland.[5]





METABOLISM


The splitting of water molecules via radiation, resulting in SO₄²⁺ availability
for D. audaxviator.

D. Audaxviator is a sulfate reducing chemoautotroph, capable of fixing nitrogen
and carbon.[4] It is capable of forming endospores when the environmental
conditions become too unfavorable.[6] D. audaxviator has the cellular machinery
to sense nutrients and is flagellated, allowing the bacteria to perform
chemotaxis.[4] The cell also has every pathway needed to synthesize all amino
acids.[4]


ENERGY

It is thought that a long-term seclusion from O₂ has lead to the complete loss
of a system for oxygen tolerance, making D. audaxviator an obligate
anaerobe.[4][8] Since the bacterium cannot utilize O₂ as its electron acceptor,
it uses sulfate (SO₄²¯), an energetically favorable alternative to O₂.[4]
However, it is generally sulfide (H₂S) that is naturally available from the
surrounding rock. Radioactivity generated by the decay of Uranium in the rocks
splits ambient water molecules into H⁺ and hydroxide (OH), which then forms
hydrogen peroxide (H₂O₂). The hydrogen peroxide reacts with the environmental
sulfide to form sulfate. The bacteria can then use the sulfate for dissimulatory
sulfate reduction, returning the compound to sulfide. Electrons from the
hydrogen left over from the splitting of water are used by the cell to reduce
sulfate.[7][8]


NITROGEN

D. audaxviator has an ammonium (NH₄⁺) transporter that takes up ammonia (NH₃)
from the surrounding environment. Ammonia is thought to be at a high enough
concentration to provide a sufficient source of nitrogen.⁽⁴⁾ The cell has
glutamine synthetases to derive nitrogen from ammonia, but it also has a
temperature resistant nitrogenase.[4] The nitrogenase allows D. audaxviator to
use the more energetically expensive method of obtaining nitrogen from N₂, if
ammonia is not available.[4]


CARBON

D. audaxviator obtains carbon from many sources. The bacterium has transporters
for both sugars and amino acids, which can be used a carbon source if they are
available in the environment.[4] This may occur when another bacterium dies.⁽⁴⁾
D. audaxviator can also obtain carbon from inorganic sources. A putative
carbonate adenosine triphosphate (ATP) binding cassette transporter or a
putative bicarbonate/Na⁺ symporter is responsible for the uptake of CO₂.[4] A
carbon monoxide dehydrogenase is used in the CoA synthesis pathway to assimilate
this inorganic carbon. Formate and CO may also be used as a carbon source.[4]





ECOLOGY

D. audaxviator lives in an ecosystem by itself, so it does not naturally
interact with any other organisms.[4] The bacterium interacts with its
environment on its hunt to find nutrients. The bacterium performs chemotaxis
along chemical gradients, and takes up nutrients it needs. This may include
taking up nutrients recycled from dead cells.[4]





REFERENCES

(1) Alawi, M., Lerm, S., Vetter, A., Wolfgramm, M., Seibt, A., and Würdemann, H.
“Diversity of Sulfate-Reducing Bacteria in a Plant Using Deep Geothermal
Energy.” Grundwasser, 2011, DOI: 10.1007/s00767-011-0164-y

(2) Balci, N., Vardar, N., Yelboga, E., and Karaguler, N. “Bacterial Community
Composition of Sediments from Artificial Lake Maslak, Istanbul, Turkey.”
Environmental Monitoring and Assessment, 2012, DOI: 10.1007/s10661-011-2368-0

(3) Bonch-Osmolovskaya, E. “High-Temperature Deep-Subsurface Microbial
Communities as a Possible Equivalent of Ancient Ecosystems.” Paleontological
Journal, 2010, DOI: 10.1134/S0031030110070130

(4) Chivian,D., Brodie, E., Alm, E., Culley, D., Dehal, P., DeSantis,T.,
Gihring, T., Lapidus, A., Lin, L., Lowry, S., Moser, D., Richardson, P.,
Southam, G., Wanger, G., Pratt, L., Andersen, G., Hazen, T., Brockman, F.,
Arkin, A., and Onstott, T. “Environmental Genomics Reveals a Single-Species
Ecosystem Deep Within Earth.” Science, 2008, DOI: 10.1126/science.1155495

(5) Itävaara, M., Nyyssönen, M., Kapanen, A., Nousiainen, A., Ahonen, L., and
Kukkonen, I. “Characterization of Bacterial Diversity to a Depth of 1500m in the
Outokumpu Deep Borehole, Fennoscandian Shield.” FEMS Microbiology Ecology, 2011,
DOI: 10.1111/j.1574-6941.2011.01111.x

(6) Lin, L., Wang, P., Rumble, D., Lippmann-Pipke, J., Boice, E., Pratt, L.,
Sherwood Lollar, B., Brodie, E., Hazen, T., Andersen, G., DeSantis, T., Moser,
D., Kershaw, D., and Onstott, T. “Long-Term Sustainability of a High-Energy,
Low-Diversity Crustal Biome.” Science, 2006, DOI: 10.1126/science.1127376

(7) Mascarelli, A. “Geomicrobiology: Low Life.” Nature, 2009, DOI:
10.1038/459770a

(8) Scalice, D. “Life Without the Sun.” NASA Astrobiology, 2008, LINK:
https://astrobiology.nasa.gov/articles/2008/10/10/life-without-the-sun/

(9) Takai, K., Moser, D., DeFlaun, M., Onstott, T., and Fredrickson, J.
“Archaeal Diversity in Waters from Deep South African Gold Mines.” Applied and
Environmental Microbiology, 2001, DOI: 10.1128/AEM.67.21.5750-5760.2001

(10) Thomas, C., and Nielsen, K. “Mechanisms of, and Barriers to, Horizontal
Gene Transfer between Bacteria.” Nature Reviews Microbiology, 2005, DOI:
10.1038/nrmicro1234

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