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Learning objectives

  • Describe the unique features of deeply branching bacteria
  • Give examples of significant deeply branching bacteria

On a phylogenetic tree (see A Systematic Approach ), the trunk or root of the tree represents a common ancient evolutionary ancestor, often called the last universal common ancestor (LUCA) , and the branches are its evolutionary descendants. Scientists consider the deeply branching bacteria , such as the genus Acetothermus , to be the first of these non-LUCA forms of life produced by evolution some 3.5 billion years ago. When placed on the phylogenetic tree, they stem from the common root of life, deep and close to the LUCA root—hence the name “deeply branching” ( [link] ).

A diagram of a phylogenetic tree. At the base is the label LUCA This branches into two branches. The branch on the left is the bacteria, the branch to the right branches again to form the archaea and Eukarya. The lowest branch of the bacteria is the acetothermus (which is starred). Branches above that include (from bottom to top): aquifex, thermotoga, green filamentous bacteria, bacteroides, cytophaga, planctomyces, gram positives, cyanobacteria, porteiobacteria and spirochetes. Branches of the arcaea from bottom to top: pyrodicticum, thermoproteus, Thermococcus, methanococcus, methanobacterium, methanosarcina, and halophies. Branches of the Eukarya from bottom to top: diplomonads, microsporidia, trichomonads, flagellates, entamoebae, slime molds, ciliates, plants, animals, and fungi.
The star on this phylogenetic tree of life shows the position of the deeply branching bacteria Acetothermus . (credit: modification of work by Eric Gaba)

The deeply branching bacteria may provide clues regarding the structure and function of ancient and now extinct forms of life. We can hypothesize that ancient bacteria, like the deeply branching bacteria that still exist, were thermophiles or hyperthermophiles , meaning that they thrived at very high temperatures. Acetothermus paucivorans , a gram-negative anaerobic bacterium discovered in 1988 in sewage sludge, is a thermophile growing at an optimal temperature of 58 °C. G. Dietrich et al. “ Acetothermus paucivorans , gen. nov., sp. Nov., a Strictly Anaerobic, Thermophilic Bacterium From Sewage Sludge, Fermenting Hexoses to Acetate, CO 2 , and H 2 .” Systematic and Applied Microbiology 10 no. 2 (1988):174–179. Scientists have determined it to be the deepest branching bacterium, or the closest evolutionary relative of the LUCA ( [link] ).

The class Aquificae includes deeply branching bacteria that are adapted to the harshest conditions on our planet, resembling the conditions thought to dominate the earth when life first appeared. Bacteria from the genus Aquifex are hyperthermophiles, living in hot springs at a temperature higher than 90 °C. The species A. pyrophilus thrives near underwater volcanoes and thermal ocean vents, where the temperature of water (under high pressure) can reach 138 °C. Aquifex bacteria use inorganic substances as nutrients. For example, A. pyrophilus can reduce oxygen, and it is able to reduce nitrogen in anaerobic conditions. They also show a remarkable resistance to ultraviolet light and ionizing radiation. Taken together, these observations support the hypothesis that the ancient ancestors of deeply branching bacteria began evolving more than 3 billion years ago, when the earth was hot and lacked an atmosphere, exposing the bacteria to nonionizing and ionizing radiation.

The class Thermotogae is represented mostly by hyperthermophilic, as well as some mesophilic (preferring moderate temperatures), anaerobic gram-negative bacteria whose cells are wrapped in a peculiar sheath-like outer membrane called a toga . The thin layer of peptidoglycan in their cell wall has an unusual structure; it contains diaminopimelic acid and D-lysine. These bacteria are able to use a variety of organic substrates and produce molecular hydrogen, which can be used in industry. The class contains several genera, of which the best known is the genus Thermotoga . One species of this genus, T. maritima , lives near the thermal ocean vents and thrives in temperatures of 90 °C; another species, T. subterranea , lives in underground oil reservoirs.

Finally, the deeply branching bacterium Deinococcus radiodurans belongs to a genus whose name is derived from a Greek word meaning terrible berry . Nicknamed “ Conan the Bacterium ,” D. radiodurans is considered a polyextremophile because of its ability to survive under the many different kinds of extreme conditions—extreme heat, drought, vacuum, acidity, and radiation. It owes its name to its ability to withstand doses of ionizing radiation that kill all other known bacteria; this special ability is attributed to some unique mechanisms of DNA repair.

A micrograph of four round cells forming a square.
Deinococcus radiodurans , or “Conan the Bacterium,” survives in the harshest conditions on earth.

Key concepts and summary

  • Deeply branching bacteria are phylogenetically the most ancient forms of life, being the closest to the last universal common ancestor.
  • Deeply branching bacteria include many species that thrive in extreme environments that are thought to resemble conditions on earth billions of years ago
  • Deeply branching bacteria are important for our understanding of evolution; some of them are used in industry

Fill in the blank

The length of the branches of the evolutionary tree characterizes the evolutionary ________ between organisms.


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The deeply branching bacteria are thought to be the form of life closest to the last universal ________ ________.

common ancestor

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Many of the deeply branching bacteria are aquatic and hyperthermophilic, found near underwater volcanoes and thermal ocean ________.


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The deeply branching bacterium Deinococcus radiodurans is able to survive exposure to high doses of ________.

ionizing radiation

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Short answer

Briefly describe the significance of deeply branching bacteria for basic science and for industry.

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What is thought to account for the unique radiation resistance of D. radiodurans ?

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Questions & Answers

reactions if viruses to physical and chemical agents?
All viruses contain the following two components: 1) a nucleic acid genome and 2) a protein capsid that covers the genome. Together this is called the nucleocapsid. In addition, many animal viruses contain a 3) lipid envelope. The entire intact virus is called the virion.
assume you have performed a gram stain on a sample of pus from a patients urethra. you see red, nucleated cells and purple rods. what can you conclude
Petty Reply
Study of micro organisation which we cannot see with naked eye...
shyam Reply
Despite their bad reputation, microbes are mostly beneficial or have a neutral effect on our lives. Microbiology is the scientific study of these microorganisms. Microorganisms are those organisms that are too small to see with the naked eye and include things like bacteria, fungi, and viruses.
not 'things'...'organisms' like bacterias, viruses and fungi
simi, it's not bacterias but bacteria which is the plural form of the word while bacterium is the singular of it
bacterium is obvio in a singular form but plural of bacterium is bacterias
Richard is right it's bacterium which is the singular form and plural is bacteria. *Bacterias* doesn't exist.
what is microbiology?
Sandeep Reply
what is microbiology
is the study of small microorganisms that can't be viewed by our naked eyes under a microscope
it's the study of microbes their classification, diversity, their cellular structure and processes, their applications and interaction with other organisms.
what is EMP pathway
which organism is implicated in meningoencephalitis
Peter Reply
Herpes virus
which mold is called dairy mold?
Geotrichum candidum
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spore or virus
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yes it can be turned as a virus
which gram positive bacteria is responsible for food poisoning?
prion is not a living organism yet it is infectoois why
prions are virus like organism. and it is a misfolded protein that may cause disease to us.
Clostridium perfringens, B.cereus
Who is the father of the biology
I have one more question. . that who is the father of botany and medicine? ? can any one tell
define polar bond in simple way
Rashmi Reply
when there is formation of positive and negative poles i.e. one atom is +ve and other is -ve which further results in the dipole moment. Such bond is know as polar bond. It is a convalent bond only.
Hi everyone, is these site only about microbiology? I want a site which I can download articles on hematological and biochemical profile
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I think it is only about micro biology.
so whats happening
I am about to enter school and am going for MCB
Olugbenga Reply
What is DNA damage and repair
Sree Reply
removal of nucleotide bases from DNA is damage n addition or insertion of bases in DNA is repair.
replication is a term related to generation of new DNA from a parental DNA.
role of microorganism as pathogens
Parteek Reply
to live inside the host body, reproduction n destroy the immune system of host body.
what is replication
please give me ans
replication is a term related to generation of new DNA from parental DNA.
difference between endotoxin and exotoxin
Binkheir Reply
toxins released inside the cell or a body is endotoxin n toxins released outside the cell or a body is exotoxin.
Generally, exotoxins are produced by gram positive bacteria and endotoxins are the integral part of the cell wall of gram negative bacteria. But some gram negative bacteria can also produced exotoxins such as E.coli.
Yeah u r ryt aakarti exotoxins screted by gram postive bacteria nd secreted by cell surface and they are secondary metabolites ....On the other hand endotoxin released by gram negtive bacteria nd released inside the cell (lipopolysaccharide) ..
Heat stable lipopolysaccharides to be exact.
please I want the names of bacteria and the diseases they cause.
Ibrahim Reply
there are many we classify it according to their shapes gram negative or gram positive
so u may refer the book u guess or by when u want I can do a list for you and send u here
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I have the lost but it's too much to type ..
MSc entrance prepare books and questions LA etha base pani irukum..
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certain cancer or pneumonia 🤔
penicillium crysogenum
Kushal Reply
Practice MCQ 2

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Source:  OpenStax, Microbiology. OpenStax CNX. Nov 01, 2016 Download for free at http://cnx.org/content/col12087/1.4
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