Download biological diversity: bacteria and archaeans

BIOLOGICAL DIVERSITY: BACTERIA AND ARCHAEANS
Table of Contents
Monera: the Prokaryotic Kingdom | Bacterial Structure | Bacterial Reproduction
Classification of Bacteria | The Archea | The Fossil Record | Links | References
Monera: the Prokaryotic Kingdom
The taxonomic Kingdom Monera consists of the bacteria (meaning the true
bacteria and cyanobacteria, or photosynthetic bacteria). Organisms in this
group lack membrane-bound organelles associated with higher forms of life.
Such organisms are known as prokaryotes. Bacteria (technically the Eubacteria)
and blue-green bacteria (the blue-green algae when I was a student), or
cyanobacteria are the major forms of life in this kingdom. The most primitive
group, the archaebacteria, are today restricted to marginal habitats such as hot
springs or areas of low oxygen concentration. Their small size, ability to
rapidly reproduce (E. coli can reproduce by binary fission every 15 minutes),
and diverse habitats/modes of existence make monerans the most abundant and
diversified kingdom on Earth. Bacteria occur in almost every environment on
Earth, from the bottom of the ocean floor, deep inside solid rock, to the cooling
jackets of nuclear reactors. Possible bacteria-like structures have even been
recovered from 3 billion year old Martian meteorites. If these turn out to be
fossils, then the bacterial form of life would have existed simultaneously on
both Earth and Mars. However, the cellular nature of those structures has not
been conclusively established.
Two cyanobacteria, Oscillatoria (left) and Nostoc (right). The above left image is
cropped from
gopher://wiscinfo.wisc.edu:2070/I9/.image/.bot/.130/Cyanobacteria/Oscillatoria_130.
The above image right is cropped from
gopher://wiscinfo.wisc.edu:2070/I9/.image/.bot/.130/Cyanobacteria/Nostoc_130.
Bacterial Structure | Back to Top
Bacteria lack a nuclear membrane and membrane-bound organelles.
Biochemical processes that normally occur in a choloroplast or mitochondrion
of eukaryotes will take place in the cytoplasm of prokaryotes. Bacterial DNA is
circular and arrayed in a region of the cell known as the nucleoid. Scattered
within bacterial cytoplasm are numerous small loops of DNA known as
plasmids. Bacterial genes are organized in by gene systems known as operons.
Note the nucleoid region (n) where DNA is located as well as the electron
dense areas of the cytoplasm (dark areas) on these two cells of Neisseria
gonorrhoeae. This image is from: http://129.109.136.65/microbook/ch002.htm
Structure of a "typical" bacterium. Image from Purves et al., Life: The Science of
Biology, 4th Edition, by Sinauer Associates (http://www.sinauer.com/) and WH Freeman
(http://www.whfreeman.com/), used with permission.
Plasmids, small DNA fragments, are known from almost all bacterial cells.
Plasmids carry between 2 and 30 genes. Some seem to have the ability to move
in and out of the bacterial chromosome.
Structure of a bacterium highlighting the bacterial plasmid. The above is from
http://www.biosci.uga.edu/almanac/bio_103/notes/may_30.html.
The operon model of prokaryotic gene regulation was proposed by Fancois
Jacob and Jacques Monod. Groups of genes coding for related proteins are
arranged in units known as operons. An operon consists of an operator,
promoter, regulator, and structural genes. The regulator gene codes for a
repressor protein that binds to the operator, obstructing the promoter (thus,
transcription) of the structural genes. The regulator does not have to be adjacent
to other genes in the operon. If the repressor protein is removed, transcription
may occur.
Structure of a typical operon. Image from Purves et al., Life: The Science of Biology,
4th Edition, by Sinauer Associates (http://www.sinauer.com/) and WH Freeman
(http://www.whfreeman.com/), used with permission.
Operons are either inducible or repressible according to the control mechanism.
Seventy-five different operons controlling 250 structural genes have been
identified for E. coli.
Bacteria have flagella with a different microtubule structure than the flagella of
eukaryotes. Cell walls of bacteria contain peptidoglycan instead of the cellulose
found in cell walls of plants and some algae. Ribosomes are the structures in
cells where proteins are assembled. Bacterial ribosomes have different sized
ribosomal subunits than do eukaryotes.
Transmission electron micrograph of several bacterial flagella. This image is
from: http://www.slic2.wsu.edu:82/hurlbert/micro101/pages/Chap3.html
Bacteria typically have one of three shapes: rods (bacilli), spheres (cocci) or
spiral (spirilla). Unicellular, they often stick together forming clumps or
filaments.
Rod-Shaped Bacterium, hemorrhagic E. coli, strain 0157:H7 (division) (SEM
x22,810). This image is copyright Dennis Kunkel at http://www.denniskunkel.com/, used
with permission.
Scanning electron m icrographs illustrating external features of the rod-shaped
bacterium E. coli. The top image above is from:
http://www.uq.oz.au/nanoworld/t4bphage.jpg. The lower image above is from:
http://www.slic2.wsu.edu:82/hurlbert/micro101/pages/Chap2.html#two_bact_groups
Coccoid-shaped Bacterium(causes skin infections), Enterococcus faecium
(SEM x33,370). This image is copyright Dennis Kunkel at
http://www.denniskunkel.com/, used with permission.
Left, a cross-section of a cell illustrating the location of a flagella inside the
cell; Center, Borrelia burgdorferi, the organism that causes Lyme disease; and
Right, Treponema pallidum, the spirochete that causes the venereal disease
syphilis. The image above is from
http://www.bact.wisc.edu/Bact303/MajorGroupsOfProkaryotes.
Shapes and grouping forms of various bacteria. This image is from:
http://129.109.136.65/microbook/ch002.htm
Some bacteria are photosynthetic autotrophs, others are heterotrophs.
Bacterial Reproduction | Back to Top
Prokaryotes are much simpler in their organization than are eukaryotes. There
are a great many more organelles in eukaryotes, also more chromosomes. The
usual method of prokaryote cell division is binary fission. The prokaryotic
chromosome is a single DNA molecule that first replicates, then attaches each
copy to a different part of the cell membrane. When the cell begins to pull
apart, the replicate and original chromosomes are separated. Following cell
splitting (cytokinesis), there are then two cells of identical genetic composition
(except for the rare chance of a spontaneous mutation).
Animated GIF of binary fission. Image from:
http://www.slic2.wsu.edu:82/hurlbert/micro101/pages/Chap2.html#two_bact_groups
Rod-Shaped Bacterium, E. coli, dividing by binary fission (TEM x92,750). This
image is copyright Dennis Kunkel at http://www.denniskunkel.com/, used with
permission.
The prokaryote chromosome is much easier to manipulate than the eukaryotic
one. We thus know much more about the location of genes and their control in
prokaryotes.
One consequence of this asexual method of reproduction is that all organisms
in a colony are genetic equals. When treating a bacterial disease, a drug that
kills one bacteria of a specific type will kill all other members of that clone
(colony) it comes in contact with.
Certain types of bacteria can "donate" a piece of the their DNA to a recipient
cell. The recombination is the bacterial equivalent of sexual reproduction in
eukaryotes. Note that the entire DNA is not usually transferred, only a small
piece.
Donation of DNA. This image is from:
http://www.slic2.wsu.edu:82/hurlbert/micro101/pages/Chap9.html
Diagram of bacterial conjugation. Images from Purves et al., Life: The Science of
Biology, 4th Edition, by Sinauer Associates (http://www.sinauer.com/) and WH Freeman
(http://www.whfreeman.com/), used with permission.
E. coli strains undergoing conjugation (TEM x27,700). This image is copyright
Dennis Kunkel at http://www.denniskunkel.com/, used with permission.
Endospores are a method os survival, not one of reproduction. Certain bacteria
will form a spore within their cell membrane (an endospore) that allows them
to wait out deteriorating environmental conditions. Certain disease causing
bacteria (such as the one that causes the disease Anthrax) can be virulent
(capable of causing an infection) 1300 years after forming their endospore!
Electron micrographs illustrating formation of an endospore. Note, the sequence
illustrated here goes from left to right. The above image is from:
http://www.bact.wisc.edu/Bact303/MajorGroupsOfProkaryotes.
Classification of Bacteria |
Eubacteria (eu = true) are the majority of bacteria and are subdivided by their
method of energy acquisition into chemosynthetic, photosynthetic, and
heterotrophic.
Chemosynthetic Bacteria
Chemosynthetic bacteria are autotrophic, and obtain energy from the oxidation
of inorganic compounds such as ammonia, nitrite (to nitrate), or sulfur (to
sulfate).
Photosynthetic Bacteria
Photosynthetic bacteria carry out conversion of sunlight energy into
carbohydrate energy. Cyanobacteria are the major group of photosynthetic
bacteria. Some early cyanobacteria may have formed the oxygen released into
the early atmosphere. In addition to chlorophyll a, cyanobacteria also have the
blue pigment phycocyanin and the red pigment phycoerythrin.
Filamentous Cyanobacterium, Anabaena sp. (SEM x5,000). This image is
copyright Dennis Kunkel at http://www.denniskunkel.com/, used with permission.
Heterotrophic Bacteria
Members of this large and diverse group must derive their energy from another
organism by feeding. Two main types: saprophytic and symbiotic. Saprophytes
feed on dead or decaying material and are important nutrient recyclers.
Symbiotic bacteria live within a host multicellular organism and contribute to
the health of the host. Examples include cows and other grazing animals: the
bacteria convert cellulose from plant leaves and stems eaten by the animal into
glucose for digestion by the animal. Normally cellulose is nondigestible.
Possible symbiosis of bacteria within early eukaryotic cells was a major step in
the evolution of eukaryotic cells. In 1980, Lynn Margulis proposed the theory
of endosymbiosis to explain the origin of mitochondria and chloroplasts from
permanent resident prokaryotes. According to this idea, a larger prokaryote (or
perhaps early eukaryote) engulfed or surrounded a smaller prokaryote some 1.5
billion to 700 million years ago.
Steps in endosymbiosis. Image from Purves et al., Life: The Science of Biology, 4th
Edition, by Sinauer Associates (http://www.sinauer.com/) and WH Freeman
(http://www.whfreeman.com/), used with permission.
Instead of digesting the smaller organisms the large one and the smaller one
entered into a type of symbiosis known as mutualism, where both organisms
benefit and neither is harmed. The larger organism gained excess ATP provided
by the "protomitochondrion" and excess sugar provided by the
"protochloroplast", while providing a stable environment and the raw materials
the endosymbionts required. This is so strong that now eukaryotic cells cannot
survive without mitochondria (likewise photosynthetic eukaryotes cannot
survive without chloroplasts), and the endosymbionts cannot survive outside
their hosts. Nearly all eukaryotes have mitochondria. Mitochondrial division is
remarkably similar to the prokaryotic methods that will be studied later in this
course. A summary of the theory is available by clicking here.
Many heterotrophic bacteria also cause diseases such as strep throat, rheumatic
fever, cholera, gonorrhea, syphilis, and toxic shock syndrome. Bacteria can
cause disease by destroying cells, releasing toxins, contaminating food, or by
the reaction of the body to the infecting bacteria. Bacterial infections can be
controlled by vaccinations and antibiotic treatments. Antibiotics interfere with
some aspect of the replication of bacteria, and are produced by microorganisms
such as fungi, that compete with bacteria for resources. Penicillin, the first
antibiotic discovered, inhibits the synthesis of new cell walls in certain types of
bacteria. However, the overuse of antibiotics during the past fifty years has led
to natural selection favoring antibiotic resistance. There are reportedly more
than 50 strains of antibiotic resistant bacteria, necessitating the development of
new antibiotics and the frequent change of antibiotics in treatment.
The Archea | Back to Top
Archaebacteria (now more commonly referred to as the archaea) are considered
the oldest and most primitive organisms known. They have significant
differences in their cell walls and biochemistry when compared to bacteria.
Many scientists propose placing archeans into a separate kingdom, the Archaea.
Under the three domain model, they are the taxonomic equivalents of the other
bacteria and the eukaryotes. The archeans are life's extremists, occupying
environments that "normal" organisms find too harsh.
Three types of archaebacteria: methanogens, halophiles, and thermacidophiles.
They live in extreme habitats.
Sulfolobus acidocaldarius , an
extreme thermophile occurs in
geothermally-heated acid springs,
mud pots and surface soils; it can
withstand temperatures from 60 to
95 degrees C, and a pH of 1 to 5.
Left: Electron micrograph of a thin
section (X85,000); Right:
Fluorescent photomicrograph of
cells attached to a sulfur crystal.
The image above is from http://www.bact.wisc.edu/Bact303/MajorGroupsOfProkaryotes.
The Fossil Record | Back to Top
Fossil evidence supports the origins of life on earth earlier than 3.5 billion years
ago. Specimens from the North Pole region of Western Australia of of such
diversity and apparent complexity that even more primitive cells must have
existed earlier. Rocks of the Ishua Super Group in Greenland come possibly the
fossil remains of the earliest cells, 3.8 billion years old. The oldest known rocks
on earth are 3.96 Ga and are from Arctic Canada. Thus, life appears to have
begun soon after the cooling of the earth and formation of the atmosphere and
oceans.
These ancient fossils occur in marine rocks, such as limestones and sandstones,
that formed in ancient oceans. The organisms living today that are most similar
to ancient life forms are the archaebacteria. This group is today restricted to
marginal environments. Recent discoveries of bacteria at mid-ocean ridges add
yet another possible origin for life: at these mid-ocean ridges where heat and
molten rock rise to the earth's surface.
Many of the ancient phototrophs and heterotrophic bacteria lived in colonial
associations known as stromatolites. Cyanobacteria are on the outer surface,
with other photosynjthetic bacteria (anoxic) below them. Below these
phottrophs are layers of heterotrophic bacteria. The layers in the stromatolites
are alternating biogenic and sedimentologic in origin.
Image of Sharks Bay, Australia stromatolites, a cross section of one of these
structures, and a closeup of the cyanobacteria that make up the bulk of the
feature. Image from http://www.dme.wa.gov.au/ancientfossils/sharkbay2.jpg.