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Halobacterium and the
Archaebacteria
Above: a 3D computer model of Halobacterium. (Click image to enlarge). As we shall see, Halobacterium is a very unusual organism which thrives in very harsh conditions and is able to endure tremendous hardship.
Halobacterium
is a member of the Archaea or Archaebacteria. This intriguing group of
organisms do many things very differently to other cellular
lifeforms. As we shall see, certain features of their biochemistry
and molecular structure are very different from those of other
bacteria.
Exotic
lifeforms: Archaea or Archaebacteria?
Archaebacteria
were once thought to be exotic and ancient relics of the earliest
cells, predating bacteria. The truth is more subtle. To understand
what follows the reader should be aware of the key distinctions
between prokaryotes, such as bacteria, and eukaryotes, such as
animals, algae, plants, protozoa and fungi. Recall that a prokaryote
has a nucleoid rather than a nucleus to house its DNA. Bacteria and
archaebacteria are prokaryotes. Animals, plants, fungi, protozoa and
most algae are eukaryotes, possessing a nucleus bounded by a lipid
membrane.
Modern
classification systems, as given in textbooks, typically have
three principal domains of life: bacteria, archaea and
eukaryotes. A domain is the largest grouping, larger than a
kingdom, such as the animal kingdom (Animalia) and organisms
within each domain are made of a certain general type of cell,
so we have 3 principal cell types: bacterial, archaeal (both
prokaryotic) and eukaryotic. Furthermore, phylogenetic trees
(evolutionary trees) often have archaea more closely related to
eukaryotes than bacteria. I have never been convinced by this
and recently this triple scheme has been called into question.
Although
they are prokaryotes, archaeans are no longer classified as bacteria
by most scientists on the basis that they are 'genetically very
distinct' and so appear distantly related, or so it was claimed.
Hence the name change from 'archaebacteria' to 'archaea'. It was
also thought, based on incomplete molecular data, that archaea were
more closely related to eukaryotes than to bacteria and an
evolutionary scheme was devised in which bacteria gave rise to a
form that later split into archaea and eukaryotes. However, although
archaebacteria do share certain core features with eukaryotes
(including features of DNA and RNA packaging and processing) this is
only the minority of their genetic makeup. Most of their genes,
based on those that have been fully sequenced, are more closely
related to those of bacteria. The same is also true for eukaryotes,
which also share more genetic similarity to bacteria than to
archaebacteria (typically about 50 to 60% of eukaryote genes are
derived from bacteria, https://doi.org/10.1093/gbe/evaa047).
On this basis, archaea are more closely related to bacteria.
There
are additional problems with constructing phylogenetic trees based
on genetic similarity. The first problem: using different
genes can give different results. For example, genes involved in
genetic expression such as that which encodes the ribosomal RNA
(rRNA) will place archaea closer to eukaryotes, whereas a
gene for a metabolic pathway involved in nutrition may well place
archaea closer to bacteria. The rRNA genes have been traditionally
used to classify bacteria as all bacteria have them and they tend to
differ less than many other genes dues to their essential and highly
evolved functions. The rRNA forms part of the ribosomes,
machines that manufacture proteins within the cell. Ribosomes are
almost certainly one of the first parts of the cell to have evolved
and are the most essential component along with the genetic material
itself.
Many such trees used by instructors are therefore misleading. A typical phylogenetic tree may have a common ancestor as the stem and bacteria on one main branch and the other main branch forking again into 'archaea' and eukaryotes, implying the closer relationship between the latter two groups. Such a typical tree is illustrated below:
This tree implies that archaea are more closely related to eukaryotes by placing more emphasis on genes involved with DNA organization and gene expression than to the actual number of genes in common. This may seem reasonable, considering the importance of 'DNA managing' genes as opposed to 'housekeeping' genes but it creates an evolutionary picture that is misleading.
However,
it is also not satisfactory to group the domains according to the
fraction of genes shared in common. Parasitic eukaryotes tend to
lose genes as part of parasitic degeneration, due to their
dependence on host metabolism and nutrition. The parasitic fungus Encephalitozoon
intestinalis, which can cause serious disease in humans with
weakened immune systems, has lost many of its ancestral housekeeping
genes of bacterial origin and now has the smallest known number of
genes of any eukaryote. Furthermore, some 86% of its genes are of
archaea origin, and only 14% derived from bacteria. There is clearly
no satisfactory answer, and it is better to place the archaea and
bacteria on a more equal footing and regard eukaryotes as derived
from both.
Furthermore, horizontal gene transfer (HGT) has been demonstrated between archaea and bacteria (https://doi.org/10.7717/peerj.3865). This involves living cells of different species swapping genes and is horizontal as it occurs within a generation, whereas classical inheritance occurs from parents to offspring. HGT is common within prokaryotes, making the term 'species' as defined in animals void for prokaryotes. The picture that emerges is one in which eukaryotes are a chimera of bacteria and archaea whilst archaea and bacteria exchange genetic information with one-another. There is no linear tree connecting these three groups reliably. Rather than a phylogenetic tree, we require a phylogenetic network. As an example, one proposed model imagines an early bacterial cell absorbing an archaebacterial cell which largely became the nucleus. This accounts for the closer relationship between archaebacterial and eukayrote gene processing machinery, as the nucleus houses most of the genes and manufactures ribosomes.
A more realistic reticulated phylogeny might look like this:
Given
the lack of clarity regarding the closeness of the relationships
between the three domains I prefer to keep the term 'archaebacteria'
over 'archaea'. Originally bacteria included the archaebacteria and
the rest as 'eubacteria' which is acceptable when bacteria is used
as a descriptive term, but I think the terms 'archaebacteria' and
'bacteria' are quite sufficient. Both these groups are prokaryotic,
share very similar morphologies and may readily exchange genes by
HGT. The term archaebacteria is thus preferable to archaea as it
stresses the affiliation of this domain with the (eu)bacteria. It
certainly is not 'wrong' to use the term archaebacteria.
Origins
of Archaebacteria
I
would therefore caution against accepting 'official' taxonomies as
written in stone; science is not determined by committee! Some
scientists hypothesize that many of the unusual features of archeans
are due to their evolution in harsh environments, as archaeobacteria
were first discovered from harsh environments as extremophiles.
In other words their exotic nature may be due more to selection
pressure and less to evolutionary relatedness. More recently, it has
been discovered that archaebacteria are found in many more usual
habitats too, including the oceans, but were previously overlooked
due to the difficulties growing them by traditional culture methods.
We now know that not all of them are extremophiles and about 20% of
oceanic prokaryotes are archaebacteria, perhaps the majority beneath
the surface of the sediments. Interestingly, however, no known
archaebacteria are human pathogens. Another theory attributes their
odd biochemistry to selection pressure to avoid antibiotic attack
from other micro-organisms. Many bacteria and fungi secrete
antibiotics to kill their competitors.
The chief problem with phylogenetic trees is the assertion that life evolves as an organic tree, with each branch representing a lineage and dividing when a species evolves into two or more new species. This linear and vertical view of evolution is a considerable approximation, especially near the base of the tree which grew in ancient times when prokaryotes dominated the world. Horizontal gene transfer, or the exchange of genes between prokaryotes is common place and suggests the tree of life is more of a reticulated web, at least where the prokaryotes are concerned (see: Mallet et al. 2015, https://onlinelibrary.wiley.com/doi/epdf/10.1002/bies.201500149). Thus, the base of the tree is something of a mythical construct, since the relations between the various groups is better described as a web or net. As discussed above, Archaebacteria and bacteria also evidently exchange genetic information according to recent evidence.
One
plausible scheme has early life consisting of several main groups of
prokaryotes. Horizontal gene transfer by several means, including
endosymbiosis (the incorporation of one cell inside another)
produced the eukaryotes as a chimeric mixture of these types.
I
prefer to keep 'bacteria' as a descriptive term (in the same manner
as the word 'worm') and personally prefer the designation
Archaebacteria, with Eubacteria (or simply 'bacteria') referring to
the remaining 'true bacteria' in the prokaryote kingdom.
Archaebacteria have the morphological features of bacteria and just
as the various groups of worms are not all closely related, they are
nevertheless worms. Given the overall genetic similarities between
archaebacteria and eubacteria, the term 'archaebacteria' certainly
seems justified. As an example of an archaebacterium, we shall
consider Halobacterium.
Halobacterium requires a solution of at
least 2 M (2 Molar) NaCl (sodium chloride, salt) in order to grow,
and grows best in 4 to 5 M NaCl (compared to 0.6 M for typical sea
water). Such extremely salty conditions occur in the Dead Sea and
evaporating salt lakes, which are environments with high
light-intensity. In bright sunlight, Halobacteria are brick=red due
to the presence of red carotenoid
pigments
in their cell membrane, which protects the cells from damage
(especially against UV light). they often bloom, turning the water
red.
Archaebacteria are often (though not exclusively) found in harsh
environments - many are extremophiles. They appear to be a very
ancient group and may have evolved on Earth at a time when
conditions were quite different and harsh to life as we know it and
continue to dominate harsh environments. That said, it could be that
much of their unusual chemistry has been acquired through secondary
adaptation to harsh extremes and they are also being increasingly
discovered in more widespread habitats. Many species of bacteria and
archaebacteria no doubt remain to be discovered. Many do not grow in
standard culture conditions and plating out environmental samples
onto agar typically fails to detect many strains and species.
Archaebacteria include a number of extremophiles, such as
archaebacteria that live in hot springs (thermophiles), in highly acidic
solutions (acidophiles). Halobacterium thrives in high salt
concentrations, and so is a halophile, and grows best at a very
warm 42C. The study of extremophiles is a very rewarding one. It is
remarkable to see the adaptations of living organisms to extremes
and is of great importance to the study of evolution and
astrobiology.
Photophosphorylation
in
Halobacterium - making ATP from light - Solar Power
Plants,
algae and some bacteria undergo photosynthesis, the process of harvesting
light-energy to drive
growth. Halobacterium is not photosynthetic,
however it has a novel mechanism for making ATP by using sunlight to
generate a proton gradient (an electrical potential difference or
voltage) directly. Recall, that in respiration mitochondria and
bacteria use protons as a form of positive electricity to drive the
ATPase, an enzyme which spans the membrane and spins like an
electric motor when the protons flow through it. This rotary energy
is then used to generate ATP, with the ATP storing some of the
rotational energy as chemical energy (from ADP and inorganic
phosphate). Thus, we have the conversion or transduction of energy
from electrical, to rotary mechanical, to chemical; and this process
drives living cells. ATP is the universal
energy currency
of the living cell. Most active processes in most cells require ATP
as a source of energy.
Some strains of Halobacterium have a novel mechanism for
generating ATP. In aerobic conditions they will undergo normal
respiration using oxygen to synthesise ATP. However, if starved of
oxygen, they will synthesise a new membrane component, the purple
pigment bacteriorhodopsin, which is deposited in
dense patches in the cell membrane, forming patches of purple membrane. Purple membrane may
account for half of the total cell membrane surface area. Although
the organism appears not to grow under these conditions, it is able
to keep itself ticking-over. Purple membrane is 25% lipid and 75% of
a single type of protein, bacteriorhodopsin (consisting of a form of
the carotenoid chromophore retinal bound to an opsin protein).
Vertebrate retinas contain rhodopsin as a visual light-sensitive
pigment. Bacteriorhodopsin is the bacterial form of rhodopsin and
associates into trimers (groups of three bacterirhodopsin molecules)
that span the membrane. A Schiff base links the retinal to the opsin
protein (via a lysine amino acid residue). A Schiff base is an
organic molecule containing a nitrogen atom bound to a C atom on one
side by a C=N double bond and to a carbon on the other side (in this
case to the lysine residue) by a C-N single bond (the N is not bound
to hydrogen). A proton can reversibly attach to the N, which thus
acts as a proton store.
When the retinal absorbs a photon of light of the right wavelength
band, it becomes energised and undergoes a shape-change
(conformational change to a higher energy state) which causes the
stored proton to be released. Thus, when light strikes the purple
membrane protons are released into the space outside the cell
membrane. These protons do not leak far, but are conducted back into
the cell - protons are positively charged and the inside of the cell
membrane is negatively charged with a voltage drop of about -200 mV
across the membrane (the cell membrane is acting as an electrical
capacitor or a store of electric charge difference).
In most bacteria and in mitochondria, ATPase has a
proton-permeable channel running through its centre, and the protons
flow into the cell (completing the circuit by flowing towards the
negatively charged cell interior) through the ATPase. Many thousands
of ATPase molecules span the non-purple patches of membrane (the
'brown membrane'). It has been shown that ATPase in general (not
specifically that of Halobacterium) spins around like an
electric motor as the proton current flows through it and this
rotational mechanical energy is used to make ATP.
Another light-driven transport system, independent of
bacteriorhodopsin is found in Halobacterium. This system is driven by
the retinal-containing pigment halorhodopsin (peak absorption at 590
nm, yellow light). However, the contribution of halorhodopsin to ATP
synthesis appears to be small. Mutants lacking bacteriorhodopsin,
but not halorhodopsin do not release protons to the outside upon
stimulation by light, but do take-up protons on illumination. This
appears to be a passive transport process (see transport across
membranes) and is accompanied by sodium ion export (we have a
proton-sodium antiporter). This generates a sodium ion gradient and
it is thought that in Halobacterium, sodium ions, rather than
protons (as is usual) carry the charge that drives the ATPase. Thus,
light stimulates bacteriorhopsin to release protons and light also
stimulates halorhodopsin to open a proton-sodium importer which
allows the protons to flow back into the cell whilst sodium ions
flow out and then the sodium ions flow back in to the cell through
the ATPase, synthesising ATP. Bacterirhodopsin in Halobacterium also drives passive
potassium import accompanied by sodium export (a potassium-sodium
antiporter) which also contributes to the sodium gradient and ATP
synthesis. Protons are still needed by Halobacterium, however, for pH
regulation.
The solubility of oxygen in brine is very low, so oxygen starvation
must be a common occurrence in the very salty waters in which Halobacterium lives. Oxygen is required
for the synthesis of the retinal in bacteriorhodopsin, so the cells
can not grow without oxygen, however they may grow by using light to
generate ATP if small amounts of oxygen are available, and then they
may grow photoheterotrophically as photoheterotrophs (that is by using light as
a source of energy but not for carbon assimilation from inorganic
sources, as the cells would require pre-assimilated organic carbon
as a food-source). Amino acids (the building blocks of proteins) are
the preferred carbon and energy source.
Flagella
Halobacterium
is typically bipolarly flagellated (though sometime monopolarly)
with a bundle of 5-10 flagella at each end of the cell.
The filaments form right handed helices. The flagella are inserted
into a distinct polar cap structure in the cytoplasm, which
presumably anchors the flagella (and may contain the molecular
switches to switch from clockwise to counterclockwise rotation).
Halobacterial cells are pushed forward by clockwise rotation and
pulled backward by counterclockwise rotation of the right handed
flagellar bundles. Thus, one bundle rotates clockwise (the trailing
bundle) and the other rotates counterclockwise (the leading bundle)
with periodic synchronized reversals in rotation direction of both
bundles. However, in bipolarly flagellated forms, the leading bundle
is depicted as curved backwards, so it is not clear what role it
plays.
The eubacterial flagellum filament is thicker than that of
archaebacteria like Halobacterium, as it contains a central
channel through which flagellin monomers are transported to the
growing tip. It seems as though the archaebacterial flagellum is
more like a (type IV) pilus and so likely grows from
the base. Bacterial flagella are typically powered (like ATPase) by
proton currents, which cause the flagella to rotate by driving an
electric motor at the base of each flagellum. In Halobacterium, it is possible that
sodium currents drive the flagella motor instead (as they do in some
alkalophilic bacteria). ATP is required for flagella rotation in Halobacterium, though this might be an
indirect requirement on ATP to establish/maintain ion gradients.
The
distinctive nature of the archaebacterial flagellum has produced the
suggestion that flagellum should refer to the bacterial
structure whereas the term archaellum refers to the
flagellum of archaea (archaebacteria). This leads to awkward terms
such as archaellation instead of flagellation. Even
more proposterous is the accompanying suggestion that all such
appendages in eukaryotes be called cilia, whether they are in fact
cilia or flagella! All three kingdoms may possess flagella,
according to the original definition of the term, which is
independent of microscopic structure and simply describes a long
whip-like appendage. Microbiologists should not interfere with a
definition that is well understood in zoology and botany! I
sincerely hope that some committee does NOT redefine these terms!
That is not how science progresses, so please stop it! An archaellum
is still a type of flagellum and personally I prefer the term archaebacterial
flagellum or the suitable contraction archaeflagellum.
Photokinesis
Click
here for
an explanation of the usage of the terms taxis and kinesis.
The optimum wavelength of light absorbed as an energy source by
bacterirhodopsin in Halobacterium is about 560 nm
(yellow-green, almost yellow light). Halobacteria are motile, they
can swim equally well in either direction by means of a tuft of
flagella on each cell pole. Every 10-50 seconds or so a Halobacterium cell spontaneously
reverses direction. The trailing flagella come together to form a
propulsive bundle.
A sudden increase of yellow-green light leads to a suppression of
spontaneous reversals for about 10 seconds. A sudden decrease in
yellow-green light elicits an extra reversal response (after a few
seconds). The end result of these behaviours is that the bacteria
tend to congregate in areas of bright (yellow-green) light, which
their purple membranes can utilize.
Conversely, a sudden increase in blue or UV light elicits an extra
reversal and a decrease in blue/UV
suppresses spontaneous reversals (again for about 10 seconds). This
leads to avoidance of harmful UV light.
When a Halobacterium cell reverses, it does not
set-of in exactly the opposite direction (if they could only move
along a straight line they would have problems!) but rather the new
direction of locomotion is something like 160 to 200 degrees offset
from the original direction of motion - the cell turns slightly
during reversal. Our stochastic computer simulations of kinesis
suggest two optimal strategies for finding or avoiding a source of
light (or chemicals) - random tumbling (as in chemokinesis in Escherichia coli and Salmonella) or a set change in
direction a few degrees either side of 180, as in Halobacterium. (Persistently making
small turns of less than 90 degrees appears to be highly
inefficient) so kinesis in Halobacterium is still optimised (though
very different from the tumbles of Escherichia
coli).
Two photosensory pigments have been found in Halobacterium. PS 565 (photosystem 565)
has peak absorption at 565 nm and is used to respond to yellow-green
light to enable the cells to find optimum conditions for
photophosphorylation and ATP synthesis. The second system, PS 370
has peak absorption at 370 nm and so appears responsible for the
protective avoidance of blue/UV light. these photosensors also
contain retinal and are more variants of bacteriorhodopsin.
Cell
Wall and Cell Membrane
Outside
the cell membrane is a cell wall that is quite different to the
usual peptidoglycan cell wall of eubacteria. The Halobacterium cell wall is made of
glycoprotein. This glycoprotein can be removed by placing the cells
in dilute solution (salt appears to be essential for the
glycoprotein's stability in the wall) and such cells lose their
shape, rounding up and become susceptible to osmotic stress (they
will lyse (burst) in very dilute solution).
The cell
membrane
is overall about 20% lipid (the remainder being protein). This is
low, but similar values may be found in mitochondria and other
bacteria - bacteria (and their mitochondrial descendants) have
membranes with high protein contents. The lipids
of archaebacteria, including Halobacterium, are very different to
those in other cells. Most organisms have cell membranes composed of
glycerol esters, with two fatty acid carbon-chains or tails attached
to a glycerol backbone by ester linkages (-C-(C=O)-O-C-) forming a
glycerol diester, and a charged head that usually contains
phosphorous. In archaebacteria, the carbon tails are instead bound
to glycerol by ether linkages (-C-O-C-), the so-called glycerol
diethers. In both types of membrane the lipids arrange into two
opposing leaflets. However, in some archaebacteria, the lipids are
diglycerol tetraethers, in which two carbon chains join two
glycerols together, so that a single layer can form a membrane, with
one glycerol at each surface. In all cases, the glycerols contain
charged (polar) heads, usually containing phosphorous, which are
'water-liking' (hydrophilic) and so sit on the surfaces of the
membranes, which are in contact with water, with water largely
excluded from the 'water-hating' (hydrophobic) membrane cores formed
from the fatty-acid carbon tails/chains.
The membrane of Halobacterium
halobium
(Halobacterium
salinarum)
the main lipid is a phosphatidylglycerophosphate diether, with
two carbon tails and a phosphate head joined to a glycerol molecule
by bonds (ether bonds for the tails). Detergent is required to
solubulize most cell membranes, but that of Halobacterium
halobium
dissolves in water (though not in strong saline of course!)! It is
thought by some that the unusual ether lipids of archaebacterial
cell membranes is evidence for their distant relationship from other
cellular life (Eubacteria and eukaryotes), however, it could be a
secondary adaptation to extreme conditions, with the ethers (and
perhaps especially the tetraethers) being more stable in high
temperatures.
Genome
Halobacterium has a genome size of about
2.5 Gbp (Gbp = giga-base-pairs or 10^9 bp). There is a main
chromosome and a large plasmid. The large plasmid encodes
gas vacuoles and the pigment bacteriorhodopsin. There are many
different transposable sequences (transposons), up to some 5000 bp in
length, and each present as 20 or more copies. These transposons are
highly mobile, being able to create a copy that leaves the host DNA molecule (the original
stays where it is) and reinsert back into the host DNA molecule in a
different position (a process termed replicative recombination).
Remarkably it has been found that even if the DNA of Halobacterium is completely fragmented
by radiation, the cell can repair and rebuild it within hours! Such
radiation would destroy the vast majority of living organisms. Salt
and UV can damage DNA in a way similar to ionizing radiation and so
such versatile repair mechanisms are probably a part of the survival
kit of these halophiles. Often, when water evaporates, cells of Halobacterium may become encased in salt
(along with a small amount of water). This is a very harsh
environment (salt desiccates living cells which is why it helps
preserve food), but the layer of salt may prevent further
dehydration and in this state Halobacterium can remain dormant,
repairing its DNA and recommencing growth in more favourable
conditions. There are controversial claims that Halobacteria have
been revived from salt deposits that are 250 million years old.
Could the cells have remained dormant for this length of time? With
such efficient DNA repair mechanisms it seems likely to me.
We have seen that Halobacterium is remarkable in a number
of ways and is certainly an extremophile able to thrive in
conditions that would destroy most living organisms. It can tolerate
extreme radiation, high light intensities and very salty conditions
whilst being able to endure desiccation in a dormant state. However,
it is worth remembering that dilute water kills it! In non-salty
water its cell wall dissolves and then it will swell as water enters
the cell by osmosis and burst. Extremophiles are superbly adapted
for their extreme environments, but our own environment may well be
extreme and intolerable to them. Organisms are each adapted to their
own environments and an unfamiliar environment may often seem
extreme! extremophilism is relative to some extent.
Gas
Vacuoles
Halobacterium frequently possesses gas
vacuoles
- arrays of gas-filled proteinaceous cylinders in the cytoplasm
which confer buoyancy, helping it float upwards and maintain its
position in oxygenated waters, away from the anoxic (oxygen-free)
sediments.
Halophiles
Halobacterium is one of a number of
halophilic (salt-loving) bacteria abundant in very salty
(hypersaline)
waters. Halophilic archaebacteria, including Halobacterium, are sometimes generically
called halobacteria.
these include Many are extreme halophiles, meaning they thrive in
salt concentrations greater than 3.4 to 5.1 Molar (20-30%). To
prevent excessive movement of water into or out of the cell, these
archaebacteria have a high concentration of potassium chloride salt
in their cytoplasm, achieving isotonicity with their environment
(see osmosis). In contrast non-halophiles thrive below 0.2 Molar.
(Sea water is 3.5% salts). Halobacterium thrives in a variety of
natural hypersaline waters and also in solar salterns - vast
artificial pools of water which are slowly evaporated by the Sun to
precipitate salt which is harvested. In these salterns Halobacterium may reach a concentration
of 10 million cells per ml and turns the water red!
Such hypersaline pools are often covered in a microbial mat,
essentially a thick biofilm formed from several species.
Photosynthetic Cyanobacteria, like Aphanothece
halophytica,
forms the top brown layer of these microbial mats and thrives in 2-5
Molar salt and lyses (bursts) in distilled water due to osmosis
(this is unusual for bacteria because of their tough cell walls, but
just goes to show how concentrated the cytoplasm is, which in this
case contains high concentrations of an osmolyte called glycine betaine
rather than potassium chloride which is the major osmolyte in Halobacterium). Filamentous
cyanobacteria, like species of Oscillatoria, form the second layer in
the microbial mats, which is green and these thrive at 1-2.5 Molar
salt concentration and possess nitrogen-fixing heterocysts.
Beneath the cyanobacterial-layer are phototrophic bacteria, such as
green sulphur-bacteria (e.g. Chlorobium), green non-sulphur
bacteria (e.g. Chloroflexus), purple sulphur-bacteria
(e.g. Chromatium) and purple non-sulphur
bacteria (e.g. Rhodospirlillum species).
Beneath the phototrophic layer are sulphur-oxidising bacteria, such
as Beggiatoa
alba and
Thiobacillus
halophilus.
Some of these fix carbon-dioxide, and oxidise sulphides and sulphur
to sulphate.
Finally, anaerobes and archea inhabit the anoxic bottom sediments.
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Article updated: 5th July 2025