Haloarchaea, or halobacteria, are a group of microorganisms that can grow in extremely salty environments exceeding normal food pickling levels. They have unusual features such as red, orange, or purple pigmentation and the ability to use sunlight to power proton pumps. Haloarchaea are found in hypersaline lakes and salterns, and have adapted mechanisms to balance osmotic pressure through accumulating potassium ions intracellularly. They have potential applications in biotechnology due to their stable enzymes that function in high salinity conditions unsuitable for most other organisms.

1. INTRODUCTION:
The halobacteria are a group of microorganisms with so many unusual features growth at salt
concentrations higher than those used in any food pickling processes, striking pigmentation in
red, orange or purple, obligately salt-dependent enzymes, possessors of the first known proton
pump, bacteriorhodopsin, which is driven just by sunlight that early researchers became almost
desperate about “the halobacteria’s confusion to biology”, which was the title of a lecture given
by Larsen (1973) and which described what was known at the time about that “life in the
borderland of physiological possibilities”. While some halobacterial features have turned out to
be not completely singular and while the molecular basis for halophilism is being unraveled,
there are still many characteristics which are unique, and new ones have been added since e.g. a
square and flat morphology, potential longevity of halophilic microorganisms in salt sediments
for millions of years, implications of the discovery of extraterrestrial halite.
Fig.1; Haloarchea
Haloarchaea can grow aerobically or anaerobically. Parts of the membranes of haloarchaea are
purplish in color and large blooms of haloarchaea appear reddish, from the
pigment bacteriorhodopsin, related to the retinal pigment rhodopsin, which it uses to transform
light energy into chemical energy by a process unrelated to chlorophyll-based photosynthesis.
Haloarchaea have a potential to solubilize phosphorus. Phosphorus-solubilizing halophilic
archaea may well play a role in P (phosphorus) nutrition to vegetation growing in hypersaline
soils. Haloarchaea may also have application as inoculants for crops growing in hypersaline
regions.
ISOLATION:
These microorganisms were first isolated and identified as the causative agents of spoilage of
salted materials, when conditions were such that the microbes had enough moisture to come out

2. of brine inclusions in salt used in either preservation (such as in the preparation of salted codfish)
or production (such as in the curing of hides in the leather industry) processes.
TAXONOMY, PHYLOGENY AND METABOLIC DIVERSITY:
The extremely halophilic archaea (also called haloarchaea or, traditionally, “halobacteria”)
belong to the order Halobacteriales, which contains one family, the Halobacteriaceae (Grant et
al. 2011); since the publication of Bergey’s Manual of Systematic Bacteriology (2001), which
listed 14 recognized haloarchaeal genera, the number has increased to 19 genera, according to
the International Committee on Systematics of Prokaryotes. The number of validated species at
this time is 57.Archaea share many characteristics with both Bacteria and Eukarya. Archaea are
split into two major groups
o Crenarchaeota
o Euryarchaeota
Bioenergetics and intermediary metabolism of Archaea are similar to those found in
Bacteria.Except some Archaea use methanogenesis. Autotrophy via several different pathways is
widespread in Archaea.
Fig. Phylogeny of Haloarchea
MORPHOLOGY:
Haloarchaea are often considered pleomorphic, or able to take on a range of shapes—even within
a single species. This makes identification by microscopic means difficult, and it is now more
common to use gene sequencing techniques for identification instead.

3. One of the more unusually shaped Haloarchaea is the "Square Haloarchaeon of Walsby". It was
classified in 2004 using a very low nutrition solution to allow growth along with a high salt
concentration, square in shape and extremely thin (like a postage stamp). This shape is probably
only permitted by the high osmolarity of the water, permitting cell shapes that would be difficult,
if not impossible, under other conditions.
The morphology of non-coccoid haloarchaea can change, dependent on the salt concentration of
the environment. With increasing dilution of salt, club-shaped, swollen and bent rods or spheres
appear.
INTERNAL STRUCTURES:
Internal gas vesicles are only produced by prokaryotes; several bacteria and also some halophilic
archaea are capable of synthesizing these flotation devices (Walsby 1994). Gas vesicles are filled
by diffusion with gases dissolved in the environment; their function is apparently to provide
buoyancy and enabling cells to regulate their position in the water. The identification of gas
vesicle genes and their regulation is carried out in the laboratories of Pfeifer) and DasSarma
(Shukla and DasSarma 2004). Walsby pointed out that without the presence of gas vesicles,
which can be detected easily by phase contrast microscopy due to their refractivity, the square
haloarchaeal sheets described above would hardly have been recognized as living entities.
Inside the cytoplasm of Halobacterium salinarum, fibrillary structures were identified, which
apparently consist of a bundle of hollow tubes and were termed “fibrocrystalline bodies” (Cho et
al. 1967). Recently, the isolation of these structures was reported and their sensitivity to the drug
vincristine .This feature, together with the appearance of the fibrils, could indicate the presence
of a cytoskeleton-like organelle in haloarchaea.
LIVING ENVIRONMENT:
Haloarchaea require salt concentrations in excess of 2 M (or about 10%) to grow, and optimal
growth usually occurs at much higher concentrations, typically 20–25%. However, Haloarchaea
can grow up to saturation (about 37% salts).
Haloarchaea are found mainly in hypersaline lakes and solar salterns. Their high densities in the
water often lead to pink or red colourations of the water (the cells possessing high levels of
carotenoid pigments, presumably for UV protection).Some of them live in underground rock salt
deposits, including one from middle-late Eocene (38-41 million years ago). Some even older
ones from more than 250 million years ago have been reported.
WATER BALANCE IN EXTREME HALOPHILES:
Halophiles need to maintain osmotic balance.This is usually achieved by accumulation or
synthesis of compatible solutes. Halobacterium species instead pump large amounts of K+ into
the cell from the environment.Intracellular K+ concentration exceeds extracellular Na+
concentration and positive water balance is maintained. Proteins of halophiles are highly acidic
and contain fewer hydrophobic amino acids and lysine residues.

4. Fig.3; Salt ponds with pink colored Haloarchaea on the edge of San Francisco Bay,
near Fremont, California
ADAPTATIONS TO ENVIRONMENT:
Haloarchaea can grow at an aw close to 0.75, yet a water activity (aw) lower than 0.90 is
inhibitory to most microbes.The number of solutes causes osmotic stress on microbes, which can
cause cell lysis, unfolding of proteins and inactivation of enzymes when there is a large enough
imbalance.Haloarchaea combat this by retaining compatible solutes such as potassium chloride
(KCl) in their intracellular space to allow them to balance osmotic pressure. Retaining these salts
is referred to as the “salt-in” method where the cell accumulates a high internal concentration of
potassium.Because of the elevated potassium levels, haloarchaea have specialized proteins that
have a highly negative surface charge to tolerate high potassium concentrations.
Haloarchaea have adapted to use glycerol as a carbon and energy source in catabolic processes,
which is often present in high salt environments due to Dunaliella species that produce glycerol
in large quantities.
REPRODUCTION:
They reproduce by binary fission. They do not form resting stages or spores. Mostly they are
non-motile.
APPLICATIONS IN BIOTECHNOLOGICAL AND ENVIRONMENTAL PROCESSES:
Halophilic microorganisms possess stable enzymes that function in very high salinity, an
extreme condition that leads to denaturation, aggregation, and precipitation of most other
proteins. Genomic and structural analyses have established that the enzymes of halophilic
Archaea and many halophilic Bacteria are negatively charged due to an excess of acidic over
basic residues, and altered hydrophobicity, which enhance solubility and promote function in low
water activity conditions. Here, we provide an update on recent bioinformatic analysis of
predicted halophilic proteomes as well as experimental molecular studies on individual

5. halophilic enzymes. Recent efforts on discovery and utilization of halophiles and their enzymes
for biotechnology, including biofuel applications are also considered.
Applications (current and potential) of halophilic microorganisms can be divided into a number
of categories:
1.Centuries‐old processes such as the manufacturing of solar salt from seawater and the
production of traditional fermented foods. Such processes existed long before the nature
of the microorganisms involved became known, and little if anything is done to control
these microorganisms to improve the production processes.
2.Utilization of the salt tolerance of halophilic microorganisms and of enzymes produced by
them to catalyze processes in high salt environments.
3.Exploitation of the properties of specific compounds produced by certain types of
halophiles that enable them to withstand the high salt concentrations in their medium
(ectoine, glycerol and others).
4.Applications of unique compounds made by some halophiles, not directly connected with
their life in high salt environments. The prime example is bacteriorhodopsin in the purple
membrane of Halobacterium salinarum, which is not essential for its growth, and is stable
and active also in the absence of salt.
5.Industrial uses of compounds present in halophiles as well as in many non‐halophilic
counterparts. Compounds such as β‐carotene, poly‐β−hydroxyalkanoate,
exopolysaccharides, etc. found in some halophiles are also made by many other
microorganisms. Sometimes there is a clear advantage to using halophiles for their
production (e.g., β‐carotene production from Dunaliella); in most other cases, however,
the superiority of the halophiles as producers of such compounds is yet to be proven.
REFERENCES:
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fibrocrystalline body, a structure present in haloarchaeal species, from Halobacterium
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 Boring J, Kushner DJ, Gibbons NE. Specificity of the salt requirement of Halobacterium
cutirubrum. Can J Microbiol. 1963;9:143–154.
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Biol. 2002;12:516–522.
 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3188376/
 https://link.springer.com/article/10.1007/s10295-011-1021-9
 https://www.sciencedirect.com/science/article/abs/pii/S1369527415000600