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European Journal of Applied Sciences – Vol. 12, No. 3
Publication Date: June 25, 2024
DOI:10.14738/aivp.123.17003
Olukunle, O. F., & Oyegoke, T. S. (2024). The Potentialities of Extremophiles from Hypersaline Habitats- A Review. European
Journal of Applied Sciences, Vol - 12(3). 88-117.
Services for Science and Education – United Kingdom
The Potentialities of Extremophiles from Hypersaline Habitats- A
Review
Oluwatoyin Folake Olukunle
Department of Biotechnology, School of Life Sciences,
Federal University of Technology, P. M. B. 704, Akure, Nigeria
Oyegoke, T. S.
Dept of Quality Assurance, Kroger,
Dallas Fulfilment Center, Dallas,Texas
ABSTRACT
Extremophiles are organisms that are capable of living in extreme environments
(radiation, temperature, pH, salinity etc). Fascinating questions on the
potentialities of extremophiles from hypersaline habitats are yet to be fully
answered. Hence, it is an intriguing study that needs continuity. Most recent
studies, on extremophiles, reveal the intrinsic benefits of halophiles in many
biological and biotechnological processes ranging from several medical
applications to a wide variety of industrial processes. In most cases, the
applications of these extremophilic microorganisms and their metabolites (e.g.
enzymes, inhibitors, compatible solutes, etc) in biotechnology have been the
driving force to understanding these organisms, and have contributed
immeasurably to several biological types of research. Despite the justifications
from several studies on the potentialities of extremophiles, relatively little
information is still provided on the biotechnological applications of halophiles,
hence the optimal capabilities of these diverse populations of microorganisms
remain clandestine and need to be explored. This review is aimed at discussing
recent applications of extremophiles from hypersaline environments in order to
provide a basis for researchers to explore these halophilic organisms for newer
applications in many fields such as in agriculture, pharmaceuticals, medicine,
food, and industries.
Keywords: Extremophiles, Hypersaline environment, Halophiles, Biotechnological
applications and Metabolites.
INTRODUCTION
In relation to the abiotic factors, such as temperature, pH, humidity, pressure, salinity, etc, an
environment is regarded as extreme if it is not conducive to human habitation. However, such
environments have been found habitable for microorganisms. These microbes are ubiquitous
and therefore found to inhabit any environment. They are recognized as the most numerous
and diverse species on the planet, making up a sizable portion of the biomass. They are found
in all kinds of environments – high/low temperature, high/low pressure, high/low pH,
high/low salt concentration, etc. due to the fact that they have developed special adaptive
mechanisms to enable them survive in these harsh environments. Recent advances in
Environmental Microbiology, Microbial Ecology, coupled with genome/metagenomic
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Olukunle, O. F., & Oyegoke, T. S. (2024). The Potentialities of Extremophiles from Hypersaline Habitats- A Review. European Journal of Applied
Sciences, Vol - 12(3). 88-117.
URL: http://dx.doi.org/10.14738/aivp.123.17003
sequencing have revealed unexpected biodiversity in these environments (Ferrer et al., 2007).
Therefore, an extreme environment can be defined as habitats previously considered not
suitable for life survival and these include extremes of acidic to basic, toxic wastes, organic
solvents, heavy metals, pressures; hydrothermal vents, frozen sea water niches of ice, and
high salt solutions (Seckbach, et al., 2006; Lawlor, 2017). Hypersaline habitats are intense
environments in the universe with an elevated level of salt concentration constituting
relatively broad consortia of organisms (Waditee-Sirisattha et al., 2016). These organisms are
generally referred to as halophiles due to their adaptability to an environment with high salt
concentration. They are relatively found in all domains of life existence which includes
Prokarya (archaea and bacteria) and Eukarya (Oren 2015). For their survivability in regions
of very high salinity, they produce high concentrations of compatible solutes in order to
maintain an osmotic equilibrium of their cytoplasm with the hypersaline environment (Abed
et al. 2013). The adaptability of halophiles in hypersaline habitats ranges from a salinity level
of 0.6 M up to saturation salinity of less than 5 M NaCl (Ventosa et al. 1998). According to
reports, halophilic bacteria live in hypersaline settings including salty foods, the sea, saline
lakes, saltern ponds, the desert and hypersaline soils (Chen and Jiang 2018; Vera-Gargallo et
al., 2023; Sánchez-Porro, 2023). These extremophiles are considered to be of great relevance
for their potentialities to excite several promising applications in biotechnology. Despite
several biological products obtainable from these extremophiles (proteins, extremozymes,
biopolymers, and compatible solutes), they possess useful physiological properties which can
facilitate their exploitation for inhibitory compounds with wide variety of applications
spanning medical field, environmental strata, and to a large extent in food sectors (Ventosa et
al. 1998; Sysoev et al., 2021; Ali et al., 2023). The outlines below provide a detailed review of
the biotechnological potentialities of halophiles in industrial and medical biotechnologies.
HYPERSALINE ENVIRONMENTS
Hypersaline systems are considered to be extreme habitats or harsh environments having
high salinity, much higher than that of seawater, even exceeding salt saturation. Arahal and
Ventosa (2002) classified hypersaline environments into two main types, depending on their
origin.
1. Thalassohaline – originates from seawater. Examples include solar salterns, salty lakes
like Utah's Great Salt Lake, or salty soils.
2. Athalassohaline soils come from sources other than the ocean. Examples are the Dead
Sea in Israel and Soda lakes (alkaline lakes – pH is between 9 and 11.5, poor in Mg+ or
Ca+ ions, but abundant in carbonates).
Other examples of hypersaline settings include Lake Tyrell in Australia (29% salinity), Santa
Polasaltern in Spain (13 to 37 % salinity) and crystallizer ponds in the United States (18 to 38
% salinity) (Fernández et al., 2014; Cowan et al., 2015). All of these hypersaline systems
contain organisms, which are known as halophiles. Halophiles are today classified into three
major groups according to the salt concentrations (NaCl) of their location: slight halophiles
(0.34 to 0.85M or 2 to 5% NaCl w/v concentration), moderate halophiles (0.85 to 3.4M or 5 to
20% w/v concentration) and extreme halophiles (3.4 to 5.1M or 20 to 30% w/v
concentration) with slight variations in the salt concentrations. It is important to note that the
halotolerants do not need salt for their development but have mainly developed special traits
to survive in the presence of high salt concentration. It is also worthy to note that other salts
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aside NaCl are contained in hypersaline environments but the classification of halophiles is in
relation to NaCl (Sysoev et al., 2021).
Extremophilic Microorganisms in Hypersaline Environments
Halophiles are categorized according to their salt requirements and exist in all three spheres
of existence, including Bacteria, Archaea, and Eukarya. Halophiles thrive in hypersaline
conditions at 3.5M or 30% w/v NaCl concentration or high salt concentration, which may also
be characterized by additional severe circumstances, such as low oxygen concentration (< 6.6
mg/L at 25°C), high or low temperatures (thermophilic or psychrophilic respectively) and
alkaline pH of ≥ 7.5 (Vreeland and Hochstein, 1993). Apart from these aforementioned harsh
conditions, halophiles are also remarkably tolerant to multiple stresses such as radiation
(sunlight exposure), extreme pressures (DasSarma and DasSarma), dessication and toxic
metals (Vauclare et al., 2014).
Hypersaline Archaea
The main population in the hypersaline environment contain usually the haloarchaea group in
the families Halobacteriaceae and Haloferacaceae; and the phylum Euryarchaeota (Oren,
2002, Gupta, 2016). The main population in this environment contains usually the class,
Halobacteria, which is regarded as one of the most significant groupings in the domain
archaea with Halobacteriales and Halobacteriaceae only, as order and family respectively
(Grant et al., 2001). However, new classification for two new orders Natrialbales ord. nov and
Haloferacales ord. nov in the families Natrialbacea ord. nov. and Haloferaceae ord. nov.
respectively within the class Halobacteria were proposed by Grupta et al., 2015. This,
proposal was however, not formally recognized in Subcommittee on the Taxonomy of
Halobacteriaceae (Quadri, et al., 2016). In a study carried out on extremely halophilic archaea
from the hypersaline environment of Algerian Sahara, ten (10) archaeal genera: Natrinema,
Natrialba, Haloterrigena, Halorubrum, Haloarcula, Halostagnicola, Halopiger, Natronococcus,
Haloferax, and Halogeometricum, belonging to the class Halobacteria were identified based on
the sequencing of 16S rRNA gene (Quadri et al., 2016). Most archaea are able to utilize glucose
as a carbon source while some, such as Natrialba (Xu et al., 2001), Natronomonas (Falb et al.,
2005; Falb et al., 2008) and Halobacterium (Javor, 1984) are unable to do so. Halobacteria
possess charged amino acids on their surface and acidic proteins, which enable them keep
water molecules around cellular components and resist the denaturing effects of salts while
Halobacterium has a high tolerance for elevated levels of salinity. Halobacteria also produce
brilliant pink, red and orange colours which are primarily produced by bacterioruberins.
Bacterioruberin is a 50-carbon pigment that has been proposed to protect cells from osmotic
stress or damage from ultraviolet light (Peck et al., 2019). Halophilic archaea are found to
dominate the populations of aerobic hypersaline environments. Most phylotypes obtained
from hypersaline environments did not correspond to any previously described organism and
most of the community is composed of uncultured organisms (Ochsenreiter et al., 2002).
Hypersaline Bacteria
In the study conducted by Kumar et al (2012), one hundred and eight (108) moderately
halophilic bacteria with the ability to produce hydrolase were isolated from diverse saline
habitats of India, out of which 21 potential isolates were characterized based on
morphological, biochemical and 16S rRNA gene analysis to be related to the genera
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Olukunle, O. F., & Oyegoke, T. S. (2024). The Potentialities of Extremophiles from Hypersaline Habitats- A Review. European Journal of Applied
Sciences, Vol - 12(3). 88-117.
URL: http://dx.doi.org/10.14738/aivp.123.17003
Virgibacillus, Marinobacter, Geomicrobium, Oceanobacillus, Bacillus, Halobacillus
Staphylococcus, and Chromohalobacter. Phototrophic sulfur bacteria have been reported to
occur massively in hypersaline brines of high alkalinity (Larsen, 1980). The comparative
sequence analysis in the study of Kumar et al., (2012) revealed the presence of several novel
clusters of sulfur-reducing bacteria from the order Desulfovibrionales and the family
Desulfobacteraceae. Desulfotignum balticum, Desulfosarcina variabilis, Desulfosalina
propionicus, and Desulfobacter halotolerans; Firmicutes of the Halanaerobiales, Spirochetes,
Planctomycete and members of the Bacteroidetes and α-proteobacteria are abundant in
hypersaline environments. Ollivier et al (1994) described six anaerobic fermentative genera,
containing nine species, two of which are homoacetogens while the rest belong to the family
Haloanaerobiaceae, as indicated by their unique 16S rRNA oligonucleotide sequences. Salicola
marasensis sp. IC10 is an extremely halophilic bacterium that has the ability to produce
promising enzymes (lipase and protease) in terms of its resistance to salinity and
temperature (Moreno, 2013).
Eukaryotic Halophiles
Algae:
Algae are obligately aerobic, photosynthetic microorganisms that survive at moderately high
salinities. Apart from Dunaliella salina and Asteromonas gracilis, which can grow in an
extremely high concentration of salts, most of the green algae species are moderate
halophiles. They are found at the bottom of the food chain, where they usually serve as the
primary producers in most hypersaline environments (Mcgenity, 2012). They are the main
source of food for larvae of brine flies and brine shrimps e.g. Dunaliella salina, D. viridis and D.
parva. (DasSarma and Arora, 2001). Examples of common halophilic diatoms include
Nitzschia sp., Navicula sp. and Amphora coffeaeformis. (DasSarma and Arora, 2001). Halophilic
algae have been exploited for the production of biofuels (Amoozegar et al., 2019) and the
commercial production of Beta-carotene and other essential products under controlled
environments (Oren, 2014).
Protozoa:
Protozoa have been observed in hypersaline environments, particularly the
chemoheterotrophic protists that ingest algae and bacteria e.g. Fabrea salina (moderate
halophile) and Porodon utahensis (extreme halophile), identified from a west Australian lake
and the Great Salt Lake respectively (Post et al., 1983).
Hypersaline Fungi:
Fungi are worldwide distributed, found inhabiting diverse extreme ecotypes from desert to
hypersaline environments. Hypersaline fungi can be found in three divisions viz;
1. Ascomycota that includes Sarocladium strictum of the class Ascomycetes; Cladosporium
cladosporioides, Alternaria clamydospora, Alternaria rhizophorae, Bipolarisprieskaensis;
Embellisia clamydospora, Embellisiatellustris, Ulocladiumalternariae of the class
Dothideomycetes; Penicillium expansum of the class Eurotimycetes; Botrytis cinerea of
the class Leotimycetes and Acremonium larvarum, A. Potronii, Acrostalagmus
luteoalbus, Tricoderma atroviride and T. Harzianum of the class Sordariomycetes.
2. A basidiomycetous fungus known as Wallemia ichthyophaga requires at least 1.5 M
NaCl for in vitro growth (Asem and Eimanifar, 2014). In fungi, obligate requirement for
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salt is an exception, for instance, Hortaea werneckii is reported to grow well in
standard microbiological media without salt (Gostincar, 2010). Yeasts and molds are
chemoheterotrophic eukaryotes that have been found to tolerate hypersaline
environments. A halotolerant yeast, Debaromyces hansenii was isolated from sea water,
saprophytic hyphomycete and Cladosporium glycolicum, were found growing on
submerged wood panels in hypersaline environment while Basipetospora halophila
and Polypaecilum pisce were isolated from salted fish (DasSarma and Arora, 2001).
Gunde-Cimerman et al. (2000) reported on polymorphic black yeasts, Hortaea
werneckii, Phaeotheca triangularis, Trimmatostroma salinum, Aureobasidium pullulans
and Cladosporium spp., isolated from hypersaline waters (3-30% NaCl) of a saltern.
Viruses in Hypersaline Environments
Viruses are dependent on their hosts for replication and have been found in all three domains
of life, unlike bacteria and archaea that are ubiquitous in nature. It is known that they are
more abundant than even cells, but details about the ubiquity and diversity of viruses are
lacking, particularly in extreme environments. Due to the lack of a universal marker gene for
viruses, it is often difficult to track viral populations across samples in hypersaline
environment (Emerson et al., 2012). However, reports on viruses detected from marine
environments showed their abundance and diversity with other forms of life in the
environment (Suttle, 2005; Edwards and Rohwer,2005; Breibart and Rohwer, 2005). Viruses
in the hypersaline environments are known as haloviruses. Viruses are found to exhibit a
large genomic diversity in hypersaline environments and based on pulsed-field
electrophoresis analysis, the viral population is directly proportional to the hosts’ ecology as
suggested by (Sandaa et al., 2003). The halophilic viruses have been characterized and they
belong to the three main families Myoviridae, Siphoviridae and Podoviridae, they are found to
infect archaea and bacteria found in hypersaline environment. Their genome consists of linear
double-stranded DNA. Amongst bacteria, bacteriophages (environmental halophage) and
eukaryotic viruses, it has been observed that there is less than 10% sequence similarities in
their genome, which could be adduced to the environment from which the viruses are isolated
(Dyall-Smith et al., 2003).
Multicellular Eukaryotes
Few organisms apart from microorganisms have the ability to tolerate hypersaline conditions.
For instance, Tilapia species have been observed to survive in about 1mol/L NaCl, which is the
highest salinity for any vertebrates (DasSarma and Arora, 2001). A surprising number of
invertebrates such as Atriplex halimus and Mesembryanthemum crystallinum which are
obligate and facultative halophylic plants, rotifers such as Keratella quadrata and Brachionus
angularis, worms such as Macrostomum species, copepods such as Robertsonia salsa and
Nitocra lacustris, ostracods such as Diacypris compacta, Cypridis torosa, Reticypris herbsti and
Paracyprideinae spp. are known to survive in hypersaline environments. Examples of insects
found in this environment include brine shrimp Artemia franciscana and brine flies Ephydra
gracillis and E. hians. Birds are not left out. One common example is the pink flamingo
(DasSarma and Arora, 2001). Clausen et al. (2014) reported cyst formation in a marine
heterotardigrade, i.e., Echiniscoides sigismundi, which constitutes a cryptic species complex
present worldwide in tidal zones. Tardigrades are microscopic metazoans that can withstand
extreme environmental conditions by entering dormant states. They may also form cysts.
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Olukunle, O. F., & Oyegoke, T. S. (2024). The Potentialities of Extremophiles from Hypersaline Habitats- A Review. European Journal of Applied
Sciences, Vol - 12(3). 88-117.
URL: http://dx.doi.org/10.14738/aivp.123.17003
MICROBIAL DIVERSITY OF HYPERSALINE ENVIRONMENTS
In recent past, ecological studies and microbial communities of hypersaline environment
were mainly based on culture-dependent techniques otherwise known as
classical/traditional techniques. This method of identification of microorganisms includes
isolation, morphological and biochemical characterization, which forms the basis of
classification for bacterial genera (Cowan and Steel, 1993) and has been used for many
decades, however, with its limitations (Wahler and Reymond, 2001). Some microorganisms
that are still uncultured cannot easily be identified by conventional techniques (Kamagata
and Tamaki, 2005). Also, only about 1% bacteria can be detected from many environments
when traditional culture-based approaches are used (Pace,1997; Amann,2000; Kamagata
and Tamaki, 2005). As a result of the limitations associated with traditional techniques used
to identify microbes, new strategies and approaches are being implemented for the rapid,
sensitive and specific detection of microorganisms in the environment. The molecular
analyses have been found to be more appropriate than the traditional approaches. With the
advent of molecular approaches to studying microbial communities, there is increased
knowledge in the microbial diversity of hypersaline environment. Recent studies have
shown biodiversity to be higher in hypersaline environments than the reports obtained from
culture-dependent techniques. It is reported that Salinibacter ruber, a bacterial species
represents a high proportion of this environment. Although, aerobic archaea, member of the
Halobacteriales, are very abundant in the alkaline lakes, e.g. species belonging to the genera:
Halobacterium, Haloferax, Halorubrum, Haloarcula, as well as Natronobacterium or
Natronococcus (Arahal and Ventosa,2002). Only a few extreme halophiles are found to grow
in thalassic brine such as Halobacterium, Dunaliella, and a few species of bacteria (DasSarma
and Arora, 2001).
MICROBIAL ADAPTATION TO HYPERSALINE ENVIRONMENTS
The hypersaline environments are highly challenging in several ways, for instance, the low
availability of water which is an essential requirement for life and exposure to UV radiation
poses challenge to non-halophiles because the UV radiation damages the bonds between the
thymine molecules in their DNAs (thymine dimers). These challenges and many others, cause
only few biological activities to occur in these environments. However, the halophiles have
special ways by which they can adapt to this harsh environment. In this review, such
adaptations are discussed
Osmotic Pressure Balance
Permeability of water characteristic in biological membranes is evident in a non-halophile
found in a high-salt environment because water will exit the cell due to osmotic principle
(plasmolysis), and the resultant process will leave behind a highly concentrated cytoplasm
where proteins, nucleic acids and other macromolecules will definitely lose their structure
and function. But, bacteria adapted to living at low water activity may possess a cytoplasm
that has the same osmotic pressure as that of the salty medium. For turgor pressure of the cell
to be maintained (as displayed by all cells but with the possible exception of some halophilic
archaea of the family Halobacteriaceae), the intracellular osmotic pressure should even
exceed that of the extracellular environment (Oren, 2014). In order to achieve a balanced
osmotic pressure, halophiles make use of two distinct strategies which may depend on the
environment and or the genetic constitution of the bacteria. The combination of these two
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Olukunle, O. F., & Oyegoke, T. S. (2024). The Potentialities of Extremophiles from Hypersaline Habitats- A Review. European Journal of Applied
Sciences, Vol - 12(3). 88-117.
URL: http://dx.doi.org/10.14738/aivp.123.17003
halophiles. The investigators also reported that Halobacterium salinarum, an archaeon thrives
in such environments and that it is among the most UV-resistant of any organism. Hypersaline
environments are found deep in the earth’s crust and at the bottom of the deep sea, and high
pressure is known to significantly influence metabolism (Mcgenity and Oren, 2012).
Hypersaline acidic environments are rare, but Minegishi and Mizuki (2008) reported the
isolation and capability of the members Halobacteriaceae to grow between pH 4 and 6
describing the twin East Lake in Australia as both highly acidic (pH 3) and hypersaline (160 g
l
−1) and harbours many bacteria, such as Sphingomonads that are not typically found in
neutral lakes of equivalent salinity (Mormile and Hong, 2009). There are reports on microbial
activity in cold, hypersaline environments; the culture of moderately halophilic
psychrotolerant strains; and the psychrotolerant extreme halophile (Halorubrum
lacusprofundi) from Deep Lake, Antarctica (Franzmann et al., 1988). Although its temperature
optimum is 31−37°C, growth is still possible at temperatures as low as 4°C, and so it could
grow during the short periods when the temperature of Deep Lake increases from subzero to
a maximum of 11.5°C (Oren, 2002b). Many members of the Halobacteriaceae can grow up to
50°C, the most thermophilic halophiles are bacteria. For example, Halothermothrix orenii,
from a Salt Lake in Tunisia, grows anaerobically up to 68°C (optimum 60°C) and up to 200 g
l
−1 salt (Cayol et al., 1994). An isolate from Wadi A Natrun, Natranaerobius thermophilus, a
representative of a new order, the Natranaerobiales, grows optimally at 54°C (up to 56°C) at
pH 9.5, and at a total Na+ concentration between 3.3 and 3.9 M (range: 3.1 – 4.9 M) (Bower
and Mesbah, 2009; Oren, 2002b). A better understanding of the compounding and in some
cases off-setting effects of multiple extremes on organisms is needed if the frontiers of life
going to be identified and extended.
Pigmentation
Microbes are able to survive hypersaline environments by synthesizing some pigments. This
is necessary because of their exposure to ultraviolet radiations that can cause damage to their
DNA if not protected (Oren, 2008; Ghai et al., 2011). Therefore, rich array of carotenoid
pigments is synthesized by some halophilic microbes that live in these environments.
Halophilic archaea have been reported to contain among others, two types of pigments
namely bacteriorhodopsin and halorhodopsin (Fathepure, 2014). The former is a vehicle in
the export of hydrogen ions in the ATP generation process (this type of energy production
does not involve chlorophyll as found in plant or photosynthetic microbes). Exposure to UV
light is necessary for the activation of bacteriorhodopsin, a purple chromoprotein located
within the cell membrane, which acts as a proton pump and drives ATP synthesis (Song and
Gunner, 2014). The latter, on the other hand, exports hydrogen ions and import chlorine ions
into the cells. For example, bacteriorhodopsin that is reportedly found in the genus
Halobacterium, allows for energy production under reduced oxygen or anaerobic conditions
and high light intensities. This gives a competitive advantage against organism with aerobic
energy production mechanism (Baxter and Litchfield, 2005).
Halotaxism
This is a mechanism whereby sensitivity and response to salt concentration beyond halophilic
bacterial adaptation is developed by migrating to preferred saline (Holland, 2013). For
example, filamentous cyanobacterium Coleofasciculus (Microcoleus) Chthonoplastes is
reported to migrate from a less salt concentration to a higher concentration and required
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Olukunle, O. F., & Oyegoke, T. S. (2024). The Potentialities of Extremophiles from Hypersaline Habitats- A Review. European Journal of Applied
Sciences, Vol - 12(3). 88-117.
URL: http://dx.doi.org/10.14738/aivp.123.17003
Xylanase aids in the breakdown of xylan and is also employed in the production of coffee and
animal feed (Butt et al., 2008).
Production of Biomolecules
Bacteria, archaea, and fungi that grow optimally at high saline concentrations and are able to
produce biomolecules, are of pharmaceutical interests for therapeutic applications (Corral et
al., 2019). Macrolactins, polypeptides, polyketides, lipopeptides, and isocoumarins are among
the bioactive produced by halophilic bacteria as secondary metabolites (Hamdache et al.,
2011; Ladeira et al., 2015). These metabolites and compatible solutes are biomolecular or
stress protective agents during metabolic process (Bose et al., 2015). Halophilic and
halotolerant bacteria are novel sources of anticancer enzymes such as L-asparaginase and L- glutaminase. These enzymes possess the ability to inhibit acute lymphoblastic leukemia and
other cancer cells (Corral et al., 2019). A study on the cytotoxic effect of metabolites from a
halophilic fungal strain, Aspergillus sp. F1 reported the production of three compounds with
anticancer activity which include cytochalasin E, ergosterol, and rosellichalasin. It was also
observed that there was increased production of these bioactive compounds at higher salt
concentrations (Xiao et al., 2013).
Production of Biopolymers
Biosurfactants:
Halophilic microbes are producers of biosurfactants (Waditee-Sirisattha et al., 2016), of which
some have been documented to have broad-spectrum antibiotic characteristics (Kaya et al.,
2006; Alit-Susanta and Takikawa, 2006; Imadalou-Idreset al., 2013). Biosurfactants are
surface-active biomolecules with great applicability in the food, pharmaceutical and oil
industries. Endospore-forming bacteria, which survive for long periods in harsh
environments, are described as biosurfactant producers (Couto et al., 2015). It is employed in
the laundry industries because it reduces the surface tension in order to form stable
emulsions. Biosurfactants may be employed in oil recovery and emulsifiers in food industries.
Couto et al (2015) isolated several Bacillus strains from sediment and mud samples from
Vermelha Lagoon, Massambaba, Dois Rios and Abraão Beaches (saline environments) and
Seca salterns (hypersaline environments) with production of high stable emulsion-forming
biosurfactants.
Polyhydroxyalkanoates (PHAs):
PHAs are groups of naturally-occurring biopolyesters synthesized by various microorganisms,
in response to harsh environmental condition such as an unbalanced nutrient state, exposure
to toxicants, or hypersalinity (Tan et al., 2014; Koller, 2017). A single molecule of PHA is
typically made up of about 600 to 35,000 (R)-hydroxy fatty acid monomeric units with each
monomer harbouring a R group (side chain) which may be saturated, unsaturated, branched,
or a substituted alkyl groups, with the first being the predominant form and the later less
commonly occurring (Khanna and Srivastava, 2005; Lu et al., 2009). Although PHAs are
reported to have similar properties to those of common petrochemical-based synthetic
thermoplastics, their peculiarity is their ability to become completely mineralized to carbon
dioxide and water under aerobic conditions and to methane and carbon dioxide under
anaerobic conditions by diverse populace of microorganisms in the environment (Anderson
and Dawes, 1990; Lee, 1996).
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BIOTECHNOLOGICAL APPLICATIONS OF HALOPHILES
Agriculture
Today, the production of food is a major concern in many parts of the world. The failures in
ecological pressures that affect global agriculture productivity can be classified as biotic and
abiotic. Plant diseases are one type of stress (Maheshwari 2013) and others such as salinity,
drought, nutrient deficiency and temperature are abiotic stresses (Atkinson and Urwin 2012).
The use of chemical fertilizers and pesticides to combat these stresses, though successful has
proven to be non-ecofriendly. These chemical fertilizers, pesticides and herbicides have a
deleterious effect on the nutritional value of foods as well as on the health of farmers and
consumers. Microbial inoculants that can act as biofertilizers, bioherbicide, biopesticides, and
biocontrol agents can be employed as alternative solutions to the problems created by
synthetic chemicals. They can be applied to the soil or the plant in order to improve crop
productivity and health (Alori and Babalola, 2018).
A newer method of chemical fertilizers and pesticides is the use of genetically modified
organisms (GMOs) including crops. However, controversial reports of possible health and
ecological consequences as a result of the consumption of these GMOs have been documented.
Moreover, the world's cultivable land could not yield efficiently due to the high salinity of the
soil (Selvakumar et al. 2014). Consequently, an alternative to chemically enhanced increasing
agricultural output and reducing the negative consequences of salt stress can be through the
use of beneficial microbes for agriculture (Strap, 2011). Bacteria known as plant-growth
promoting rhizobacteria (PGPR) are found in the rhizosphere or in plant tissue as endophyte
which promotes plant growth directly by the synthesis of hydrogen cyanide (Voisard et al.,
1989), secretion of antibiotics, extracellular cell wall hydrolyzing enzymes or siderophore,
removal or detoxification of organic and inorganic pollutants, stress tolerance, and or
indirectly by competing for spaces or nutrients (Nadeem et al., 2013). Application of
halotolerant plant growth promoting rhizobacteria (PGPR) such as Micrococcus sp. on cowpea,
Brachybacterium saurashtrense on groundnut (Dastager, 2010) in saline soil could promote
plant growth through the degradation and detoxification of soil pollutants (Jha et al., 2012).
Plant-growth promoting rhizobacteria (PGPR) which are halotolerant such as Bacillus pumilus,
Pseudomonas mendocina, Arthrobacter sp. have been isolated by Tiwari et al., (2011).
Salinization of arable land due to the constant accumulation of harmful ions in the soil is a
growing concern, harming the world's most productive agricultural zones. Current crops are
salt-sensitive and traditional breeding has failed to improve their salinity tolerance.
Halophytes can be grown in salt-contaminated soil for soil desalination. Salt tolerance can
therefore be achieved by genetically engineering halophylic enzymes encoding DNA into
crops. In both aerobic and anaerobic conditions, halophylic bacteria can promote the
biodegradation of some harmful organic and inorganic substances. In hyper saline conditions,
wastewater with high amounts of such substances is treated with these
bacteria. Halophylic bacteria are used in the bioremediation of heavy metals such as cadmium,
mercury, arsenic, and others from wastewater. These potential applications of halophytes in
phytoremediation, bioremediation, and desalination can lead to the rehabilitation of saline
and degraded lands, paving the way for sustainable agriculture.
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Sediment sample
collected in the
Japanese Sea
Verrucosispora
strain AB 18-032
Multidrug-resistant
(MDR) bacteria
Abyssomicin C Bister et al.
(2004)
vancomycin- resistant S. aureus
Salt pan environment
of Kodiakarai,
Vedaranyam,
Nagapattinam, and
Tamilnadu, India.
Streptoverticillium
album
S. aureus, NR Gayathri et al.
K. pneumoniae, (2011)
E. coli
hypersaline
cyanobacterial mat
Human pathogens NR Abed et al.
Halomonas sp. SK- (2013)
1
Dunaliella salina
Marine sediment, La
Jolla, California
Streptomyces sp.
CNQ-418
Methicillin-resistant
S. aureus (MRSA)
Marinopyrroles A Hughes et al.
Marinopyrroles B (2008)
Mangrove sediment,
Nizampatnam,
Andhra Pradesh,
India
Pseudonocardia
VUK-10
S. aureus, NR Mangamuri et
S. mutans, al. (2012)
B. subtilis,
E. coli,
E. faecalis,
P. aeruginosa
Mucus secreted by
the box- fish
Ostracion cubicus,
Israel
Vibrio
parahaemolyticus
B2
S. aureus, Vibrindole A Bell et al.
S. albus, (1994)
B. subtilis
Sediment of the
Lagoon de Terminos
at the Gulf of Mexico
Streptomyces
B8005
Streptomyces
B4842
E. coli,
S. aureus,
S. viridochromogenes
Resistomycin 1-Hydroxy-1-
norresistomycin
Kock et al.
(2005)
Resistoflavin methyl ether
Khewra Salt Range,
Punjab, Pakistan
Aquisalibacillus
elongatus MB592,
Salinicoccus sesuvii
MB597,
Halomonas
aquamarina
MB598
B. subtilis,
B. pumilus,
B. cereus,
K. pneumoniae,
Alcaligenes faecalis,
P. geniculata,
E. faecium
E. faecalis,
NR Fariq et al.
(2019)
Saline soil of Kovalam
solar salterns, India
Nocardiopsis sp.
AJ1
E. coli,
S. aureus,
A. hydrophila,
V. parahaemolyticus,
P. aeruginosa,
Pyrrolo (1,2-A (pyrazine-1,4-
dione, hexahydro-3-(2-
methylpropyl)-)
Adlin Jenifer et
al. (2019)
Actinomycin C2
Water samples Asen
fjord in the
Trondheim fjord and
Steinvikholmen,
Norway
Streptomyces sp. Gram-negative and
Gram-positive
bacteria
NR Hakvåg et al.
(2008)
Hypersaline soil,
Xinjiang, China
Nocardiopsis gilva
YIM 90087
B. subtilis,
S. aureus
p-Terphenyl: 6’-Hydroxy- 4,2’,3’,4’’-tetramethoxy-p- terphenyl
p-Terphenyl
Shou-zheng
Tian et al.
(2013)
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