Extremophile – An Adaptive Strategy for Extreme Conditions and Applications

Abstract

The concurrence of microorganisms in niches that are hostile like extremes of temperature, pH, salt concentration and high pressure depends upon novel molecular mechanisms to enhance the stability of their proteins, nucleic acids, lipids and cell membranes. The structural, physiological and genomic features of extremophiles that make them capable of withstanding extremely selective environmental conditions are particularly fascinating. Highly stable enzymes exhibiting several industrial and biotechnological properties are being isolated and purified from these extremophiles. Successful gene cloning of the purified extremozymes in the mesophilic hosts has already been done. Various extremozymes such as amylase, lipase, xylanase, cellulase and protease from thermophiles, halothermophiles and psychrophiles are of industrial interests due to their enhanced stability at forbidding conditions. In this review, we made an attempt to point out the unique features of extremophiles, particularly thermophiles and psychrophiles, at the structural, genomic and proteomic levels, which allow for functionality at harsh conditions focusing on the temperature tolerance by them.

1. INTRODUCTION

The earth has a wide area of niches and ecosystems that are generally not habitable and referred to as extremes where microbial life systems sustain that need immediate attention of research and development. Organisms, commonly called extremophiles, thrive best under these extreme environmental conditions and represent the unique adaptability of primitive life-forms [1]. Discussing particularly about thermophiles and psychrophiles, thermophiles are the organisms accustomed to its survival at uplifted temperature conditions. In contrast, psychrophiles or cryophiles are the organisms that proliferate at frigid temperature conditions. These organisms are difficult to study because of the isolation, maintenance and preservation at their optimum extrinsic temperatures. It is important to mention that irrespective of the temperature variations, all the macromolecules of the cell such as DNA, RNA, proteins, etc. should be operational and stable. One of the exciting features of these magical extremophiles lies hidden in the enzymes they secrete. The greater intrinsic stability of extremozymes than the enzymes isolated from its mesophilic counterparts makes them wonderful candidates to be used for application based industrial work. Although the genomic, proteomic and molecular basis of their stability is not fully understood, significant research is underway to explain their mechanism of adaptability to variations in temperatures [2]. Many reports in this area have suggested that the factors responsible for the stability could be various bondings and interactions such as H-bond [3, 4], covalent bond, the composition of amino acid [5], the G+C content [6], tRNA composition and its folding patterns [7]. Pioneer efforts of some early research [8, 9] have made insight into the study of the factors responsible for the thermostability of proteins. The complete genome sequences of many hyperthermophilic archaeons, thermophiles and psychrophiles have already been reported, thus determining the aspects of thermophilicity and psychrophilicity [10]. It has been studied that the structural stability of an organism depends on the wide range of interactions [11], such as hydrogen bonding, hydrophobic and van der Waals [10]. To adjust the thermostability, various proteins use certain combinations and permutations of these interactions [12], though these interactions are sometimes temperature-dependent. The stability of proteins in extremophiles is seen primarily through changes in the residues of amino acid. It has been observed that on the molecular surface of proteins charged residues (i.e., Glu, Arg and Lys) increase in thermophiles, while the same is in lesser quantity in case of psychrophiles [13]. The subsequent rise in Gly residues in psychrophiles increases protein stability. Genomic studies on extremophiles revealed that the structural composition of DNA [14] and the arrangements of codon-anticodon interactions [15] could be the promising stability factors for their stability. tRNA sequence and its folding patterns in the complete genome of an organism could be co-related with its optimum growth temperature [7] and hence would account for organism thermostability. G+C content in the tRNA of extremophiles has also been investigated for analyzing the differences associated with stability at extremes of temperature [6, 16]. Experimental evidence has suggested that the enhanced stability of proteins and cell membranes of thermophiles or stability of unique cold shock proteins and cell membrane/ cell wall of psychrophiles could depend on interactions within proteins, DNA or on their structural stability [17-20] (Fig. 1). Thus, it can be said that there exists an interesting relationship between OGT and various intra- and intermolecular interactions [12].

Fig. (1).

Stability pattern of thermophiles vs psychrophiles. (A higher resolution / colour version of this figure is available in the electronic copy of the article).

Many biochemical mechanisms can also be related to the existence of extremophiles to adverse conditions, some of which are involvement of lipids and membranes, various stabilizing factors, rapid resynthesis of heat-inactivated molecules as in case of thermophiles, hyperthermophiles or halothermophiles, allostery and highly charged macromolecular environment [21].

Environmental stress (high, low or moderate) also controls the constitution of lipid membranes in the extremophiles as well as non-extremophiles. Koga, Y. (2012) reviewed that the proportion of some lipids increases with the increase in temperature. It was investigated in his report that in thermophiles and archaea ester linkages are not heat resistant, whereas the ether lipids are stable at high temperatures. Moreover, on the increase in the growth temperature, isofatty acids increased, whereas on lowering the temperature, anteiso fatty acids increased in the case of Bacillus spp. [22]. As lipids generally function in the membrane as a group of molecules and not as a single molecule, it is reported that on changing the temperature, the organisms adapt by changing the acyl or alkyl side chain of their lipid membrane rather than changing to different lipid class [23].

As mentioned above, thermostable biomolecules have found notable importance in the field of biotechnology; and this has spurred research into an organism’s capability to thrive best at exotic temperatures [24, 25] (Fig. 2). Extremozymes have been employed in diverse industrial applications, some of which are starch processing, cellulose degradation, leather, baking and detergent industry [17]. These organisms also connect to the primitive forms of life present billion years before on our planet and help to study the evolutionary process [26].

Fig. (2).

Applications of extremophiles. (A higher resolution / colour version of this figure is available in the electronic copy of the article).

2. STRUCTURAL DEPENDENCY ON EXTREMOPHILE STABILITY

Structure and protein sequence studies of extremophiles relate to its stability in comparison to its non-extremophilic counterparts. The sequence generally alters as a part of evolution when the organism inhabits different habitats. As explained in the introduction, various structural factors such as hydrophobicity [10], lengthening or shortening of loops, the pattern of atom packing, increased or decreased surface area, hydrogen bonding [4] and salt bridges [10] attribute to the structural stability of an organism (Fig. 1). The Boltzmann factor defines the strength of any type of interaction relates to the organism’s stability as analyzed by Miyazawa and Jernigan (1985), the advantage of this approach is that the large database of the structures and the strength of temperature-dependent interactions could be studied [27].

For studying the stability in extremophilic protein, hydrophobicity is one of the main stabilizing factors in proteins. It has a direct relationship with the enthalpy and entropy of the system, as, at room temperature where hydrophobicity is found to be maximum, the solvent molecules that are in contact with the non-polar molecules organize themselves in such a way that the enthalpy of the system is reduced and hence the entropy of the solvent decreases [28]. The enthalpy and entropy together contribute to the free energy of stabilization, which has a relationship with the optimum growth temperature of an organism [29]. The relevance of entropy and enthalpy in thermophiles and psychrophiles is mentioned below.

2.1. Thermophiles and Stability

When the temperature is increased above room temperature, as in the case of thermophilicity, hydrophobicity is one of the dominant factors that stabilize proteins of thermophiles [12]. The stability can be provided by a hydrophobic factor of a single protein or due to multiple protein chains. Razvi and Scholtz (2006) found that most thermophiles use the simple method that raises the ΔG at all temperatures as the principal way to increase their melting temperature [30] (Fig. 1). Perutz and Raidt (1975) compared the structure of ferredoxin from the thermophiles and mesophiles and found that in between the polar groups, extra salt bridges were present in the structure of ferredoxin from thermophiles. Besides this, there were more side chain-hydrogen bonds in thermophilic ferredoxin that jointly increased stability [8]. In glutamate dehydrogenase isolated from thermophile Pyrococcus furiosus, salt bridges form a highly stable network and account for the stability of the protein in contrast to the glutamate dehydrogenase from non-thermophile Clostridium symbiosum, where the salt bridges form fewer networks [10]. It has been reported that deleted or shortened loops [31], greater rigidity, small surface area to volume ratio, more disulphide bonds, increased intracellular ionic concentrations, increased cationic proteins and supercoiling are proposed mechanisms of increased thermostability [32].

2.2. Psychrophiles and Stability

Similar to thermophiles, psychrophiles also have a sequence and structure relationship. The entropy of the system decreases with the decrease in temperature, explaining the phenomenon of psychrophilicity. For adjusting membrane fluidity in psychrophiles, there should be the presence of unsaturated and branched fatty acids chains, and the length of fatty acids has to be shortened [33]. Psychrophiles synthesize heat-labile and cold-active enzymes that have high enzymatic efficiency and activity at low temperatures. Experimental evidence proves that psychrophilic enzymes catalyze the reaction at lower ΔG values than the reaction catalyzed by its mesophilic counterpart, suggesting that psychrophilic enzymes are more active [34]. It is also found that the active site for the binding of enzymes in the case of psychrophiles is heat-labile in comparison to the mesophilic enzymes. Another assumed structural adaptability for the catalytic action of the psychrophilic enzyme is that the catalytic cavity of psychrophile is comparatively more than mesophiles or thermophiles by deletions of certain residues in loops bordering active site [35] or by the replacement of bulky side

chains with smaller groups [36], thus making the active site more accessible to ligands. Papaleo et al. (2006) noted that the sites of cold stable enzymes not responsible for catalysis are more rigid than the mesophilic homologs [37]. All the weak interactions such as hydrogen bonds, proline content, ion pairs, etc. are minimally found and have non-polar core clusters, with weaker hydrophobicity to make the protein interior less compact.

3. AMINO ACID COMPOSITION AND EXTREMOPHILE STABILITY

The composition of amino acid, as reported by several studies, is linearly related to the OGT of an organism [38]. It has been found that thermophilic proteins prefer to contain charged, aromatic and hydrophobic residues compared with its mesophilic proteins. Goldstein, R.A. (2007) analyzed the database of proteins for studying the interaction of amino acids from psychrophiles, thermophiles and hyperthermophiles [12]. When analyzed on the Ramachandran plot, the authors reported that the amino acids of the thermostable protein occur more on the helical and the sheet region [39].

3.1. Thermophiles and Stability

Jaenicke and Bohm (1998) studied the relationship of amino acids and stability of protein at extrinsic temperature. They observed in their study that when thermophiles/hyperthermophiles are kept at a temperature of 100°C or more, Val and Leu are the more thermostable amino acids followed by Ile, Tyr, Lys, His and apparently Asp, Glu, Arg and Cys are the least thermostable of all the common amino acids. They also compared the genome of thermophiles with the specific genes from mesophiles, and it was found that the thermophilic genome encodes for higher charged amino acids and lower polar/uncharged residues. They also observed that at a higher temperature, deamidation of glutamine increased [40]. Goldstein, R.A. (2007) supported the study of Jaenicke and Bohm (1998) and reported that a marked increase in the frequency of aromatic and charged amino acids (Tyr, Phe, Glu, Lys and Arg) was found in thermophiles and hyperthermophiles whereas the frequency of uncharged residues significantly decreased (Gln, Asn, His, Thr) [12]. Similar results were obtained by other workers [41, 42]. Zeldovich et al. (2007) also observed a significant rise in the presence of Val, Tyr, Trp, Arg, Glu and Leu in thermophiles [5]. The increase in the frequency of charged residues defines the role of salt bridges in protein stability and its dielectric properties in the case of extremophiles [43]. Russell et al. (1994) studied the comparison of the amino acid of citrate synthase from Thermoplasma acidophilum which is a thermophile and Pyrococcus furiosus, a mesophile and found that as optimum temperature increased Ile, Tyr, Lys and Glu content increased, but Asn, Gln and Cys residue decreased [35].

3.2. Psychrophiles and Stability

The frequency of uncharged polar residues is declined in psychrophiles. Unlike thermophiles, a lesser number of charged and aromatic residues are observed in psychrophiles. Violot et al. (2005) observed that when the strength of various interactions is decreased, the flexibility of psychrophilic protein increased [44]. The subsequent rise in the frequency of Gly is notified, which increases the stability of the protein by raising the entropy of the protein system. Metpally and Reddy (2009) found that Ala, Asp, Ser and Thr were preferred significantly while Glu and Leu were less preferred in psychrophiles when compared to mesophilic counterparts [45]. If the frequency of occurrence and alignment of proteins are compared in psychrophiles, it is reported that over aromatic, charged and hydrophilic groups, the tiny and neutral groups of amino acids are preferred. While considering the secondary structural elements, it is analyzed that α-helices of psychrophilic proteomes have fewer residues in comparison to coil regions. Psychrophiles, when grown at low temperature, show the production of cold-acclimation proteins (CAPs) whereas no CAPs are seen when grown at a milder temperature [46]. These CAPs function in maintaining cell cycle and growth of the organism at low temperature, though their functions are not yet fully understood.

4. GENOMIC ADAPTATIONS FOR EXTREMOPHILE STABILITY

Comparative genomics studies on thermophiles, psychrophiles and mesophiles disclose that a series of co-ordinated changes are linked with an organism’s genome thriving at extreme conditions. OGT of an organism could be correlated with the arrangements and frequency of the presence of tRNAs, the G+C content of tRNA genes, secondary structures and its folding patterns. As studied before, the sequence and structure data study for any bacteria (be it hyperthermophile, thermophile, mesophile or psychrophile), revealed proportional linkages between OGT and DNA’s dinucleotide compositions.

4.1. Thermophiles and Stability

It is known that the tRNA content of thermophilic organisms is less as compared to the frequency of tRNAs in mesophiles and psychrophiles (Fig. 1). Dutta and Chaudhuri (2010) reported that tRNAs that were found to be reduced in number amongst thermophiles had the anticodons of hydrophilic (i.e., Asp, Gln, Tyr, Val, Asn, Gly) and few had hydrophobic residues (i.e., Met, Ile, Leu) [7]. Though there was no increase in the number of tRNA amongst any thermophile studied, some tRNAs did not alter (Ala^-, Ser^-, Thr^-, Cys^-, His^-). tRNAs of thermophiles showed greater structural stability than psychrophiles and mesophiles [47]. Experimental results with a group of psychrophilic and thermophilic genomes confirmed that there was a directly proportional correlation between the G+C content of tRNAs and OGT of an organism, though thermophilic genes corresponding to tRNA have been reported to exhibit a much higher G+C content in comparison to psychrophiles and mesophiles [14]. With the increase in temperature, tRNA of psychrophiles tends to fold its structure in such a way that it results in increasing more loops than stems whereas thermophiles remain folded into a structure that is stable at all temperature ranges, and this tendency of extremophiles might be a reason for the stability of RNA’s secondary structure at adverse temperatures. Moreover, important to note that the G+C content of ribosomal RNA was linearly correlated with growth temperature, while the G+C content of genomic DNA hardly shows any difference in hyperthermophiles, thermophiles as well as in mesophiles [7, 14]. It is believed that the higher G+C content in

thermophiles might be a mechanism by which the organism facilitates intramolecular stabilization of the RNA secondary structure. As discussed, a direct connection subsists between DNA’s dinucleotide compositions and growth temperature, so changes in dinucleotides are way good than alterations in mononucleotides (G+C content). The reason for this is that altering the dinucleotide sequence may produce different sets of DNA sequences, those codes for amino acids [14]. It is also proposed that all the genes and proteins might acquire stability toward temperature in bacteria, and for this, the dinucleotide composition of DNA would be biased in such a direction that an overall increase in charged residues occurred at the protein level.

When the uracil content of 16S rRNA was considered, it was found that it had an inverse correlation with OGT, i.e., as the temperature increased, A:U base pair content of RNA decreased [48]. Thus, 16S rRNA’s uracil content acted as a useful reader of OGT in thermophiles and psychrophiles. It is found that rRNA stems also had some mismatched base pairs and on the thermodynamic grounds, as the temperature increased, G:U mismatches that are less stable are selected

over A:U pairs, which are more stable [49]. Studies have been conducted to see if changing the content of uracil has any relationship with rRNA stability as RNAs show sensitivity to chemical hydrolysis at a temperature of more than 50ºC [50]. For the stability of RNAs at low temperature, Dalluge et al. [51] observed various modifications that maintained the conformational flexibility of RNA.

4.2. Psychrophiles and Stability

In psychrophiles, it is found that the tRNA content is higher in comparison to its thermophilic counterpart (Fig. 1). As the temperature of the psychrophilic organism is increased, its tRNA folds in an unstable structure that has more loops than stems. It is also observed that the low temperature strengthens the relationship between the double helix and the supercoiled state of DNA. The presence of nucleic acid-binding protein, which plays a central role in relieving the adverse effects of low temperature on psychrophiles, has been analyzed [51, 52].

5. APPLICATIONS REVEALED BY EXPLORING EXTREMOPHILES

The isolation and characterization of the extremophilic prokaryotes have proven that these organisms have engrossing metabolic features and fascinating evolutionary past [53]. The novel methods for extremophile’s isolation, characterization and the utilization of molecular tools for analyzing and understanding their phylogeny and diversity paved the way for searching organisms with diverse applications in research, health, evolution and biotech industries (Fig. 2).

Lately, a novel perspective towards genome sequencing has opened the doors for the investigation of these magical organisms for various academic, researches, industrial and biotechnological applications. Most organisms, specifically pointing hyperthermophiles and thermophiles, inhabit closely to “universal ancestor” of all extant of life on Earth. Thus, their origin under stressed conditions would give useful hints on the evolution of our planet and, likewise, others in the universe. Extremophiles, as explained in the introduction, produce extremely stable enzymes known as extremozymes, whose catalytic properties are used in conditions that were initially nominated as unsuited and harsh [54]. Extremozymes are of particular interest in several industrial processes like baking, saccharification [55], detergent, pharmaceutical, food, beverage or textile industries [56] (Fig. 2). These enzymes are evolved by nature and do not require manipulations to get adapt to temperature, solvent-tolerance, or other extremities in comparison to enzymes tailored synthetically from non-extremophiles. Some examples of enzymes isolated from thermophiles and psychrophiles, along with their sources and the industries wherein they are used, are listed in Tables 1 and 2, respectively.

Table 1. Enzymes isolated from thermophilic bacteria and their industrial applications.

Enzyme	Industries Wherein Enzyme is Used	Bacterial Species	References
Lipase	Baking industry, Cosmetic industry, Dairy industry, Detergent industry, Leather industry, Paper industry, Pharmaceutical industry, Pulp industry	Anoxybacillus flavithermus WK1 Geobacillus stearothermophilus 5 Geobacillus sp. SBS-4S Geobacillus zalihae sp. Nov Pseudomonas aeruginosa BTS-2 Thermosyntropha lipolytica	[60] [61] [62] [63] [64] [65]
Protease	Baking industry, Brewing industry, Dairy industry, Detergent industry, Food and Feed, Pharmaceutical and Biotech industry	Bacillus brevis Bacillus HUTBS62 Bacillus HUTBS71 Bacillus sp. JB-99 Bacillus stearothermophilus Chaetomium thermophilum Geobacillus collagenovorans MO-1 Paenibacillus tezpurensis sp. nov. AS-S24-II Pyrodictium sp. Thermococcus onnurineus NA1 Thermus aquaticus YT-1	[66] [67] [68] [69] [70] [71] [72] [73] [74] [75]
Xylanase	Baking industry, Bioprocessing of fabrics, Biobleaching of pulp, Detergent industry, Paper industry, Pulp industry, Waste paper recycling industry	Actinomadura sp. strain Cpt20 Anoxybacillus kaynarcensis sp. Bacillus halodurans Bacillus sp. Caldocellum saccharolyticum Dictyoglomus thermophilum Dictyoglomus thermophilum Rt46B.1 Geobacillus sp. MT-1 Thermoanaerobacterium saccharolyicum NTOU 1 Thermomyces lanuginosus	[76] [77] [78] [79] [80] [81] [82] [83] [84] [85]
α- Amylase	Bakery industry, Cellulose and chitin processing industry, Detergent industry, Textile industry	Bacillus sp. isolate A3-15 Bacillus thermooleovorans NP54 Bacillus stearothermophilus Geobacillus stearothermophilus Halothermothrix orenii Streptomyces sp. TO1	[86] [87] [88] [89] [90] [91]
β- Glucosidase	Biofuel industry, Biorefining industry, Brewing industry, Chemical industry	Aureobasidium pullulans Bacillus thuringiensis Fervidobacterium islandicum Thermoanaerobacter brockii	[92] [93] [94] [95]
Enzyme	Industries Wherein Enzyme is Used	Bacterial Species	References
Chitinase	Agriculture industry, Chitin modification for food and health products	Aeromonas sp. DYU-Too7 Bacillus licheniformis Bacillus sp. HSA,3-1a Bacillus sp. Hu1 Microbispora sp. V2 Ralstonia sp. A-471 Silanimonas lenta Streptomyces roseolilacinus	[96] [97] [98] [99] [100] [101] [102]
Cellulase	Agriculture industry, Animal Feed industry, Bioethanol industry, Brewing industry, Paper processing industry, Pulp industry, Textile industry, Wine industry	Acidothermus cellulolyticus Clostridium thermocellum Clostridium thermocellum Cel9I Geobacillus pallidus Moorella sp.F21 Rhodothermus marinus Thermobifida fusca Cel9A	[103] [104] [105] [106] [107] [108] [105]
Esterase	Detergent industry, Food industry, Paper industry, Pharmaceutical industry	Alicyclobacillus acidocaldarius Anoxybacillus gonensis A4 Anoxybacillus gonensis G2 Bacillus circulans Caldocellum saccharolyticum Fervidobacterium nodosum Rt17-B1 Thermobacillus xylanilyticus Thermus sp. P1074 Thermus thermophilus HB27	[109] [110] [111] [112] [113] [114] [115] [116] [117]
Urease	Agriculture industry, Automobile industry, Biofuel and Chemical industry, Crop, Biorefinig	Bacillus sp. strain TB-90 Campylobacter laridis Campylobacter larus Campylobacter sputorum biovar paraureolyticus Campylobacter sp. Streptococcus sp.	[118] [119] [120] [121] [122] [123]

Table 2. Enzymes isolated from psychrophilic bacteria and their industrial applications.

Enzyme	Industries Wherein it is Used	Bacterial Species	References
Lipase	Baking industry, Cosmetic industry, Dairy industry, Detergent industry, Leather industry, Paper industry, Pharmaceutical industry, Pulp industry	Acinetobacter calcoaceticus LP009 Acinetobacter sp. RAG‐1 Moraxella sp. Pseudomonas aeruginosa NCIM 2036 Pseudomonas sp. B11-1 Psychrobacter glacincola Psychrobacter immobilis B10 Psychrobacter sp. TA144 Psychrobacter okhotskensis	[124] [125] [126] [127] [128] [129] [130] [131] [132]
α-Amylase	Bakery industry, Cellulose and chitin processing industry, Detergent industry, Textile industry, Wine industry	Aeromonas veronii NS07 Alteromonas haloplanktis Pseudoalteromonas haloplanktis Pseudoalteromonas haloplanktis TAC 125	[133] [134] [135, 136] [137]
Protease	Baking industry, Brewing industry, Dairy industry, Detergent industry, Food and feed, Pharmaceutical and Biotech industry	Alteromonas sp. Bacillus sp. Bacillus sp. 158 Clostridium sp. LP3 Colwellia sp. NJ341 Clostridium schirmacherense Flavobacterium YS-80 Halomonas sp. Pseudoalteromonas sp. NJ276 Pseudomonas strain DY-A Pseudomonas fluorescens 114 Streptomyces sp. Rheinheimera sp.	[138] [126] [139] [140] [141] [140] [142] [138] [141] [143] [144] [145] [138]
β-galactosidase	Biofuel, Biorefining, Brewing industry, Chemical industry	Arthrobacter psychrolactophilus Arthrobacter sp. SB Bacillus sp. Bacillus subtilis KL88 Carnobacterium piscicola BA Paenibacillus sp. strain C7 Pedobacter cryoconitis Pseudoalteromonas haloplanktis Shewanella atlantica Shewanella canadensis	[146] [147] [54] [148] [149] [150] [151] [152] [153]
Enzyme	Industries Wherein it is Used	Bacterial Species	References
Xylanase	Baking industry, Bioprocessing of fabrics, Biobleaching of pulp, Detergent industry, Fruit juice processing, Paper industry, Pulp industry, Waste paper recycling industry	Clostridium strain PXYL1 Flavobacterium sp. Flavobacterium frigidarium Glaciecola mesophila KMM 241 Paenibacillus curdlanolyticus B6 Paenibacillus sp. KIJ1 Pseudoalteromonas haloplanktis	[154] [155] [156] [157] [158] [159] [160]
Cellulase	Agriculture industry, Animal feed industry, Bioethanol industry, Brewing industry, Fruit juice processing, Paper processing industry, Pulp industry, Textile industry, Wine industry	Arthrobacter sp. Cadophora malorum Fibrobacter succinogene Flavobacterium sp. Geomyces sp. Paenibacillus sp. Pedobacter sp. Pseudoalteromonas haloplanktis Rhodotorula glutinis Shewanella sp. G5	[161] [162] [163]
Chitinase	Chitin modification for food and health products	Aeromonas veronii CD3 Alteromonas sp. strain O-7 Arthrobacter sp. TAD20 Glaciozyma antarctica PI12 Moritella marina Verticillium lecanii A3 Vibrio sp. Fi:7	[164] [165] [166] [167] [168] [169] [170]
Esterase	Detergent industry, Food industry, Paper industry, Pharmaceutical industry,	Oleispira antarctica RB8 Oleispira antarctica Pseudoalteromonas arctica Pseudomonas sp. B11-1 Psychrobacter sp. Ant 300 Streptomyces coelicolor A3	[171] [172] [173] [174] [175] [176]

Extremophiles have also shown applications in the production of single-cell protein (SCP), bioremediation, petroleum industry, biomining, biosensors, medicines and antibiotic production. Raja et al., (2010), isolated Dactyl sporangium from Rohtang hill soil and found that it produces a proteinaceous substance, which showed antimicrobial activity against streptomyces and thus acted as an antibiotic [57]. The species of Bacillus and Geobacillus that can degrade hydrocarbon can be effectively used for bioremediation.

FUTURE WORK AND CONCLUDING REMARKS

In summary, current knowledge of extremophiles is scarce, and very few of them are cultivable in the defined environment. More development in gene expression studies, like developing new heterologous systems, will increase the investigation of microbial diversity [58]. More novel extremozymes with diverse catalytic activity can be isolated, characterized and purified if their gene expression libraries are screened with fast and accurate detection technologies. Studies are going on to discover microbial communities in the environment, once considered to be harsh for any form of life to exist [59]. With the advancements of new techniques, remote areas such as ice-covered ocean and deep-sea could be explored; organism could be tapped at structural, genomic and proteomic levels, and could be exploited to provide a precious resource for biotechnology and industry.

LIST OF ABBREVIATIONS

OGT

Optimum Growth Temperature

DNA

Deoxy Ribonucleic Acid

RNA

Ribonucleic Acid

CONSENT FOR PUBLICATION

Not applicable.

FUNDING

The study was supported by the Department of Science and Technology (DST) SERB (Grant no. PDF/2017/001367) and DST-FIST (Grant no. SR/FST/LSI-590/2014), Govt. of India.

CONFLICT OF INTEREST

The authors declare no conflict of interest, financial or otherwise.