Keywords

13.1 Ecological Diversity of Cold-Adapted Microorganisms

Low temperature suits best to psychrophiles for their growth and reproduction. The cold-adapted microorganisms are present at higher altitudes and in deep blue seas where the temperature is below 15 °C. Though these ecosystems are too harsh for survival, still diverse microbial communities survive facing all the challenges with the help of adaptations at various levels. The challenges include availability of nutrients, IR radiations, excessive UV radiations, change in pH and very high osmotic pressure (De Maayer et al. 2014). Psychrophiles may be autotrophic, chemotrophic or heterotrophic based on their mode of nutrition. They play an important role in nutrient turn over and production of biomass in low-temperature environments and have significant role in various industries. Psychrophilic microbes find their importance in food preservation and degradation of organic matter in low temperatures where artificial cold environmental conditions are generated in vitro. Ironically most of the food spoiling bacteria are adapted to man-made cryo-environments. Pseudomonas, Psychrobacter, Staphylococcus and Photobacterium have often been isolated from psychrotolerant bacteria in artificial cold environments. Some of the factors that govern the existence of the genera in artificial cold environments have been summarized below:

  • Possibility of shift of ambient environment to cold environment

  • Possibility of invasion by their own basic cellular and molecular components

  • Ability to propagate rapidly

  • Presence or absence of oxygen at low temperatures

These genera are considered to be genetically diverse and have mechanisms for adapting to cold environments.

In addition to the above attributes, marine psychrophiles have cell membranes made up of lipids that do not harden in cold environment. Moreover, the presence of catalase atoms in psychrophiles help them to adapt to the natural conditions (higher concentration of hydrogen peroxide at low temperatures) under which these microbes endure. Three types of psychrotolerant H2O2-safe microscopic organisms have been disengaged from channel reservoirs of a fish egg preparing plant that utilizes H2O2 as a fading operator. Certain varieties of obscure bacterial species exist with specific varieties of natural adjustment systems (e.g., enzymatic efficiency of catalase and its cellular localization) contingent upon the natural H2O2 focus and delicacy of cells. Therefore, it is really hard to surmise general rules that may clarify the limit with respect to numerous psychrophiles to adjust their genomic and metabolic highlights to their local cold natural surroundings. The physiological studies of individual strain of proteins and genes show high level of psychrophilic adaptation (Rodrigues and Tiedje 2008; Casanueva et al. 2010). Various omics technologies have been utilized to ponder different capacities in microorganisms developed under various cold temperatures (Allen et al. 2009; Fondi et al. 2016). These adaptations work in a synergistic way at both genomic and metabolic levels to help the microorganism lead a smooth life in cold environment (Math et al. 2012). One of the example is the adenylate cyclase present in the cell membrane which gets activated at low temperature, aiding in smooth functioning of metabolic pathways. Various such cold adaptations will be discussed in detail in this chapter.

13.2 Effects of Low Temperature on Microbes

Low temperature can affect the microorganism in various ways. Reduction in growth rate and number of cell, variation in cell composition and nutritional requirements are some direct effects while other indirect effects include solute solubility, cell density, nutrient distribution and osmotic adjustment of the membrane. Microbes sense the decrease in environmental temperature with the help of their cellular responses like stiffness in their membrane, which is a very important membrane-associated sensor. Cold signal transduction pathway in microorganism is a two-component system. The signal with the help of sensors reaches the response regulator which in turn upregulate the genes involved in membrane fluidity in cold-adapted microorganism. The lipid bilayer maintains the cell permeability and transportation of essential solutes in liquid–crystalline phase. When temperature decreases, the functional phase of lipid transits into gel form due to which membrane fluidity is lost. Gene for fatty acid desaturases includes membrane lipid and protein phosphorylation and dephosphorylation, which induce phosphorylation of cytosolic protein (Jagtap and Ray 1999). Composition of fatty acid varies according to external temperature. At low temperature, there is more unsaturation owing to saturases, more methyl branching, alteration in fatty chain length, increase in the ratio of ante-iso to iso branching and change in the ratio of sterol and phospholipid contents. In 2008, Coa-Hoang et al. stated that cold shock induced membrane injury which triggered high rate of cell inactivation in microbes like Escherichia coli and Bacillus subtilis. Other adapting features like secretion of cold-shock proteins (Czapski and Trun 2014), molecular RNA chaperones, osmotic solutes (cryoprotectants) (Kawahara et al. 2008), enzymatic denaturation, incorrect protein tertiary and quaternary structures and intracellular ice formation play a pivotal role in the existence of microbes in cold environments.

13.2.1 Cell Membrane-Associated Changes

Microorganisms exhibit significant tolerance to chilling by reducing the damage in their membranes. Downshift in temperature reduces membrane fluidity and induce permeability in response to increased phase transition of membrane phospholipids (Cao-Hoang et al. 2010). Cells growing at 37 °C have more saturated fatty acid content (laurate) while at low temperature the content of laurate decreases and is substituted by unsaturated fatty acid (palmitoleate), which increases membrane fluidity and decreases membrane phase separation. Enzyme fatty acid desaturase causes unsaturation of fatty acids in Bacillus subtilis in preexisting membrane phospholipids (Aguilar et al. 2001). The gene of enzyme desaturase is regulated by a sensor called DesK kinase which activates the transcriptional activators DesR at cold temperature (Albanesi et al. 2004). Figure 13.1 illustrates the mechanism of unsaturation of already present fatty acids and maintenance of membrane fluidity by desaturase enzyme.

Fig. 13.1
figure 1

Mechanism of unsaturation of already present fatty acids by desaturase enzyme at low temperature

Full size image

13.2.2 Role of Cryoprotectants

Cryoprotectants (CRPs) are small molecules or chemical chaperones that provide defence mechanism against cold stress (Kawahara et al. 2008). These compounds include sugars like monosaccharides (glucose, fructose), disaccharides (sucrose, trehalose, etc.), polyamines, polyols (alcohol sugars such as glycerol and sorbitol) and amino acids (glycine, alanine, and proline). CRPs may be secreted outside the cell or may be located at intracellular level. Secreted CRPs can lower the freezing of water (Bouvet and Ben 2003) while their intracellular counterparts increase the total internal solute concentrations so as to regulate the osmotic pressure and maintain the osmolarity prior to freezing. CRPs have been reported in various bacteria like Lactobacillus, Pseudomonas and Pantoea. During cold shock, glycine betaine controls the aggregation of cellular proteins and regulates the fluidity of the membrane (Chattopadhyay 2002). Almost similar conditions have been reported in food-borne pathogen L. monocytogenes, where glycine betaine maintains high osmolarity at chilling stress (Angelidis and Smith 2003). Another cryoprotectant trehalose accumulates on both sides of the cell membrane and conserves intracellular water to stabilize cell membrane against freezing (Sano et al. 1999). Exopolysaccharides (EPS) synthesized by psychrophiles in cold environment have polyhydroxyls which prevent ice nucleation of water, enzyme denaturation and lysis of cell (Feng et al. 2014). ESPs store water and minerals and assist in cell aggregation, cell coating and formation of biofilm (microbial cells adhere to each other within an indigenous matrix of extracellular polymer) and maintain the viability of cells (Qin et al. 2007). Fungi Mortierella elongata, has some characteristics which favour their growth at low temperatures. These features include increased the amounts of intracellular trehalose, stearidonic acid and absence of ergosterol lipid when subjected to cold stress (Weinstein et al. 2000). Ergosterol is the main sterol in fungi which makes lipid membranes more rigid and decrease their membrane permeability; hence, deficiency of ergosterol causes the membrane more liable to cold-induced damage. Therefore, M. elongata increases the production of trehalose as an adaptation method in low temperature. Trehalose is the most effective cryoprotectants in thermotolerance in the fungi Neurospora crassa and Cunninghamella japonica (Neves et al. 1991; Tereshina et al. 1991). Dong and Chen found that at 4 °C cultured cell extracts of Methanolobus psychrophilus R15, there is upregulation of a new type of adenosine derivative which acts as osmotic solute in cold condition. Another cryoprotectant Cor26 is accumulated in Pseudomonas fluorescens KUIN-1 bacteria and aspartate in Methanococcoides burtoni in response to cold temperature. Aspartate is known to increase the affinity of GTP binding to elongation factor 2 while the action of Cor26 is unknown.

13.3 Cold Acclimation Proteins and Cold-Shock Proteins

Psychrophiles release a group of ~20 proteins during steady-state growth at cold temperature referred as cold-acclimation proteins (CAPs). The level of these proteins increases constitutively at low temperatures which help microorganism to adapt in cold climate (Phadtare 2004). They regulate protein synthesis and are essential for viability in cold condition. RNA chaperone CspA are usually cold-shock proteins reported in mesophiles and function as Caps in cold-adapted bacteria. The function of CAPs is not yet explored much; however, it has been revealed that these proteins regulate cell cycle and cell growth at lower temperature. Pantoea ananas KUIN-3 release a cold acclimation protein, Hsc 25, which has the potential of refolding the cold-denatured enzymes (Kawahara et al. 2008).

When environmental temperature comes down suddenly, psychrophilic bacteria show cold-shock response and release cold-shock proteins (Csps). These are (65–75 aa in length) nucleic acid-binding proteins (Czapski and Trun 2014). Cold-shock proteins neutralize various detrimental effects of fall in temperature and hence facilitate the cells to adjust with a transient overexpression that affect a number of molecular and cellular processes (Phadtare 2004). At cryo-temperature, RNA structures stabilize and become non-dynamic that induce premature transcription and translation termination. However, protein folding is disorganized, and ribosome function is hindered. Csps function as RNA chaperones helping in the sliding of ribosomes on target mRNA. This activity can be inhibited due to secondary structures of RNA at cold stress. Due to chaperone activity of Csps, single-stranded state of RNA is maintained (Barria et al. 2013). All Csps are ancient proteins which have some key conserved structure which includes five antiparallel strands that makes a ß-barrel. Csps that comprise a single nucleic acid-binding domain are known as cold-shock domain (CSD). CSD consists of two RNA binding motifs referred as ribonucleoprotein 1 and 2 (Lee et al. 2013). These binding motifs open tightly packed nucleic acid molecules which are inaccessible for translation (Chaikam and Karlson 2010).

Proteins that are constitutively synthesized in cell are called housekeeping proteins. Cold shock does not lower the production of these proteins, whereas the expression of Csps is enhanced with aggravated cold shock (Ermolenko and Makhatadze 2002). These CSPs reduce the expression of housekeeping gene and maintain the folding of important proteins. Hence cells adapt for temperature downshift at slower rate. Chaikam and Karlson (2010) reported that Csps are actively associated with the maintenance of chromosome folding. CspA was the first reported cold-shock protein in Escherichia coli (Goldstein et al. 1990). Previously it was reported that E. coli CspAs consist of nine homologous proteins (CspA to CspI). CspA consists of 13% of total cell proteins at cold condition while at 37–40 °C it is declined to lower levels (Lee et al. 2013). During cold or before freezing, Csp proteins are overexpressed in Lactobacillus strains which increase the survival rate of the cells. Human pathogen Listeria monocytogenes becomes less virulent under refrigerated condition due to the removal of cspA, cspB, and cspD genes which regulate the synthesis of the virulence factor listeriolysin O (Schärer et al. 2013). CspA from psychrophilic Psychromonas arctica was overexpressed in E. coli which increases the rate of cell survival and cold resistance in hosts by tenfold after repetitive freezing and thawing in polar environments (Jung et al. 2010). In Antarctic bacterium Psychrobacter sp. G, three CSP genes Csp1137, Csp2039 and Csp2531 have been identified with their regulatory sequence (Song et al. 2012). Csp genes of Yersinia enterocolitica 8081 and Yersinia pseudotuberculosis IP32953 share the maximum homology with csp genes of E. coli K-12 W3110 (Kanehisa et al. 2016). Enteropathogenic Yersinia psychrotrophs (spread by eatables and cause enteric illness yersiniosis) bear a locus having CspA duplication gene (cspA1 and A2) (Neuhaus et al. 1999).

13.4 Ice Nucleators and Antifreeze Proteins

Ice nucleators are the proteins that act as an ice crystal surface at low temperature (0 °C). They induce freezing and control the energy required for ice formation by ice crystal surface arrangement on water. Some bacteria have the potential of ice formation at low temperature. These bacteria are reported as “ice plus” bacteria. They have ice nucleation-active protein (Ina protein) located on the outer bacterial wall, which act as potent nucleating centre for ice crystals. Erwinia herbicola produces highly potent ice nucleators which show optimum activity at subfreezing temperature (Kozloff et al. 1983). Hirano et al. (1985) found that ice-nucleating bacteria live on the surface of leaves and induce frost damage when the temperature goes down. Pantoea (Lindow 1983), Xanthomonas (Kim et al. 1987) and Pseudomonas (Obata et al. 1987) are some examples of cryotolerant ice-nucleating bacteria.

Antifreeze proteins (AFPs) are ice-binding proteins that inhibit ice crystal formation and growth in any bacterium (Gilbert et al. 2005). They control the ice from melting by binding irreversibly to its surface. AFPs induce high thermal hysteresis activity and inhibit ice recrystallization even at milli-molar concentrations. Fungal AFP from a mold Typhula ishikariensis also possesses ice-binding properties (Cheng et al. 2016). Re-crystallization of ice is robustly reduced due to binding of antifreeze proteins with multiple ice planes. Ice-nucleating and antifreeze activities of AFP have also been identified in Arctic rhizobacterium Pseudomonas putida GR12-2, which is an extracellular glycolipo protein (Muryoi et al. 2004). There are three domains in ice-nucleating proteins, namely N, R and C. N domain facilitate the binding of ice-nucleating proteins (INP) to lipids and carbohydrates, and ice formation and ice nucleation activity are related to R-domain and C-terminal domain (Kawahara et al. 2008). Yamashita et al. (2002) reported that Moraxella was the first reported bacteria in Antarctic region that synthesizes an AFP for its survival in extreme cryo-environment. Ca2+-dependent AFPs have been reported from Marinomonas primoryensis bacteria which is dominantly found in Antarctic lake (Gilbert et al. 2005). Psychrophilic phytopathogenic fungi have extracellular AFPs which check the freezing of hyphae (Robinson 2001) and make sure the accessibility of substrate by checking the rate of freezing of nutrients. Hoshino et al. (2009) reported that various genera belonging to Basidiomycetes, Oomycetes and Ascomycetes release AFPs which control freezing of extracellular environment and check the growth of mycelia at very low temperature.

13.5 Cold-Adapted Enzyme

In low temperature, the rate of chemical reactions is very slow because there is inadequate kinetic energy to conquer enzyme activation barriers (in ground state (substrate) and activated state). Psychrophiles release enzymes that show high specific activities at lower temperatures called as cold-adapted enzymes. These include cellulases, lipases, proteases, amylases, xylanases, pectinases, keratinases, esterases, catalases, peroxidases and phytases are perform important role under very harsh climatic as they represent low activation energy and high catalytic activity (Kuddus et al. 2011). Reaction rate (kcat) of cold-adapted enzymes is highly independent of temperature.

Cold-adapted enzymes increase structural flexibility to cope with freezing at low temperature (Collins et al. 2008). These adaptations involve molecular dynamic simulations of distinct stabilizing interactions either in the enzyme or at the active site of enzyme (implicated in catalysis). Some relevant factors include electrostatic interactions like reduced no of ion pairs, hydrogen bonds and hydrophobic interaction, reduced proline and arginine residues in loops, location of glycine residues, decreased cofactor binding, increased interaction with the solvent and reduced inter-subunit interactions (Siddiqui and Cavicchioli 2006). Amylases are very common enzyme found in microorganisms, plants and animals. The α-amylase reported in Pseudoalteromonas haloplanktis (AHA), also the most studied cold-adapted enzyme is monomeric, has multiple domains and shows Ca2+- and Cl−-dependent properties (Siddiqui and Cavicchioli 2006). D’Amico et al. (2003) reported that activation energy is reduced in psychrophilic α-amylase (35 kJ mol−1) as compared to thermophilic α-amylase (70 kJ mol−1), so kcat of psychrophilic α-amylase is increased 21-folds at low temperature. Binding of substrates require low energy, so binding affinity of cold-adapted enzymes is lesser, and substrate binding is highly accessible. Several cold-adapted enzymes comprise a more labile and localized flexibility (flexible catalytic site) than other protein structure (Siddiqui et al. 2005). In cold-adapted enzymes, buried amino acids are smaller and show lesser hydrophobicity than their mesophilic and thermophilic counterparts. Hydrolysis and transesterification of fatty acid esters are catalysed by another class comprising of hydrolytic enzymes called esterases and lipases. Esterases differ from lipases on mode of their kinetics and specificity of substrate (Chahiniana and Sarda 2009). EstSL3 esterase, a novel cold-adapted enzyme from Alkalibacterium sp. SL3, shows close similarity to lipases extracted from Alkalibacterium and Enterococcus (Wang et al. 2016). Many cold-adapted enzymes have broad cavities to contain H2O molecules and/or ligands (Giordano et al. 2015). Cold active pectinases abundantly used in the food-processing industry isolated from Cryptococcus have pectinolytic activity (35–36 U/mL at 9 °C) and synthesizes pectinase by glucose as carbon substrate (Birgisson et al. 2003). Some psychrophilic yeasts reported in Japan have pectinolytic activity only at 5 °C and are not capable to survive at high temperature (Tomoyuki et al. 2002). Aureobasidium pullulans strain produces pectinase enzyme at cryo-temperature which shows higher pectinase activity of 0.7–0.8 U/mL at 12 °C (Merín et al. 2011). Fungal strains Aspergillus awamori isolated from Himalayan region not only has maximum pectinase activity but also produce high amount of psychrophilic xylanases and cellulases (Anuradha et al. 2010)

13.6 RNA Degradosomes in Psychrophiles

Psychrophilic microbes have a multiprotein complex called degradosome which is engaged with the debasement of delivery moiety RNA and the handling of ribosomal RNA which is directed by non-coding RNA. The degradosome consists of enzymes like RNA helicase B, polynucleotide phosphorylase and RNase E (Carpousis 2002; Feng et al. 2001; Cho 2017). The amount of RNA in any cell varies with time for instance, in Escherichia coli, the time period of messenger RNA is approximately in the range of 2–25 min, whereas it may live more in other microscopic organisms. RNA is degraded even in resting cells, and the resulting nucleotides are reused for crisp rounds of nucleic acid synthesis. The amount of RNA formed by degradosomes is significant as related to quality guideline and quality control.

All life forms have numerous enzymatic tools for debasing RNA, for example, ribonucleases, helicases, 3′-end nucleotidyltransferases, 5′-end topping and decapping catalysts and RNA-restricting proteins which utilize RNA as substrate. RNA degradosome of Escherichia coli comprises four essential parts: (1) hydrolytic endo-ribonuclease RNase E, (2) phosphorolytic exoribonuclease PNPase, (3) adenosine triphosphate (ATP)-subordinate RNA helicase (Rh1B), and (4) glycolytic compound enolase. The degradosome of Antarctic bacterium Pseudomonas syringae contains ribonuclease E and RNA helicase. Polynucleotide phosphorylase is present in Escherichia coli, and this enzyme controls the quality of ribosomal ribonucleic acid. But the composition of degradosome of the Antarctic counterpart possess another exoribonuclease, ribonuclease R. In Escherichia coli, it is well known that ribonuclease R can degrade RNA molecules, and in this process, it does not require (ATP) but helicase requires ATP due to which energy is conserved by cells at low temperatures as well (Purusharth et al. 2005; Hardwick et al. 2010; Carpousis et al. 2009). The metabolic enzyme aconitase is found in C. crescentus and phosphofructokinase is found in B. subtilis degradosomes though enolase enzyme is present in both the psychrophiles. The bacteria regularly adjusts the gene expression to survive in the harsh environments. Ribonucleases (RNases) control the expression of regulatory proteins and protein-coding RNA by degradation and maturation. Exoribonucleolytic capacities are available in polynucleotide phosphorylase (PNPase) and RNase R in the human pathogen Streptococcus pyogenes. An exoribonuclease, PNPase is the focal 3′-to-5′ exoRNase partaking in RNA damage (Lécrivain et al. 2018; Chandran and Luisi 2006; Chandran et al. 2007). A considerable number of the Csp family proteins are in charge of RNA adjustment and debasement. Moreover, mRNA is stable in cold conditions due to the presence of chaperon functions of Csps. CspA also destabilizes the secondary structure and maintains its structure in a single-stranded state, which is necessary for its degradation. CspE acts in the opposite manner, and it stops RNA degradation. CspE binds to poly-A tails, which interfere with their degradation by PNPase, and it stops RNA cleavage by RNase E (Feng et al. 2001; Khemici et al. 2008; Prud’homme-Généreux et al. 2004). A DEAD-box helicase called DeaD in E. coli is added into the degradosome which can degrade RNA, under cold conditions.

13.7 Plant Growth Promotion by Agricultural Microbes in Cold Climate

The psychrophilic microorganisms help in the growth of plants under adverse conditions, their promotion and adaptations under harsh environments such as extremes of temperatures, high salt conditions, extremes of pH and drought stresses and are termed as plant-associated extremophilic microorganisms. They possess diverse plant development advancing characteristics, and hence, these productive and potential organisms might be used as biofertilizers to enhance the productivity and maintain the well-being of soil giving a push to the highly talked sustainable agriculture (Verma et al. 2016).

Certain strains of rhizospheric microorganisms, known as plant growth-promoting bacteria (PGPB), invigorate plant development and wellness. Various microorganisms promoting the yields are significant for keeping up the supportability of harvest generation in horticulture. Microorganisms related with harvests can be rhizospheric, phyllospheric, and endophytic based on their location. The rhizosphere contains roots and is affected by the addition of substrates that influence microbial action. A number of microorganisms are found to be associated with the plant rhizosphere which helps in the growth and development of the plant belonging to genera Azospirillum, Bacillus, Pseudomonas, Rhizobium, etc. (Verma et al. 2014). The epiphytic microorganisms are most versatile in nature as they endure high temperature (40–55 °C) and UV radiation. The phyllospheric microorganisms include Agrobacterium, Pseudomonas, etc. which can survive in harsh conditions such as extremes of temperature (Nutaratat et al. 2014).

The endophytic living beings are those microorganisms that colonizes in various aerial and subaerial parts of the plant, viz. root, stem or seeds without expediting any ruinous effect on the host plant. These microorganisms have been extracted from plants including wheat (Verma et al. 2013), soybean, pea, common bean, chickpea, pearl millet and rice (Suman et al. 2016). Various examples of endophytic microbial species are Achromobacter, Azoarcus, etc. (Verma et al. 2014). Microscopic organisms isolated from harsh temperature conditions are adjusted to live under stressful temperature conditions. Many optimizations have been used to isolate psychrotolerant and psychrophilic microbes from soil. The growth of cold-tolerant Antarctic bacterium can be increased by supplementing the minimal media supplements like amino acids which improved the growth rate of psychrophilic bacteria when the temperature was lowered from 11 °C to almost freezing point of water, i.e. 5 °C.

Indole-3-acetic acid (IAA) is a vital phytohormone secreted by PGPR which enhances overall plant development (Selvakumar et al. 2008). This IAA-secreting capacity of psychrophilic microorganisms acts as a marker tool for their identification while looking at the physiological or environmental conditions. Auxin production in microscopic organisms is controlled by the proline amino acid-dependent pentose phosphate pathway (Sahay et al. 2017). Pantoea dispersa and Serratia marcescens show their maximum IAA-creating capacity at 4 and 15 °C, respectively. Seed treatment with these bacterial strains significantly improved plant biomass and supplement take-up of wheat seedling developed at cold temperatures. Introduction of seeds with these mentioned strains upgraded the seed germination, root growth and shoot lengths of wheat plantlets developed at low temperatures (Sahu and Ray 2008).

Another bacterial framework that influences plant advancement is the nearness of compound 1-aminocyclopropane-1-carboxylate (ACC) deaminase. This catalyst enhances the overall development and improvement of plants. Bacterial strains that have ACC deaminase can diminish the ethylene combination even in virus infections, thus curbing the negative impact on plants. Plants having ACC deaminase may adjust to this troublesome situation by cutting down ethylene level similar to normal stresses. Few psychro-tolerant bacteria producing ACC deaminase promote plant development even at low temperature that too under high osmotic pressure.

13.7.1 Nitrogen Fixation

Nitrogen fixation is a very essential process in the soil which is performed by many bacterial species that have the capacity to absorb atmospheric nitrogen and convert it into nitrogenous substances that furnish important nutrients for plants. These microbes synthesize the nitrogenase enzymes that form ammonia from nitrogen N2. These processes require biological energy in the form of adenosine triphosphate (ATP). The nitrogen-fixing microbes may be free-living or symbiotic. The basic source of energy for some of the nitrogen-fixing microbes living freely is sunlight while others depend on organic matter present in soil. Soil microorganism Azotobacter is an aerobic heterotroph, and Clostridium species are active in conditions that do not have oxygen. Both the groups of microorganisms (free-living and symbiotic) can fix only minimal amounts of nitrogen, but still they are important for the survival of various plants in the environment.

Symbiotic microbes thrive on plant roots, forming root nodules. Rhizobium is an important member of this group which lives symbiotically with various members of leguminaseae family (peas, clover, beans, peanuts, soybeans). Frankia is an actinomycete associate with several plant families including species of temperate region trees, for example, Alnus and Myrica, the arid-region Acacia, and the tropical-region Casuarina and Ceanothus. Crop rotation helps to introduce good amounts of N2 into the soil for efficient crop production.

Cyanobacteria have a pioneering role in fixing atmospheric nitrogen. They are very active in media which is very shallow such as flooded rice fields and marshy areas. An aquatic microbe, Anabaena, lives in a shallow medium in association with Azola which is a water fern, and its symbiosis can produce a high quantity of nitrogen per hectare annually, which is sufficient for rice production.

Nitrogen cycle involves various processes such as fixation, ammonification, nitrification and denitrification. Each step involves specialized microbes, and the consequences depend on the physiological state of the soil. The nitrogen cycle in the soil is also affected by the atmospheric processes. Partial removal of nitrogen in the soil releases N2O, which is a very harmful and strong greenhouse gas responsible for global warming. Carbon dioxide and methane are other greenhouse gases that come out from the soil in special circumstances. N2 fixation in the deep Arctic or Atlantic Ocean is the most important source of nitrogen where nitrogen is limited in system. N2 fixation that occurs in ice-free summer waters contributes up to 30% of the N2 fixation in the Arctic Ocean. Nitrogen fixation in freshwater is relatively common at high altitudes, but still nitrogen fixation in oceans is considered to be common. In Arctic region, when ice melts due to increase in temperature, the net production of nitrogen is increased in Arctic Ocean due to marine nitrogen fixers (Arrigo et al. 2012). A sufficient amount of nitrogen fixers is required to increase the productivity of nitrogen via Arctic nitrogen cycle which affects the primary producers that form the foundation of the food chain (Popova et al. 2012). This data helps us to conclude that N2 fixation can also occur at minimum temperatures (Moisander et al. 2010) and at very high altitudes (Sohm et al. 2011; Díez et al. 2012) where it was believed that nitrogen fixation would not be possible.

13.7.2 Phosphate Solubilization

The availability of phosphorus is very important for the cultivation of healthy crops to cope up with the universal requirement of food. Various metabolic and physiological processes like energy transfer, photosynthesis, respiration, signal transduction and nitrogen fixation in plants of the family leguminaceae require P as essential macronutrient. Although P is the most abundant macronutrient found in almost all types of soils, it acts as the major limiting factor for the plant growth because of its unavailability to plants. Inorganic P occurs mostly in insoluble mineral complexes in soil, some are present in chemical fertilizers, and the plants are unable to absorb the insoluble and precipitated forms. Soil microorganisms help in the transformation of phosphorus and make it easily available to plant roots as they possess the ability to solubilize and mineralize phosphorus from inorganic phosphorus (Rodriduez et al. 1999). P-solubilizing bacteria and fungi have been isolated from both rhizospheric and non-rhizospheric soils and phyllosphere (Zaidi et al. 2009). In addition to bacteria and fungi, various microbial species that exhibit P solubilization capacity are actinomycetes and algae. Examples of P solubilization microorganisms are Pseudomonas species, Bacillus species, Rhodococcus species, Arthrobacter species, Serratia species, Chryseobacterium species, etc. (Wani et al. 2005; Chen et al. 2006), Azotobacter species (Sharma et al. 2013), Xanthomonas species (Srinivasan et al. 2012), Enterobacter species, etc., (Zhu et al. 2012), and Vibrio species and Xanthobacter species (Babalola and Glick 2012). The Rhizobium species that fixes atmospheric nitrogen to the host plants also show P solubilization property. Rhizobium species and Crotalaria species (Jorquera et al. 2011) increase the P content in plants by making P easily available to plants. Kushneria species is a halophilic bacteria that was extracted from the soils of Daqiao saltern on the eastern coast of China, which have proved to be very beneficial for saline soils. Phosphate-solubilizing fungi include strains of Fusarium, Alternaria, Sacchromyces, etc. For better usage of amassed phosphorus in soils their use is very promising in the form of biofertilizers enhancing sustainable agriculture one step further (Richardson and Simpson 2011).

One of the enzymes that cause P solubilization is glucose dehydrogenase which is a membrane-bound enzyme and causes oxidation of glucose to gluconic acid. The gluconic acid is then converted to 2-ketogluconic acid and 2,5-diketogluconic acid by the action of enzymes. P is solubilized effectively by 2-ketogluconic acid as compared to gluconic acid. Although most of the studies on P solubilizing microorganisms were performed at mesophilic temperatures but some reports are also available of studies at low temperatures such as 10 °C (Vassilev et al. 2006).

P mineralization means debasement of the remaining portion of the molecule after solubilization of organic phosphorus which results in dissolution of Ca-P compounds. Phytase is another enzyme responsible for organic P mineralization. This enzyme causes the formation of phosphorus from organic materials which are stored in the form of phytate in soil (Yi et al. 2008). Other enzymes involved in P mineralization are NSAPs (non-specific acid phosphatases) which remove the phosphate group from phosphoester bonds of organic compounds. Various non-specific acid phosphatase (NSAPs) enzymes released by P-solubilizing microorganisms belonging to the family of phosphormonoesterases. The acid phosphatase enzymes play an important role in solubilization, though alkaline phosphatases are also present. Various solubilization and mineralization processes that involve different enzymes play an important role in recycling of phosphorus.

13.7.3 Stress Management

Cold-tolerant microbes permanently sustain low temperature in cryo-environments such as deep sea, mountains, and polar regions. These organisms are also known as psychrotolerant, psychrotroph, or psychrophiles as they grow better at very low temperatures (Morita 1975). Psychrophiles can overcome two main challenges because of their unique properties: First challenge is the survival of psychrophiles at very low temperatures because if there is decrease in temperature, the biochemical reactions are affected exponentially. Second, the viscous aqueous environments are considerably increased as temperature is decreased. Their growth rate is maximum between temperatures of 2 and 12 °C (Xu et al. 2003).

The membrane functions are also affected, which leads to decreased membrane fluidity and the loss of membrane functions. The physical properties of membranes are affected by fatty acid composition, and it changes with the environment of the microbes. In general, reduced temperature produces a higher content of branched fatty acids both saturated and unsaturated (Pandey et al. 2004). Another adaptation of psychrophiles is an increased content of big and more compact head groups of lipids, proteins, and carotenes (Deming 2002). In some psychrophiles, there is less non-polar carotenoid pigment synthesis (Chintalapati et al. 2004).

Microbial activity at temperatures around −20 °C occurs in normal water inside the ice. These contain increased concentrations of sodium chloride (NaCl) or other particulate matters which maintain the fluid flow. Different factors such as hydrostatic and osmotic pressure, solar radiations, availability of nutrients and stress also strongly affect the growth of psychrophilic microbes. Various specialized proteins are expressed in microbes when they are subjected to sudden change in temperatures. These proteins are involved in cellular processes like protein folding and the control of membrane fluidity (Russell 2000). In psychrophiles, Caps are expressed at low temperatures though they are similar to the Caps present in mesophiles. This shows that a sensory system that senses temperature is present in psychrophiles, and these thermosensors sense membrane fluidity as well (Arthur and Watson 1976).

Anti-freeze proteins (AFPs) or ice-restricting proteins have been recognized in microorganisms living in Antarctic lake (Gilbert et al. 2005) that can tie to ice precious stones in an expansive surface and brings down the temperature at which a life form can openly develop (Jia and Davies 2002). AFP from certain organisms is Ca2+-reliant and hyperactive ice surfaces and control ice precious. AFPs bind to stone development and recrystallization by bringing down the point of solidification (warm hysteresis) (Krembs et al. 2002).

Other molecules that have an important role in protecting psychrophiles against cold conditions are disaccharide trehalose and exopolysaccharides. Trehalose binds the molecules together and helps in the prevention of protein denaturation and protein aggregation (Nichols et al. 2005). The trehalose disaccharide also rummage-free radicals and stabilize cellular membranes under cold climatic conditions. Increased concentrations of exopolysaccharides have been found in bacteria of sea of Antarctica water (Muryoi et al. 2004) and in sea ice of Arctic water (Nichols et al. 2005). These change the physiological environment of bacterial cells, participate in adhering of cells to surfaces and retain water, increase the nutrient concentration, retain and save extracellular enzymes against cold denaturation, and most importantly, it acts as cyoprotectant (Tosco et al. 2003). EPS have elevated amounts of polyhydroxyl which brings down the point of solidification of water. EPS can likewise trap water, supplements and metal particles and encourage surface grip, cell collection and biofilm arrangement and may likewise assume a job in securing extracellular catalysts against cold denaturation and autolysis. EPS have high levels of polyhydroxyl which lowers the freezing point of water (Campanaro et al. 2011). EPS influenced the species colonization and survival of the present organisms in the natural surroundings near the oceans ice by lowering the rate of development of ice because of higher saltiness (Mykytczuk et al. 2011).

The combination of cytoplasmic ice crystals is incited by cell solidifying. The accumulation of substances like sucrose, glycine, betaine and mannitol results in the bringing down of the point of solidification of cytoplasm consequently giving assurance against solidifying.

Ongoing transcriptome investigations have demonstrated that introduction to cold temperatures initiates a fast up-guideline of qualities engaged with layer biogenesis, for example, unsaturated fat and LPS biosynthesis, peptidoglycan biosynthesis, glycosyltransferases and outer membrane proteins (Deming 2002). Similar genomic studies have additionally uncovered that genes engaged with the synthesis of cell membrane are overexpressed in the genomes of psychrophilic microorganisms. General membrane transport proteins are elevated as seen by transcriptomic contemplates, against the lower dispersion rates over the cell layers experienced at colder temperature (Qiu et al. 2006). Specifically, the dimensions of peptide transporters are expanded which encourages cold and hyperosmotic stress which improves the take-up of supplements (Reva et al. 2006).

Another class of layer smoothness modulators are carotenoid pigments. Both polar and non-polar carotenoid pigments are delivered by different Antarctic microorganisms and have been proposed to keep up layer smoothness and aid in keeping up equalization amid changes in temperatures (Fig. 13.1). Wax esters additionally assume a significant job in cool balanced film ease. In Psychrobacter urativorans, they may represent up to 14% of the cell lipid content, and in P. arcticus, the wax ester synthase is constitutively communicated, paying little heed to the development temperature (Sung et al. 2011).

13.8 Industrially Important Cold Enzymes

Psychrophilic microorganisms produce enzymes that can sustain low temperature and other stresses of cold climatic conditions. These enzymes are used in paper, pulp, pharmaceutical and food industries (Whitman 1998). Psychrozymes or cold-adapted enzymes can sustain temperatures between 10 and 5 °C. There is an increasing demand of psychrozymes in industries because of their withstanding nature in adverse conditions. Nowadays more attention is paid on the use of proteins isolated from cold-loving microorganisms as they act at their optimum temperature enhancing the recovery of the products of enzymatic reaction (D’Amico et al. 2006).

The psychrozymes have the ability to degrade a wide range of polymeric substances and the substance that can produce enzymes like amylases, cellulases, pectinases, β-galactosidase, oxidases, protease and lipase. A huge amount of money is invested in psychrozymes worldwide due to their extreme potential. The industrially important psychrozymes are used in the fields of food industry (such as pectinase, β-galactosidase), bio-polishing of textile products and detergent formulation industries. Moreover, these psychrozymes are also used in bioremediation (such as oxidases), for biotransformations (methylases and aminotransferases) (Okuyama et al. 1999) and in biomedical applications. Psychrozymes are used in:

  1. 1.

    Industrial processes including food technology

  2. 2.

    Bioremediation and other pollution control technologies

  3. 3.

    Medical and other pharmaceutical uses

Psychrozymes have many benefits such as high specific activities at low temperature, they can offer many other advantages like saving energy, saving volatile compounds, contamination prevention and easy inactivation of enzymes. Most of the food industries treat the products with psychrozymes for maintaining the quality of food during their transportation and storage. Psychrozymes are also frequently utilized in detergent and textile industries. Similarly pectinases and cellulases are used in the clarification of fruit juices; proteases helps in the removal of fish skin.

Apart from food industry, psychrozymes are used for the low-temperature biodegradation, and they are best alternatives to physicochemical methods for the bioremediation of solids and waste waters polluted by hydrocarbons, oils and lipids (Violot et al. 2005). Biodegradation with psychrozymes have several advantages over other existing traditional methods. It has been observed that the treatment of contaminated soil with psychrozymes is much more cost-effective than traditional methods such as incineration, storage or concentration. In 1997, Brun et al. studied the recombinant Antarctic Pseudoalteromonas haloplanktis which secretes toluene-0-xylene monooxygenase (T0MO). This enzyme efficiently converts several aromatic compounds into their corresponding catechols in a broad range of temperature. It has been suggested that the genetically engineered Antarctic bacterium is used in the bioremediation of contaminated marine environments. The interest on psychrozymes have increased greatly because of their high activity at low temperatures which offers potential economic benefits (Margesin and Schinner 1994). For example, the “peeling” of leather by cold adapt protease can be done with normal water instead of at 37 °C. An important achievement in the field of cold-adapted enzymes has been the construction of a host-vector system that allows the overexpression of genes in psychrophilic bacteria even at low temperatures which prevents the formation of inclusion bodies and protects heat-sensitive gene products. A single PUFA is produced using psychrozymes, rather than the complex mixture which is yielded from fish or algal oils.

13.9 Conclusion

Microbes play a vital role in sustainable environment and affect both flora and fauna of any ecosystem. Cold environment has its own challenges which can be countered by using the strategies used by nature. Exploring the adaptations used by psychrophiles help us to mimic them in our day to day life. Many low-temperature microbes have a great role to play in all low temperature-based industries.