Extremophilic Microorganisms: Introduction, Types and Applications

Introduction:

The microorganisms are present in every nitch on the earth, except volcano and vacuum. These microorganisms have high adaptability. The majority of the microorganisms survive and grow in normal conditions like room temperature, around neutral pH, etc. But some microorganism survives and grow in extreme conditions like very high or very low temperature, salt concentration, etc. These are called extremophilic microorganism. There are many extremophilic microorganisms. There are many extremophilic microorganism like thermophiles, psychrophiles, acidophiles, alkaliphiles, halophiles.

All the process of growth are dependent on chemical reactions and since the rate of these reactions are influenced by temperature, the pattern of bacterial growth can be drastically influenced by this condition. The temperature that allows for the greatest fast growth in a short period of time is known as the optimum growth temperature. On the basic of their optimum temperature, the bacteria are divided into three main groups namely psychrophiles, mesophiles and thermophiles.

Types:

Thermophiles

Organisms with T _max greater than 50^oc were termed thermophilic. Generally, thermophiles are prokaryotic organism because up to now no eukaryotic organism are known to grow above 50 oc. Generally, thermophiles can grow in the range of 55^oc to 85^oc. For example- actinomycetes (62^oc); Photosynthetic cyanobacteria (73^oc); clostridia (80^oc); Acidothermus cellulolyticus (65^oc), etc.

Thermophiles are generally isolated from hot springs and other geothermal environments. Beside this it is also present in some man-made thermal systems like coal refuse piles, self-heated hay stacks, piled organic waste materials such as compost heaps and hot eluent waters form power stations of industrial plant.

The growth range of many thermophiles extend into the mesophilic region. These species are called facultative thermophiles, while other thermophiles cannot grow in the mesophilic range, i.e., they grow strictly above 50^oc.These organisms called obligate thermophiles.

Some thermophiles grow fastest at the upper temperature border of life, around 80^oc to 100^oc. Hyperthermophiles cannot grow below 80^oc. Hyperthermophiles have been isolated from environments with apparent in situ temperature between 80^oc 0 100^oc. The low capacity of oxygen at high temperature ana of reducing gases means that most hyperthermophiles are anerobic. Some pyrophilus (95^oc); Methanothermus sociabilus (97^oc); Methanococcus Jannaschii (86^oc); Methanophyrus Kandleri (100^oc); etc.

Hyperthermophiles have a good potential for use in novel biotechnological process like heavy metal leaching, bioconversion of crude oil. Desulphurization. Etc. Again thermostable enzymes such as DNA polymerases, amylases, xylanases, proteases and lipases from these organisms are used on basic research and biotechnology process. Due to the great resistance to heat, they are the source of considerable trouble in the canning industry.

Psychrophiles

The word psychrophiles originates from Greek ‘psychros’ meaning cold. Psychrotrophs (cold seeking) are able to divide at 0^oc and have optimum temperature round 20-25 ⁰c, whereas psychrophiles have optimum temperatures below 16⁰c and they are able to grow upto 20⁰c. Facultative psychrophiles or psychrotrophs are able to grow best at temperature in the range of about 20-30⁰c. But obligate psychrophiles cannot grow at temperature above 20⁰c.

 Straka and Stokes (1959) isolated psychrophilic bacteria from soil, sea waste and other materials obtained form Antarctica. These psychrophiles were isolated by cultivation at 0 ⁰c. They consists of rods, cocci and cocoo-bacillary forms. And all are gram negative and non-motile organisms.

During isolation of obligate psychrophile’s it is usually necessary to maintain the source samples at cold temperatures form the time they are collected and also to chill all media before isolation because strick psychrophiles usually die if they are even temporarily exposed to room temperature.

These psychrophiles play a significant role in the biodegradation of organic materials in cold habitats. These organisms produce extracellular proteases. Psychrophilic proteolytic microorganism plays an important ecological role in cold environment because they take part in the nitrogen cycle. Proteases from psychrophilic and psychotropics microorganisms have been shown to be quite active at 0⁰c. Extracellular proteases of psychrophiles have generally neutral or alkaline optimum pH for activity.

Proteases have a great industrial importance. It is a very convenient tool whenever removal or degradation of protein is needed. Bacterial serine alkaline proteases are utilized in laundry, leather tanning, and food processing.

Mesophiles

(Greek ‘meso’= middle). Mesophiles are not extremophiles. They grow best within a temperature range of approximately 20 to 40 ⁰c i.e., Room temperature. The example of mesophilic organism are all the bacteria that are pathogenic for human and warm-blooded animals, which grow best at about body temperatures (37 ⁰c). Another example is all saprophytes and plant parasites which grow best at 20-35 ⁰c.

 For the survival and growth of living organism, hydrogen ion concentration is one of the factors. The hydrogen ion concentration affects the ionic state and therefore the availability of many metabolites and inorganic ions to an organism. Very high concentration of hydrogen ion (acid) or extremely low concentration of hydrogen ions (alkaline) are normally toxic to most organism. Most organisms live within the limits of pH 4 to 9 and have optimum pH around these pH limits. Highly acidic and alkaline habitats harbor different microbial populations which can tolerate these environmental extremes. Depending upon the extreme pH, optimum for many microorganisms, they are divided into acidophiles, alkalophiles and neutrophiles.

Acidophiles: Microorganisms which require pH value less than 5 for its maximum growth are called acidophiles. Acidophiles are generally found in natural environment of pH 3 to 4 like some pine soils, some lakes, acidic bogs, etc. Some examples of acidophiles are Bactoderma, Caulobacter, Microcyclus, Planctomyces, Thiovobium, etc., which grow optimum at pH 3 to 5. Acetobacter acidophilium has optimum growth at pH3.

Currently, indigenous acidophile populations are being exploited in mineral leaching. T. ferrooxidans has been used to reduce sulfur content of coal in order to reduce sulfur emissions.

Halophiles

(Salt loving organisms) The halophilic bacteria are a distinctive group of prokaryotic clearly able to adopt to the high salt concertation and high light intensities of their natural habitat. Halophiles show optimum growth between 0.2- 2.0 M Nacl. The ribosomes and enzymes of these bacteria requires high salt concentration in order to function effectively. A characteristics property of extreme halophiles is the presence of red pigment called bacteriorhodopsin and halorhodopsin in their cell membrane.  These bacteria are known to produce a typical brick red cooler in sea water, pond water etc. where it grows. This red pigment protects the halophiles against photochemical damage form the high light intensities.

Example of this group are the Halobacteria (Archaea); Ectothiorhodospira (phototrophic eubacterim) and Dunaliella (Green algae). Dunaliella strains ar epotentail producers of a range of valuable products like glycerol. It produces up to 30% w/v of its biomass as glycerol.

The salt resistance properties of halophiles are useful in the treatment of hypersaline waste streams such as slaughter house, waste chemical brines, refinery wastes and some agricultural wastes.

Alkalophiles

 Alkalophiles are the bacteria which grow best in alkaline pH, i.e., pH 8 to 11.4. These organisms are of two types, alkaline tolerant organism which grow maximum in the pH range of 7.0-9.0 but cannot grow above pH 9.5 and obligate alkalophilic organism, which grow extremely between pH 10.0 and 12.0 but cannot grow below pH 8.5.

All the extreme alkalophiles contain very high level of respiratory chain components in the membrane, generally more acidic amino acid composition of proteins and a Na+ cycle that facilitate solute uptake and pH homeostasis. Most of the alkalophilic organisms are aerobic or facultatively anaerobic. Some examples of alkalophiles are Spirulina sp. (8.0-11.0), Bacillus Strain (9.0-11.0); clostridium sp (8.0-11.3); Methanobacterine (6.5-10.0).

The degradation of cellulose is an important problem in waste treatment. Extracellular cellulase from alkalophilic, Bacillus strain N-4 has been characterized and the gene encoding the enzyme has been cloned in E.coli and sequenced. Another cellulase from alkalophilic Bacillus strain 1139 has a pH optimum of 9.0.