Alkalophiles: Meaning Molecular Adaptations and Application

In this article we will discuss about:- 1. Meaning of Alkalophiles 2. Environments Suitable for Growth of Alkalophiles 3. Molecular Adaptations 4. Biotechnological Applications.

Meaning of Alkalophiles:

Alkalophiles are a few of the extremophiles that have very high pH optima for growth, sometimes as high as pH 10. Most of the alkalophilic microorganisms are non-marine, aerobic or facultative anaerobic. The most well-studied aikalophile have been Bacillus species such as Bacillus alkalophilus, B.firmus, B. sp. No. 8-1, and B. sp. No. C-125. Alklophiles bear flagella and hence are motile.

Some extremely alkalophilic bacteria are also halophilic (salt-loving), and most of these are archaebacteria. For convenience, Natronobacierium, Natronomonas, and their relatives are halophilic (as they inhabit soda lakes possessing high concentration of salt) as well as extremely alkalophilic (as they grow optimally at high pH of 9-11).

Environments Suitable for Growth of Alkalophiles:

Though there are two kinds of naturally occurring stable alkaline environments:

(1) High C_a²⁺ environments (ground waters bearing high CaOH) (2) low c²⁺_a environments (soda lakes and soda deserts dominated by sodium carbonate). Soda lakes and soda deserts represent the most stable naturally occurring alkaline environments found worldwide. These environments characteristically contain high concentrations of Na₂CO₃ (usually as Na₂CO₃.10H₂O or Na₂CO₃.NaHCO₃.2H₂O).

The distinguishing feature of soda lakes is that significant amounts of Ca²⁺ and Mg²⁺ must be absent and Na₂O₃ must be present in high concentration. Soda lakes in the Rift Valley of Kenya and similar lakes found in other places on earth are highly alkaline with pH values of 11 -12.

Diverse industrial activities including food processing (KOH mediated removal of potato skins), cement manufacture (or casting), alkaline electroplating, leather tanning, paper and board manufacture, indigo fermentation and rayon manufacture, and herbicide manufacture generate anthropogenic, sources of alkaline type environments.

Molecular Adaptations to Alkalophiles:

Alkalophilic microorganisms possess a remarkable ability to maintain cytoplasmic pH much lower than the external pH values of 10-11. A majority of alkalophilic microorganisms need Na⁺ ions for growth. For example. Bacillus firmus and Exiguobacterium auranticum use Na⁺ / H⁺ anti-porter system in the region of pH 19.0 to 9.0, with the usual respiration-coupled extrusion of Na⁺ ions being replaced by at least 2 antiporter proteins for the uptake of protons.

Bacteria extrude protons by primary transport systems to generate a proton motive force that can be used for ATP synthesis. This connection to ATP synthesis results in the proton-motive force being maintained in a fairly narrow range during growth.

The proton motive force is the sum of the electrical membrane potential and pH gradient. Although growing microbial cells cannot greatly vary their proton motive force, they may vary the relative contributions of these two components in order to adapt to extremes of environmental pH.

Microorganisms growing at neutral pH usually maintain their external pH slightly higher than the external pH. At very high environmental pH levels, however, alkalophilic bacteria reverse their pH gradient (inside more acidic than the exterior) in order to maintain their external pH near neutral.

This reversed pH gradient contributes a negative component to the value of the proton motive force, so the electrical membrane potential must be high to compensate and give a sufficiently energetic proton motive force ATP synthesis.

It is currently not known how alkalophiles undertake processes that normally require inward translocation of proton-coupled ATP synthetase and pH homeostasis, which depend on an antiparty of sodium and protons. It has been hypothesised that alkalophilic microorganisms may have a sodium motive force instead of a proton motive force.

Research into cell wall analysts of the alkalophies indicated 4a there was some correlation between the density of high charge on the cell membrane and the degree of pHegulatm exhibited by alkalophilic species. This is one of the reasons why a cell gown in alkaline pH is stable. This causes cellular adaptation of microorganisms for growth in alkaline pH.

Biotechnological Applications of Alkalophiles:

Alkalophilic microorganisms offer a multitude of actual or potential applications in various fields of biotechnology. Not only do many of them produce compounds of industrial interest, but they a so posse useful physiological properties which can facilitate their exploitation for commercial purposes.

Following are some of the current applications and some expected future developments of alkalophiles:

1. Enzymes:

Since the discovery of serine protease, a proteolytic enzyme used in the detergent industry industrial applications of alkalophiles have been investigated and some enzymes have been commercialised.

Of the enzymes now available to industry, enzymes such as proteases, cellulases, lipases, pullulanases are by far the most widely employed and they still remain the target enzymes. Detergent enzymes account for approximately 60% of total worldwide enzyme production.

They usually have a pH of between 8 and 0.5 in solution. The main reason for selecting enzymes from alkalophiles is their long term stability in detergent products, energy cost saving by lowering the washing temperatures, quicker and more reliable product, reduced effluent problems during the process, and stability in the presence of detergent additives such as bleach activators softeners, bleaches and perfumes.

Due to the unusual properties of these enzymes they are expected to fill the gap between biological and chemical processes and have been greatly employed in laundry detergents. Many currently employed alkalophile enzymes are very useful as tools for biotechnology exploitation. The present applications of enzymes obtained from alkalophiles and their some future prospects are shown in the Table 19.3.

2. Lignin Degradation:

Lignin, a constituent of plant cell wall, is a complex polymer of phenylpropane units. It is the most abundant renewable aromatic material on earth but is difficult to biodegrade by microorganisms other than certain fungi. To date, however, no study has shown that lignin is mineralised rapidly or extensively by aerobic bacteria; filamentous bacteria (actinomycetes) can decompose lignin to some extent.

In recent years, few studies have treated alkalophilic species and especially their degradation of phenolic-lignin compounds. These species so far appear to degrade lignin, but more extensive studies focused on the capabilities of alkalophilic microorganisms to degrade lignin polymer are clearly needed.

3. Food:

In view of population trends and the current protein shortage, it is estimated that protein production for human nutrition must be increased in years to come. One of the way to augment protein production is not only to increase protein production through conventional sources but also by microbial biomass.

Spirulina, a cyanobacterium commonly called ‘single cell protein’ has long been a staple food in Africa. This microorganism is very rich in protein content hence is receiving growing attention for its nutritional characteristics.

Spirulina is commercially produced in many countries and is now being sold in health food stores as powder granules or flakes and as tablets or capsules. Spirulina powder has the highest protein content (60-70%) of any natural food, far more than fish (15-20%), soybeans (35%), dried milk power (35%), peanuts (25%), fresh eggs (12%), or grains (8-14%).

Spirulina has one very useful characteristic. It grows naturally in alkaline lakes containing NaCO, NaHCO₃ and other minerals, and is readily cultured in hypersaline alkaline environments which do not allow the invasion and growth of other containment microorganisms and the lake tends to be monospecific, permitting the recovery of Spirulina by simple filtration. The yield is very impressive. The economics of Spirulina both on small as well as big alkaline lakes (called farms) make it a potentially profitable option.

4. Antibiotic -type Compounds:

In a recent research study, microorganisms isolated from the alkaline- saline Lake Acigol in Turkey were screened for their activity against other microorganisms. The preliminary results indicated that alkaline-saline lake isolates exhibited antimicrobial activity against Bacillus subtilis Staphylococcus aureus, Micrococcus luteus, Mycobacterium smegmatis, and Candida albicans.

These results have encouraged further research work to identify the metabolites produced by alkalophilic bacteria. The discovery of these bioactive compounds provides evidence that microorganisms from such environments are also capable of producing antibiotic-type compounds. Alkalophilic producers of novel antibiotic-type compounds still await exploitation.