In this article we will discuss about:- 1. Meaning of Acidophiles 2. Microbes in Acidophilic Environment 3. Molecular Adaptations 4. Biotechnological Applications.
Meaning of Acidophiles:
pH values less than 7 are said to be acidic and the microorganisms that have their growth optimum between pH 0 and 5.5 are called acidophiles. Fungi as a group tend to be more acid-tolerant than bacteria, and many of them grow optimally at pH 5 or below while a few grow well at pH values as low as 2.
Many a number of bacteria are also acidophilic. Some of them are obligate acidophils (i.e., unable to grow at pH 7, the neutral pH) which include species of Acidithiobacillus and Leptospirillum and several genera of archaebacteria such as Ferroplasma, Picrophilus, and Thermoplasma.
Microbes in Acidophilic Environment:
Acidithiobacillus ferrooxidans is a strict acidophilic and is very common in acid marine drainages and in acid springs. A. ferrooxidans and Leptospirillum ferrooxidans thrive in environments in which sulfuric acid is the dominant acid and large amounts of sulfate are present. A. ferrooxidans dominates in the habitat at 20-30°C and moderately acidic pH (2-4), while L. ferrooxidans dominates at 30-50°C and more acidic pH (1-2).
A. ferrooxidans and L. ferrooxidans are phylogenetically, morphologically, and metabolically distinct. While ferrooxidans grows chemolithotrophically on either Fe2+ or S° (elemental suflur), L. ferrooxidans can -row only on Fe2+.
Thermoplasma, Ferroplasma , and Picrophilus are the archaebacteria among the most acidophilic of all known microorganisms. Picrophilus is capable of growth below pH 0. Thermoplasma apparently metabolises organic compounds leached from the hot coal refuse. T. volcanium has been isolated in hot acidic soils throughout the world and is highly motile by multiple flagella. Ferroplasma is a chemolithotrophic relative of Thermoplasma.
It oxidises Fe2+ to Fe3+ to obtain energy and during this process the acid is generated rerroplasma thrives in mine tailings containing pyrite (FeS), which is its energy source.
The extreme acidophily of Ferroplasma allows it to drive down the pH of its habitat to extremely acidic values. Picrophilus is even more acidophilic than Thermoplasma and Ferroplasma as it grows optimally at pH 0.7 and is capable of growth to as low as pH – 0.6. Two species of Picrophilus have been isolated from acidic solfataras in Japan.
Molecular Adaptations to Acidophiles:
The critical factor that governs acidophily is the stability of the cytoplasmic membrane. When the pH reaches neutrality (i.e., pH 7), the cytoplasmic membranes of strongly acidophilic bacteria are destroyed and the cells lyse. This indicates that strong acidophiles require high concentration of hydrogen ions for membrane stability.
Acidophiles, therefore, have evolved a unique cell membrane structure. Thermoplasma membrane contains a lipopolysaccharide-type material called lipoglycan which consists of a tetra-ether lipid monolayer membrane with mannose and glucose (Fig. 19.19). This molecule constitutes a major fraction of the total lipid content of Thermoplasma.
The cytoplasmic membrane of the latter also contains glycoproteins (but not sterols). These molecules make the Thermoplasma cytoplasmic membrane stable to hot acid condition.
Cytoplasmic membrane of Picrophilus possesses an unusual arrangement of lipids that forms a highly acid impermeable membrane at optimal pH values, which helps organism to survive only in highly acidic habitats. If, by contrast, moderate pH values (e.g., pH 4) are raised in the acidophilic environment, the cytoplasmic membranes of Picrophilus quickly become leaky and disintegrate.
Biotechnological Applications of Acidophiles:
Sulfolobus and Acidianus are thermophilic acidophilic archaebacteria as they grow at temperatures up to 90°C and at pH values of 1-5. Sulfolobus occurs in sulfur-rich acidic hot springs, while Acidianus in acidic sulfataric springs.
Sulfolobus is aerobic, whereas Acidianus is facultative anaerobic. These two archaebacteria have the potential for applied use in the recovery of metals from the ores through the process of microbial leaching.
The microbes readily adapt to the conditions of low pH and high concentrations of metals required for microbial leaching. In addition, these archaebacteria can survive at high temperatures which can occur during the oxidation of metal sulfides in bioleaching reactors.
Sulfolobus and Acidianus can also enhance the recovery of gold by oxidation of pyrite (FeS) which occludes gold preventing recovery by standard metallurgical procedures. The ability of this group of microorganisms to facilitate metals recovery is yet to be developed on a commercial scale.