Question

Can someone explain the differences between thermophiles, mesophiles, halophiles, acidophiles, etc. and under what environmental conditions they would survive successfully?

Answer 1

Microorganisms are sensitive to dramatic changes in their environments. Extreme conditions of temperature, pressure, drought, salinity, and pH affect the folding and functioning of critical biomolecules that are required for cellular integrity. However, some types of organisms can survive in these environmental conditions by adapting their cellular machinery.

They are of different types based on their environment:

Adaptation to temperature:

1) Thermophiles - Thermophiles are microorganisms that have adapted to live in extremely hot environments, for example: Thermus thermophilus that grows in Hot Springs at Yellowstone. Thermophiles can tolerate temperatures between 45-80⁰C. They have mostly chemoautotrophic mode of nutrition. Inorganic redox reactions act an energy source and utilize carbon dioxide as a carbon source (chemolithotophic). Natural environments of these bacteria are hot springs and volcanic regions in North America, Iceland, New Zealand, Japan, Italy, and the former Soviet Union. Thermophiles therefore are mostly anaerobes and can use hydrogen, sulphide, sulphur, and ferrous iron as electron donors. Some thermophiles are facultative heterotrophs and can utilize organic material provided by decaying cells while other thermophiles are microaerophile that require low amounts of oxygen. Their enzymes function at higher temperature while the lipids are rich in saturated fatty acids to maintain membrane stability at high temperature. There is increased number of ionic bonds between positive and negative charges of amino acids. Hence, densely packed hydrophobic region make them resist unfolding of proteins in aqueous environment. Increase number of disulphide bonds increases the stability of thermophillic enzymes, to help in oligomerization.

2) Psychrophiles: these are microorganisms that grow at or below 0 °C, (optimum is 15-20 °C). They live in cold environments, from the stratosphere to the deep-sea. When the seawater freezes, the salt gets concentrated in small pockets, and the freezing point of water may reach −20 °C. The bacterium Psychrobacter cryopegella can grow at this temperature, between -10 to −20 °C. Hence, most psychrophiles are also halophiles (microbes that grow in elevated salt concentrations). At low temperatures and high solute concentration, enzymes become very rigid. Further, ice crystals may pierce the cell membranes. Psychrophiles have high levels of unsaturated fatty acids in order to modulate membrane fluidity. Their enzymes are cold-adapted with high specific activities at low temperatures and can carry out transcription and translation at low temperatures.

3. Mesophiles: Mesophiles are microorganism that grow best in moderate temperature, typically between 20 and 45 °C with an optimal temperature is 37 °C. They grow in habitats such as cheese and yogurt, wine. They can be aerobic, aerotolerant, facultative anaerobes and obligate anaerobes. Hence, they can live at different oxygen levels. Mesophiles are mostly human pathogens as they live at 37 °C. They help in decomposition of organic matter and mineralization of nutrients.

Adaptation to pH:

1. Acidophiles: Acidophiles are organisms that thrive under extremely acidic conditions (pH 0-6). Acidophiles are found in Lechuguilla Cave, Carlsbad, New Mexico, where the pH is 0.0. They are found in volcanic areas, mine drainades where there is extreme acidity. The low pH is due to metabolism of the organism. Acidophiles oxidize the elemental sulfur or sulfidic minerals to obtain energy. For example, Archaea Picrophilus oshimae and P. torrid us are present in volcanically heated, dry soils in Japan. They thrive at pH 0.7 and 60 °C. Acidic environment can cause denaturation of proteins. Acidophiles have proteins with more amino acids with neutral side-groups. They also actively pump protons out of the cell, thereby maintaining constant intracellular pH levels.

2. Alkalophiles are bacteria that thrive in high alkaline conditions with enzymes are operation at high pH. Beta galactosidase in Microococcus species is functional at optimum pH 7.5. Amino acids in these proteins are incorporated into proteins at pH of 8.2-8.5. In addition to peptidoglycan, alkalophiles may contain acidic polymers galacturonic acid, gluconic acid, glutamic acid, aspartic acid, and phosphoric acid, allowing sodium and hydronium ion absorption and allowing them to grow at high pH. Sodium helps in effective transport of proteins through the membrane. Alkalophiles have high buffering capacity.

Adaptation to salinity:

1. Halophiles are bacteria that live in high salinity starting from 10% sodium chloride to saturation and can even live in salt crystals. The environments include aquatic areas of high salinity, salt marshes, surface salt lakes, subterranean salt lakes, etc. Halobacterium salinarum can live in the Dead Sea. Their cytoplasm must therefore are isosmotic with the surrounding environment. Hypersaline conditions promote protein aggregation and collapse, prevent electrostatic interactions between protein residues, and decrease the availability of water molecules. Halophile proteins, however, can fold only in high salt conditions. This binding of protein to salt is dependent on presence of large number of acidic amino acids on the surface. Further, these proteins form more random-coil structures, rather than α-helices. They have a membrane pump that pumps in potassium in while pumping sodium ion. This pump functions to maintain osmotic balance. Proteins such as cysteinyl-tRNA synthetase are active only in high salt concentration.

Adaptation to oxygen:

1. Microaerophiles are bacteria that can grow in low oxygen conditions. However, they cannot live in anaerobic conditions. They are sensitive to toxic forms of oxygen and high oxygen concentration being inhibitory. They maintain a constant turnover of primary metabolic substrates in response to different oxygen levels. As a result, they maintain catabolic enzymes, substrates, and cofactors at high steady-state levels. Under low oxygen tension, some microaerophile activate anaerobic pathways. Camplyobacter jejuni utilizes a single NrdAB-type ribonucleotide reductase for DNA synthesis. Many microaerophiles lack the enzymes that can degrade reactive oxygen species. C. jejuni has two iron sulphur cluster enzymes that are normally present in obligate anaerobes, making them more susceptible to high oxygen levels. Helicobacter pylori has a branched TCA pathway. However, both the normal and , branched pathway can be connected by α-ketoglutarate ferredoxin oxidoreductase. The branched version lacks the alpha keto glutarate dehydrogenase enzyme.

2. The facultative anaerobes can grow in the presence or absence of oxygen. They switch their metabolism according to their environment. However, they prefer aerobic environment as aerobic respiration generates the largest amount of energy for faster growth. They can use fermentation mode or other terminal electron acceptors.

3. Aerotolerant anaerobes are microorganism that can also grow in the presence or absence of oxygen but exhibit no preference. They only carry out fermentation irrespective of the oxygen levels.

4. Obligate anaerobes can only grow in the absence of oxygen. An oxygenated environment is toxic to them. They are sensitive to oxygen because they lack superoxide dismutase enzyme and catalase that protects cell from reactive oxygen species.