Question

7. Each of the following bacterial lifestyles exposes the bacteria to an environment that requires special evolutionary adaptations to survive. In each case, explain what adaptations were required for the bacteria to live in that environment, and describe how the adaptations allow survival.

A. Thermophiles

B. Microaerophiles

C. Alkalophiles

D. Halophiles

Answer 1

7A, Thermophiles grow at higher temperature above 55⁰C, and have mostly chemoautotrophic mode of nutrition. Inorganic redox reactions act a energy source. They utilize carbon dioxide as a carbon source (chemolithotophic). Natural environments of these bacteria are hot springs and volcanic regions. Volcanic gases may be present in these regions. 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. Some thermophiles are microaerophile that require low amounts of oxygen. Their enzymes function at higher temperature. 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. They have lipids with “branched” C40 hydrocarbon chains composed of repeating units isoprene, bonded by ether-linkage (-O-). Ether linkages prevent thermal breakage of membrane, making them more stable under high temperature conditions. Increase number of disulphide bonds increases the stability of thermophillic enzymes. These bonds help in oligomerization. They also have increased salt bridges that acts as a structurally stabilizing element. As a result, there is increase in the thermal capacity of proteins that favour charge-charge interactions.

B. 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. Helciobacter 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 glutarte dehydrogenase enzyme.

C. Alkalophiles are bacteria that thrive in high alkaline conditions. Their enzymes are operation at high pH. Beta galactosidase in Microococcus species is functional at optimum pH 7.5. Amino acids 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.

D. Halophiles are bacteria that live in high salinity. 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. There is decrease in hydrophobic amino acid frequency. 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.