30e.1 Using new molecular techniques, scientists are discovering more about bacteria.
In their studies, microbiologists have only identified a small number of the microorganisms that exist; there are many undiscovered species, genera, families, orders, and even phyla of bacteria.
Comparative ribosomal RNA sequencing is a powerful new tool that allows microbiologists to construct bacterial phylogenies.
These phylogenies have revealed that there are three lineages of organisms, Bacteria, Archaea, and Eukarya, and that far greater evolutionary differences exists between two bacterial species than between any two eukaryotic species.
Microbiologists have begun to use phylogenetic information about bacteria to construct specific ribosomal RNA sequences. These sequences can be added to extreme environments to probe for life within the often difficult to study environment.
Using these rRNA probes, scientists have discovered many types of extremophiles, microorganisms that have evolved to live in certain types of extremes, such as high or low temperatures, high or low pH levels, high pressures, high salinities, and even high amounts of radiation.
Studying these extremophiles may reveal how molecular stability is achieved and how enzymes function under extreme conditions. Characteristics of early life forms on earth may also be revealed.
30e.2 The different types of extremophiles have different modifications that allow them to thrive in particular extremes.
- Thermophiles are bacteria that grow optimally at temperatures between 45°C and 80°C. Hyperthermophiles have growth optima over 80°C, and some can even survive at 113°C.
- Hyperthermophiles can be found in deep sea hydrothermal vents, where a variety of temperature microhabitats exist.
- Proteins in hyperthermophiles are able to remain stable because they are more compact in their structure, they have a reduced amount of flexibility, extensive ionic bonding provides extra rigidity, and chaperonin proteins are present that refold denatured proteins into their active forms.
- DNA, which normally denatures at high temperatures, keeps its double-stranded form in hyperthermophiles due its positive supercoiling, which allows more heat stability than the negative supercoiling in DNA of other organisms. Also, unique DNA binding proteins help maintain and stabilize the DNA.
- To prevent their cell membranes from coming apart at high temperatures, hyperthermophiles form their cell membranes as lipid monolayers instead of the lipid bilayers seen in other organisms.
- Psychrophiles are microorganisms that grow optimally at temperatures of 15°C or lower, such as those found in the Antarctic or Arctic.
- In contrast to the proteins of thermophiles, proteins in psychrophiles have structures that maximize flexibility and polarity.
- The cell membranes of psychrophiles contain more unsaturated fatty acids to keep the membrane sufficiently fluid to allow the transport of nutrients across it.
- Acidophiles grow optimally at a pH of 0.7 or lower, and they survive by keeping the acid in their environment out of their cell interiors.
- Alkaliphiles live in environments with pH levels above 10. They maintain a neutral cell interior, but instead of relying on a proton gradient to drive ATP synthesis, alkaliphiles use a sodium ion gradient.
- Halophiles are organisms that live in salty environments. They prevent the tendency of water to move out of the cells by maintaining a solute concentration inside their cells equal to or slightly above the NaCl concentration on the outside.
- Some extreme barophiles grow optimally at pressures over 500 atmospheres. The transmembrane proteins in barophiles likely are modified to still function at high pressures.
- Radiation-resistant bacteria have powerful DNA repair machinery to combat the effects of the high radiation doses on the DNA.
30e.3 Continued study of extremophiles and bacterial diversity will lead to more discoveries.
- Because some hyperthermophiles may have evolved relatively little in the 3.5 billion years they have been on Earth, studying them may reveal what life on early Earth was like.
- Studying bacterial diversity and the functions of the many unique genes possessed by bacteria has exciting potential.