Introduction to extremophiles

Teacher Background information: The Great Salt Lake is an interesting ecosystem to study due to the organisms that inhabit this saline environment and due to the impact humans have had on this ecosystem. In 1952, a causeway was built to replace an existing railroad trestle. Unlike, the previous trestle, the new causeway does not allow circulation of water between the two sides of the Great Salt Lake. The causeway splits the lake into a north and south arm. Because of this and the fact there is no fresh water source into the north part of the lake, the salinity concentration drastically varies between the two sides. The north arm of the lake can be nearly 30% saline while the south may only be 5%. The ecosystem in the south part of the lake is more diverse than the north because of the lower salt concentration. Fewer species of organisms are able to survive in the hypersaline environment in the north end. However, one prominent organism in the north arm of the Lake is Halobacterium of the archaea domain. These organisms are salt loving organisms.

In this lab, the students will be working with Halobacterium sp. NRC-1. Halobacterium is an excellent model organism to use in the classroom because of its unique characteristics. They are easy to culture and are a relatively harmless organism. These archaea are prokaryotes meaning they do not have a nucleus or other membrane bound organelles. Also, these archaea have both a cell wall and membrane that have a notably different composition when compared to their bacterial counterparts. Specifically, their cell wall is composed of different amino acids and sugars. Organisms in this domain inhabit some of the most extreme environments on Earth. There are some species of archaea that can survive in deep sea thermal vents where the heat is more than 300 times that which humans can endure. Some species live in high sulfur and high pressure (7,000 lbs/in2) environments, uninhabitable to most other organisms.Halobacterium thrives in salty environments in which most other organisms cannot survive. The optimal salt concentration for Halobacterium is ~4.3 M which is about ten times the salinity of sea water. At more dilute concentrations of salt, the shape of the Halobacterium will become distorted. The cells will lyse at approximately 1 M NaCl. Halobacterium has evolved over millions of years in a high salt environment, and as a result, its physiology is best suited to function in this environment. For example, their proteins have optimal activity in high salt. Also, their cytoplasm contains at least 3M K+ and 1M Na+. This molarity is essentially isotonic to the high saline environment in which they live. At low salt concentrations, water will move into their hypertonic cell causing it to expand. When the concentration difference is great enough (as it is in a 1 M solution) the cell will rupture due to the large intake of water. Halobacterium are found all over the world in hypersaline environments such as the Great Salt Lake and the Dead Sea. Large blooms of Halobacterium appear reddish purple due to the production of the pigment bacteriorhodopsin (Fig. 1). This pigment absorbs light energy to create ATP, which can be used by the organism when oxygen levels are low. Halobacterium can also absorb and digest nutrients (amino acids) from the environment, making them heterotrophs (specifically, aerobic chemoorganotrophs). Halobacterium are limited microaerobic phototrophs, but live optimally as aerobic chemoorganotrophs and are therefore classified as such. Because they live in a harsh environment and process energy is unique ways, they are an even more interesting organism for scientific inquiry.

Figure 1. Solar evaporation ponds at the abandoned chemical plant of the Pittsburgh Plate Glass Company at Bartlett (at northwest end of Owens Lake) are colored vivid red by salt-loving organisms. From http://www.desertusa.com/mag98/april/owens/owenslake.html