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Studying Living Organisms

Using Magnetotactic Bacteria to Study Natural Selection

Tim Culp
Woodrow Wilson Biology Institute
1995

Target age or
ability group:
High School
Class time
required:
One period for the actual laboratory investigation. Additional time for discussion.
Materials and equipment: Compound microscope with 100X to 400X magnification
Bar magnet
Samples of water and sediments from marsh, bay, lake, etc.
Magnetic compass
1/2 inch rubber "O-ring"
Glass slide and cover slip
Petroleum jelly
dropper
Summary of activity: This integrated science lab dynamically demonstrates that living organisms may possess incredible adaptations which enable them to survive. In this lab students will observe bacteria that are able to orient themselves using the Earth's magnetic field. The lab should be supplemented with a discussion of the Earth's magnetic field lines. It should be stressed that magnetic force field lines are not parallel to the Earth's surface except at the equator. Both north and south of the equator these lines act inward as well as toward the respective pole.

The lab offers an excellent opportunity to discuss natural selection. Magnetobacteria require anaerobic or near anaerobic conditions. Numerous species have been discovered in both fresh and saltwater sediments. Regardless of the species, those individuals found in the northern hemisphere are "north-seeking." When disturbed they move inward along the magnetic force field lines enabling them to rebury themselves in the anaerobic environment which they require. Magnetotactic bacteria in the southern hemisphere are "south-seeking." Selection pressures act against bacteria which seek the "wrong" pole. Magnetobacteria are found at the equator in both north and south-seeking varieties. Apparently by swimming horizontally in either direction when disturbed, enough of them embed themselves in anaerobic environments to perpetuate their species.

Magnetobacteria may be divided into two basic groups: one composed of species that use magnetic iron oxide particles and the other composed of species using magnetic iron sulfide. Recently ribosomal RNA comparisons have shown that the two groups are NOT closely related. The development of their ability to construct iron-based particles for magnetaxis is currently being explained in terms of convergent evolution.

Prior knowledge, concepts or vocabulary necessary to complete activity:
The students should understand:
  • how to use a compound microscope properly
  • the difference between anaerobic and aerobic organisms
  • the concept of magnetic polarity including a discussion of how a compass works
  • the shape of magnetic force field lines around a bar magnet and around the Earth itself
  • that magnetic force field lines act inward as well as toward the geographic pole

Teacher Instructions

1.Collect samples of water and surface sediments from a pond, lake, marsh, or bay. It is best to collect this at least a week before the lab. Keep in dim light. The bacterial populations will increase dramatically over time making the lab experience much more exciting. Populations kept in dim light have been found to increase over a period of months and even years.

2.The "cookbook"style lab that follows could easily be replaced by a student-designed investigation. This would require a discussion of the Earth's magnetic field, anaerobic conditions, etc. prior to the lab.

Name

Date

Period

Magnetotactic Bacteria

Purpose:In this laboratory investigation you will observe the ability of certain bacteria to sense and respond to the Earth's magnetic field. These bacteria are anaerobic and able to swim using flagella to pull themselves through the substrate. You will then be asked to formulate a hypothesis as to how this adaptation improves their chances for survival.

Materials and Equipment:
compound microscope
bar magnet
sample of water and sediments
pond, marsh, or bay magnetic compass
1/2 inch rubber "O- ring"glass slide and cover slip
petroleum jellydropper

Procedure:
1.Rub an O-ring with a thin layer of petroleum jelly and center it on a clean slide. The petroleum jelly should hold the O-ring in place.

2.Place a drop of the pond water/sediment on the cover-slip. Invert it quickly (creating a hanging drop) and place it on the O-ring so that the drop hangs freely inside the O-ring.

3.Using a compound microscope, focus on the edge of the droplet farthest away from you. Change the objectives until you reach a magnification of 100X.

4.Hold the north end of a bar magnet next to a compass. If the marked end of the compass (north-seeking) moves toward the north end of the bar magnet you may proceed. If, however, the compass points away from the north end of your magnet, the poles on the magnet have been reversed. You will need to reverse the poles on the magnet or obtain a different bar magnet.

5.Hold the north end of a bar magnet next to the edge of the slide farthest away from you. Using at least 100X magnification you will begin to see the bacteria congregate on the edge of the drop nearest the magnet. They swim very quickly and should be evident within 5-10 seconds.

6.Once the bacteria appear, turn the magnet around. What do the bacteria do? Repeat as many times as you wish. You may also find it interesting to use a higher power objective to view the bacteria.

Thought Questions:

1.Create an hypothesis about the survival advantage of the adaptation of magnetotactic bacteria demonstrated above.

2.What packaged traits must be found together to make this adaptation possible? That is, is it enough just to be able to sense the magnetic field of the Earth? Explain.

3.What other organisms might be able to use the Earth's magnetic field as an aid in navigation? What advantages does such an ability bestow on an organism in terms of survival? Can you think of any disadvantages such an ability might carry with it?

Challenges:

1.Create a three-dimensional compass that demonstrates that magnetic force field lines act inward as well as toward the respective pole.

2. Develop an experiment to test your hypothesis (#1 under "Thought Questions") about the survival advantage of these bacteria. Perform your experiment.

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