The Cannon Fungus: Grow a Fungus That Fires Objects at Light Sources!
AbstractThe Pilobolus fungus has an interesting way of making sure the next generation has a good start on life. At high speed, the fungus shoots a sac that contains spores toward a light source. Why toward a light source? Because that is where it is most likely to find an open area with grass. Once the spore is placed on grass, it is eaten by a cow or a horse, which is a critical step in its life cycle. The spore passes through the animal's digestive track and ends up in a pile of manure. For a fungal spore, this is the perfect place to be, since it is warm and full of nutrients. The spore then grows to become a mature fungus, eventually making a new "spore cannon," and the cycle begins anew. In this biology science fair project, you will grow a Pilobolus culture (though not in manure) and test how its "spore cannon" responds to various light conditions.
David B. Whyte, PhD, Science Buddies
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ObjectiveGrow a culture of the fungus Pilobolus and experiment with how its "spore cannon" responds to light conditions.
The fungus Pilobolus is a common inhabitant of cow and horse manure, or dung. While you and I might consider that a less than ideal place to live, this is high-value real estate for a fungus. The conditions are warm and moist, and there are abundant nutrients. The fungus grows in the dung by forming a network of hyphae, which are threadlike structures composed of cells that are attached end to end. It is the main tissue of the growing fungus. After growing throughout the dung pile for two to three days, the fungus begins a process of forming structures, called fruiting bodies, that are required for asexual reproduction. Fruiting bodies contain spores. These spores will grow into new fungi under the right growing conditions.
The ideal place for a young spore to start out is on a blade of grass. From there, it can be eaten by a horse or a cow. After passing through the digestive tract unscathed, the spore will, if it is lucky, find itself in a fresh dung pile all its own. But how does the parental fungus solve the problem of getting the spores out of the dung pile and onto the grass? Pilobolus has solved this problem by developing two interesting features: a light-sensitive structure that allows the fruiting body to locate light sources, and a mechanism for firing the sac full of spores toward the light source (a spore sac is also called a sporangium). Why shoot toward the light? Well, imagine a dung pile that is surrounded by trees on three sides. If it shoots the spore sac randomly, three out of four sacs will hit a tree and be lost to the next generation. The side with light is a better bet—it has a good chance of having grass, thus putting the fungal spores in a place where they stand a chance to be eaten by a passing cow or horse.
Pilobolus has evolved mechanisms that allow it to aim and fire in order to place its spores in the best possible spot for survival and reproduction. How does it do it? The stalk below the sporangium becomes swollen with liquid (due to osmotic pressure), with a black mass of spores on the top (see Figure 1, below). Below the swollen tip is a light-sensitive area, which is critical in directing the growth of the Pilobolus so that it faces toward the light. As the fungus matures, pressure builds in the stalk until the tip explodes, launching the spores into the light.
A shiny black spore sac sits on the end of a stalk of Pilobolus. The stalk is filled with a clear liquid and resembles a drop of water turned sideways.
Figure 1. Parts of the Pilobolus fruiting body. The sporangium contains spores and sits on top of a vesicle containing fluid at high pressure. The sporangium is shot toward a light source, having been aimed in the correct direction by the sporangial stalk. The spore sac reaches accelerations that are among the fastest in nature.
In this biology science fair project, you will grow a culture of Pilobolus and experiment with how its "spore cannon" responds to various light conditions. This science fair project will involve some creative problem solving on your part, since you are working with a live organism. It would be a good idea to give yourself enough time to become adept at culturing the Pilobolus before you begin experimenting. Also, the experiments will most likely need several rounds of troubleshooting and fine-tuning, so allow time for this as well.
Terms and Concepts
- Asexual reproduction
- Fruiting body
- Spore sac
- Osmotic pressure
- Sporangial stalk
- What are the various structures that make up the fruiting body of a Pilobolus fungus?
- Based on your research, how does the fungus generate the force required to fire the spore sac?
- What is the speed with which a spore sac is launched? How far does the spore sac travel?
- What makes the spore sac stick to the surface on which it lands?
- What is the definition of positive phototropism?
- Wikipedia Contributors. (2009, August 22). Pilobolus. Retrieved September 15, 2009.
- Eddleman, H., PhD. (1999). Dung Jars. Retrieved September 16, 2009.
This paper by Coble and Bland provides background and ideas for experimenting with Pilobolus.
- Coble, C.R. and Bland, C.E. (1974). Open-Ended Experimentation with the Fungus Pilobolus. Retrieved August 30, 2009.
This paper describes how the authors used super-high-speed photography (250,000 frames per second!) to capture the entire spore launch process in four species of fungi.
- Yafetto, L., et al. (2008). The Fastest Flights in Nature: High-Speed Spore Discharge Mechanisms among Fungi. Retrieved August 30, 2009.
This website provides good background information about Pilobolus, along with a fun animation.
- Fogel, R. (1996). Pilobolus— Fungal Shotgun. Retrieved August 10, 2009.
Materials and Equipment
Pilobolus culture kit; available from Carolina Biological Supply Company, item #155800P.
- Consider ordering two kits if you plan to try variations on the basic project.
- Rabbit dung agar plates. The kit comes with 10 plates, but you will need at least 12 total. Plates are available separately from Carolina Biological Supply Company, item #821427.
- Sterile cotton-tipped applicators (e.g., Q-tips) or the sterile disposable scalpel from the kit may be used. Applicators are considered sterile if they are from an unopened box. Sterile applicators are also available from Carolina Biological Supply Company, item #703032.
- Magnifying glass
- Lab notebook
- Lamp with bulb (75W or equivalent)
- Note: the suggested wattage is for an incandescent bulb. If you purchase compact fluorescent (CFL) or light-emitting diode (LED) bulbs, their actual wattages will be lower, but they will display an "equivalent" wattage on the package.
- Optional: Camera
- Permanent marker
- Metric ruler
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For health and safety reasons, science fairs regulate what kinds of biological materials can be used in science fair projects. You should check with your science fair's Scientific Review Committee before starting this experiment to make sure your science fair project complies with all local rules. Many science fairs follow Intel® International Science and Engineering Fair (ISEF) regulations. For more information, visit these Science Buddies pages: Project Involving Potentially Hazardous Biological Agents and Scientific Review Committee. You can also visit the webpage ISEF Rules & Guidelines directly.
Important Note Before You Begin: You will need to perform a total of three trials to demonstrate that your results are accurate and repeatable. The best way to do this is to start trial #2 one day after you've started trial #1, and trial #3 one day after you've started trial 2 (two days after you started trial #1). Keep careful notes in your lab notebook for all steps.
- Using a sterile disposable scalpel (part of the kit) or a sterile cotton-tipped applicator, transfer a few sporangia from the Pilobolus plate culture to four rabbit dung agar plates. Inoculate directly on the exposed dung surface.
- Invert the plates and incubate at room temperature overnight.
- The next day, place the plates agar-side-down under a light source, such as a ceiling light fixture.
- Incubate for two days.
- Use a permanent marker to number the plates 1 to 4. Label the bottom and the lid.
- Remove the covers from plates #2, #3, and #4 and wrap the plates in aluminum foil (included in the kit).
- Make small holes in the aluminum foil with the pin, one hole per plate: 1-mm diameter for plate #2, 2-mm diameter for plate #3, and 4-mm diameter for plate #4. Put each hole in the middle of each foil disk.
- Put the plastic lids back on plates #2, #3, and #4.
- Leave the plastic lid on plate #1.
- Place the plates under a light source; for example, a lamp with a 75W (or 75W equivalent) bulb.
Day 5 and Beyond:
- Observe plate #1 with the clear plastic lid. After you see black sporangia adhering to the underside of the cover of plate #1 (4–7 days), carefully remove the aluminum foil from plates 2–4.
- Look for sporangia on the underside of the foil, near the holes. Observe the pattern around the light holes and record all observations in your lab notebook. Use a magnifying glass, if necessary.
- How accurate is the aim of the Pilobolus toward the light source?
Analyzing Your Results
- Draw concentric circles around the light source holes and count the number of sporangia. Graph the results.
- Graph the size of the light hole on the x-axis and the size of the region with sporangia on the y-axis.
- Does the accuracy depend on the size of the hole?
Bacteria are all around us in our daily lives and the vast majority of them are not harmful. However, for maximum safety, all bacterial cultures should always be treated as potential hazards. This means that proper handling, cleanup, and disposal are necessary. Below are a few important safety reminders.
- Keep your nose and mouth away from tubes, pipettes, or other tools that come in contact with bacterial cultures, in order to avoid ingesting or inhaling any bacteria.
- Make sure to wash your hands thoroughly after handling bacteria.
- Proper Disposal of Bacterial Cultures
- Bacterial cultures, plates, and disposables that are used to manipulate the bacteria should be soaked in a 10% bleach solution (1 part bleach to 9 parts water) for 1–2 hours.
- Use caution when handling the bleach, as it can ruin your clothes if spilled, and any disinfectant can be harmful if splashed in your eyes.
- After bleach treatment is completed, these items can be placed in your normal household garbage.
- Cleaning Your Work Area
- At the end of your experiment, use a disinfectant, such as 70% ethanol, a 10% bleach solution, or a commercial antibacterial kitchen/bath cleaning solution, to thoroughly clean any surfaces you have used.
- Be aware of the possible hazards of disinfectants and use them carefully.
Ask an Expert
- What happens if there are two point sources?
- Use different colors of light to determine which color the Pilobolus responds to best. You can buy colored plastic filters (or gels) online. Or you could experiment with different colors of light emitting diodes (LEDs).
- Try varying the timing of light exposure, keeping the intensity the same. For example, expose separate cultures to 2 hours of light in the morning, mid-day or evening.
- Cover a culture with a 5-gallon bucket with a hole in it (with paper on the inside and tape over the hole). How accurate is the Pilobolus when the light source is at this increased distance?
- Devise a way to measure the maximum distance (vertical and horizontal) a spore sac can be fired.
- Repeat the procedure using Pilobolus that you cultured from nature. See the website in the Bibliography, authored by Dr. Eddleman, for information on growing Pilobolus.
If you like this project, you might enjoy exploring these related careers:
- Science Fair Project Guide
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- Scientific Review Committee