Beneficial microbes are everywhere, one of the easiest place to locate these in a soil context are Rhizobia, a nitrifying bacteria (diazotrophs), that make roots its home. It inhabits the root nodules of the legume family (Fabaceae), external to a host the bacteria is unable to express genes that allow it to fix nitrogen. In return for offering a home and sugar as feed, the plant receives a reliable source of Nitrite, a macronutrient.
This activity allows participants to locate, extract and cultivate the bacterium located in these nodule. It makes something often taught as a concept, which is widely abundant yet somehow extremely inaccessible, easily available and possible to interact with.
Specific learning outcomes
- Exploring the needs of bacterium
- Observing the results of symbiotic relationships
- Testing the impact on symbiotic relationships in relation to development of plants
Equipment for a group of 5
- Petri dish with nutrient agar x5
- Distilled water (boiled and cooled water also possible)
- 5-10% bleach
- Lighter/gas stove
- Needle tips or more forceps
- Glass bowls
Estimated time 20 minutes.
- Gathering legume roots with nodules, potentially may want to gather a number from various sites for comparison.
- Remove soil from roots in order to be able to examine
- If not purchasing premade petri dishes, make up dishes with nutrient agar and try to maintain sterility as much as possible.
Introduction 5 mins, introducing role of rhizobia and tasks
If gathering root materials and cleaning 20 mins
Preparing nodules for removal 5 mins
Extracting rhizobia 5 mins
Total timing 35 minutes.
Cultivation time 5-7 days.
Background learning needs
- Understanding of symbiotic relationships
- Understanding of bacterium
- Understanding of the nitrogen cycle and nitrifying bacteria’s role
- Use a brush to clean the soil from the roots of the legume plant, ensuring it has small white to pick nodules
- Cut off leaf matter
- Submerge roots into bleach for 5 minutes
- Rinse root material in water
- Use a clean surface to place the roots on and sterilize tweezers
- Use tweezers to pull of a nodule
- Add a drop of water to the centre of the agar
- Place the nodule into the drop of water upon the agar, use the tweezers to crush the nodule- it should release a slightly cloudy liquid into the water drop
- Place lid on dish and label dish with date
- Place dish out of sunlight or cover, somewhere between 20-30oc
- After 24 hours the milky solution released from the nodule begins to become white, it starts to form a large white slimey patch. Others may form but this is contamination of the agar.
- After ~10 days use a hooked probe or a teaspoon to gently scrape the white Rhizobia and add to water at 25oc, place in a bottle and shape well. This creates an inoculant of Rhizobium; this can then be used in a bean root development experiment.
- Place compost in the microwave for 10 minutes, this sterilizes it. Place in 2 seed trays when cool and sow runner beans. Water one with tap water and the other with the rhizobia inoculant, label.
- Grow the beans for 3-4 weeks old and then remove plants, clean roots and examine the nodules formed.