The Invisible Living World: Beyond Our Naked Eye · Lesson 7 of 9
Microbes That Support Plants and Aquatic Life
“Meet helpful microbes in legume roots and water ecosystems.”
• Explain how Rhizobium in root nodules helps legumes. • Connect legume growing and crop rotation to soil fertility. • Describe how microalgae make food, release oxygen and support aquatic animals. • Identify microalgal uses and reasons to protect their habitats.
A partnership inside roots
Beans, peas, lentils and other legumes may have small swellings called nodules on their roots. In the chapter’s cowpea image, these nodules contain Rhizobium bacteria. The bacteria make nitrogen from the air available in a form the plants can use. Nitrogen is needed for growth, but a plant cannot simply use all atmospheric nitrogen directly. This relationship helps the legume and can improve the soil for later crops.
A swelling on the root of certain legumes that can contain nitrogen-fixing bacteria such as Rhizobium.
The process by which certain microorganisms make atmospheric nitrogen available in forms plants can use.
Farmers may rotate legumes with other crops to help maintain soil fertility. This does not mean every crop always needs no added nutrients; it explains why a legume crop may behave differently from a crop without this bacterial partnership.
Problem
Why might beans grow without the same nitrogen fertiliser applied to neighbouring wheat?
- 1.Look for a difference in the plants’ roots, not merely the fields’ location.
- 2.Legume root nodules may contain Rhizobium, which helps supply usable nitrogen; wheat lacks the same nodule partnership.
Tiny producers in water
Microalgae are microscopic, plant-like organisms found in water and also in some other habitats. They use sunlight to make food and release oxygen. They support aquatic food webs because many small water animals feed on them and larger animals depend on those smaller ones. The chapter emphasises their major contribution to Earth’s oxygen supply. Their small individual size does not mean their combined effect is small.
Problem
What might happen if microalgae became scarce in a pond?
- 1.Some small aquatic animals would have less food.
- 2.Animals that feed on them could also be affected; a change at the base of a food web can spread upward.
The chapter names Spirulina, Chlorella and diatoms as examples and describes uses in supplements, medicines, water cleaning and possible biofuel. Its Spirulina sidebar also discusses cultivation and livelihood opportunities. These are applications of particular organisms and processes, not a claim that every microalga is interchangeable or that material collected from an unknown pond is safe to eat.
Protecting small organisms
Pollution, habitat damage and changing conditions can reduce microalgal abundance and diversity. Protecting water habitats therefore helps preserve organisms that supply food and oxygen to aquatic ecosystems. The two stories in this lesson show different pathways by which microbes support other life: a root partnership supplies a needed nutrient, while photosynthetic microbes themselves produce food.
Rhizobium does not supply a plant with sunlight. Microalgae do not need to live in a legume root nodule to make food. Keep nitrogen fixation and photosynthesis distinct.
Check your understanding
Connect the small-scale location of each microbe to its large-scale effect on plants or aquatic life.
Quiz
Where is Rhizobium found in the chapter example?
What does Rhizobium help provide to legumes?
What do microalgae use to make food?
Why are microalgae important to aquatic animals?
Which threat to microalgae is mentioned in the chapter?
Problem
One tiny organism is inside a cowpea root nodule; another floats in sunlit pond water.
- 1.The root-nodule microbe Rhizobium helps make nitrogen usable for the legume.
- 2.The microalga uses sunlight to make food and releases oxygen; both are helpful in different ways.
Practice Problems
- Use the cowpea-root image to explain what nodules are and what lives in them.
- Explain why growing legumes in rotation can help the soil used for a later crop.
- A student says Rhizobium “creates nitrogen out of nothing.” Correct the statement.
- Trace how a decline in microalgae might affect two levels of an aquatic food web.
- Name two human uses of microalgae from the chapter and one reason to conserve them.
- Compare the mechanisms by which Rhizobium and microalgae help other living things.
Key Takeaways
• Rhizobium in legume root nodules helps make atmospheric nitrogen usable for plants. • Legume crop rotation can help maintain soil fertility. • Microalgae use light to make food, release oxygen and support aquatic food webs. • Their ecological and possible human uses make protecting microalgal habitats important.