Introduction

When people think of microorganisms, they often think of bacteria or viruses. However, many other microscopic organisms are equally important. Fungi and protists play essential roles in ecosystems by recycling nutrients, producing oxygen, supporting food webs, and, in some cases, causing disease.

Although some fungi grow into large mushrooms that we can easily see, many fungi and most protists are microscopic. Together, they help maintain healthy ecosystems and are widely used in medicine, food production, agriculture, and scientific research.

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What Are Fungi?

Fungi are organisms that obtain nutrients by absorbing them from other organisms or organic matter.

Unlike plants, fungi:

• Do not carry out photosynthesis.
• Cannot make their own food.
• Feed by breaking down organic materials.

Fungi may be:

• Single-celled (such as yeast).
• Multicellular (such as moulds and mushrooms).

Fungal cells have:

• A nucleus.
• A cell wall made of chitin.
• Membrane-bound organelles.

This means fungi are eukaryotes.

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Figure 1. Fungi include microscopic yeasts and moulds as well as larger mushrooms.

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Characteristics of Fungi

Fungi:

• Are eukaryotic organisms.
• Have cell walls made of chitin.
• Absorb nutrients from their surroundings.
• Usually reproduce using spores.
• Can live on dead or living organisms.

Many fungi grow as thread-like structures called hyphae, which form a network known as a mycelium.

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What Are Protists?

Protists are a diverse group of mostly single-celled eukaryotic organisms.

Protists are very varied.

Some:

• Carry out photosynthesis like plants.
• Move like animals.
• Absorb nutrients like fungi.

Because they are so diverse, protists do not fit neatly into the plant, animal, or fungal kingdoms.

Most protists live in:

• Freshwater.
• Oceans.
• Damp soil.
• Moist environments.

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Figure 2. Protists include many different microscopic organisms, such as amoebas, paramecia, and euglenas.

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Characteristics of Protists

Protists:

• Are eukaryotic.
• Usually consist of a single cell.
• Often live in water.
• May move using:
  • Cilia.
  • Flagella.
  • Pseudopodia ("false feet").
• May obtain food by:
  • Photosynthesis.
  • Eating other organisms.
  • Absorbing nutrients.

Protists show a wide variety of lifestyles.

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Comparing Fungi, Protists, Bacteria, and Viruses

Each group has unique characteristics that help scientists classify microorganisms.

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Figure 3. Fungi, protists, bacteria, and viruses differ in cell structure, reproduction, and how they obtain nutrients.

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Beneficial Fungi

Many fungi are extremely useful.

Examples include:

Food Production

Yeast is used to make:

• Bread.
• Beer.
• Wine.

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Medicine

Some fungi produce antibiotics.

One famous example is:

• Penicillium, which produces penicillin.

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Decomposition

Fungi break down:

• Dead plants.
• Dead animals.
• Fallen leaves.

This recycles nutrients back into ecosystems.

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Harmful Fungi

Some fungi cause disease.

Examples include:

• Athlete's foot.
• Ringworm.
• Crop diseases.
• Food spoilage.

Although some fungi are harmful, most play beneficial roles in nature.

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Beneficial Protists

Some protists are important producers.

Examples include:

Microscopic Algae

They:

• Carry out photosynthesis.
• Produce oxygen.
• Form the base of aquatic food chains.

Many fish and other marine animals depend on microscopic algae for food.

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Figure 4. Photosynthetic protists produce oxygen and support aquatic ecosystems.

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Harmful Protists

Some protists cause disease.

Examples include:

Plasmodium

• Causes malaria.
• Transmitted by mosquitoes.

Other Protists

Some infect:

• Fish.
• Livestock.
• Plants.

These diseases can affect both ecosystems and agriculture.

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Ecological Roles of Fungi

Fungi are among nature's most important decomposers.

They:

• Break down dead organisms.
• Recycle nutrients.
• Improve soil fertility.
• Support plant growth.

Some fungi also form symbiotic relationships with plant roots, helping plants absorb water and minerals.

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Ecological Roles of Protists

Protists perform several important ecological functions.

Producers

Photosynthetic protists:

• Produce oxygen.
• Capture sunlight.
• Support aquatic food webs.

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Consumers

Some protists feed on bacteria and other microorganisms.

This helps control microbial populations.

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Decomposers

Certain protists help recycle nutrients in aquatic environments.

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Figure 5. Fungi recycle nutrients, while many protists support aquatic ecosystems as producers and consumers.

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Why Fungi and Protists Matter

Together, fungi and protists help:

• Maintain ecosystems.
• Recycle nutrients.
• Produce food.
• Manufacture medicines.
• Support agriculture.
• Sustain aquatic food chains.

Without them, ecosystems would function very differently.

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Worked Example

Question

Match each organism to its description.

Solution

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Real-World Connection

Every loaf of bread owes its rise to yeast, a single-celled fungus. As yeast feeds on sugars in the dough, it produces carbon dioxide gas, causing the dough to expand. At the same time, microscopic algae in oceans and lakes produce a significant proportion of the oxygen in Earth's atmosphere, supporting life across the planet.

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Did You Know?

Scientists estimate that phytoplankton—tiny photosynthetic protists found in oceans—produce around half of the Earth's oxygen. This means that roughly one out of every two breaths you take is possible because of these microscopic organisms.

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Key Terms

Chitin – A strong carbohydrate that forms the cell walls of fungi.

Decomposer – An organism that breaks down dead organic matter and recycles nutrients.

Eukaryote – An organism whose cells contain a nucleus and membrane-bound organelles.

Fungi – A kingdom of organisms that absorb nutrients from organic matter and include yeasts, moulds, and mushrooms.

Hypha (plural: hyphae) – A thread-like filament that makes up the body of many fungi.

Mycelium – A network of hyphae that forms the main body of a fungus.

Protist – A diverse group of mostly single-celled eukaryotic organisms that do not belong to the plant, animal, or fungal kingdoms.

Spore – A reproductive cell that can develop into a new organism without fertilisation.

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Key Takeaways

• Fungi are eukaryotic organisms that absorb nutrients from organic matter and often reproduce using spores.
• Protists are a diverse group of mostly single-celled eukaryotes that may carry out photosynthesis, consume other organisms, or absorb nutrients.
• Fungi and protists differ from bacteria (prokaryotes) and viruses (non-cellular infectious particles).
• Beneficial fungi are used in food production, medicine, and decomposition, while some fungi cause disease and food spoilage.
• Beneficial protists, such as microscopic algae, produce oxygen and support aquatic food webs, while harmful protists such as Plasmodium cause diseases like malaria.
• Fungi and protists play vital ecological roles by recycling nutrients, supporting food chains, and helping maintain healthy ecosystems.