Biological Classification · Lesson 3 of 10
Eubacteria, Cyanobacteria and Mycoplasma
“How common bacteria make food, recycle nutrients, reproduce and interact with living organisms.”
• Describe bacterial nutrition and ecological roles. • Explain oxygenic photosynthesis and nitrogen fixation by cyanobacteria. • Recognise binary fission, survival spores and DNA transfer as different processes. • Compare beneficial and disease-causing bacteria. • Explain the distinctive structure of mycoplasmas.
A wide range of bacteria
Eubacteria, traditionally called true bacteria, are diverse prokaryotes found in soil, water and living organisms. A typical bacterial cell has DNA, ribosomes, cytoplasm, a plasma membrane and a wall containing peptidoglycan. Some move with flagella. Small cell size does not imply simple chemistry: bacteria can obtain energy in several ways and make an enormous variety of organic and inorganic transformations possible.
Bacteria can live as decomposers, producers, parasites or partners of other organisms. Decomposers digest dead material and make nutrients available again. Other bacteria fix nitrogen, help ferment foods or produce useful compounds. Some are pathogens. An individual species' ecological role depends on its metabolism and environment rather than its shape alone.
Cyanobacteria and the significance of photosynthesis
Prokaryotic organisms capable of oxygen-producing photosynthesis using chlorophyll a and associated pigments.
Historically called blue-green algae, cyanobacteria belong among bacteria, not eukaryotic algae. Some are single cells, whereas Nostoc and Anabaena commonly form chains of cells, sometimes held within a gelatinous sheath. They may live in fresh water, salt water or moist soil. Large nutrient-rich water bodies can experience substantial proliferation of cyanobacteria, producing visible blooms.
Cyanobacteria contain chlorophyll a but lack chloroplasts. Their photosynthetic membranes are located within prokaryotic cells. The main sequence is light capture, conversion of that energy into chemical energy and synthesis of organic molecules from carbon dioxide. Oxygen is released when water is split during oxygenic photosynthesis. Thus, photosynthetic ability does not require a eukaryotic nucleus.
Heterocysts and nitrogen fixation
Some filamentous cyanobacteria contain larger specialised cells called heterocysts. These cells create a low-oxygen environment that protects nitrogenase, the enzyme needed to convert atmospheric nitrogen into ammonia and related usable nitrogen compounds. The process requires energy; neighbouring photosynthetic cells provide carbon compounds, while fixed nitrogen can support the filament. Not all cyanobacteria have heterocysts or fix nitrogen.
Chemosynthetic and heterotrophic bacteria
Certain bacteria obtain energy by oxidising inorganic chemicals, such as reduced nitrogen, sulphur or iron compounds. This energy helps drive carbon fixation in chemoautotrophs. Nitrifying bacteria, for instance, participate in transformations of ammonia and nitrite. The energy source is chemical oxidation rather than sunlight; this matters in environments with little or no light.
Many bacteria are heterotrophs and use organic materials from the environment. Decomposers break down dead bodies and wastes, returning substances to nutrient cycles. Bacteria also have useful roles in food production: lactic acid bacteria transform milk components during curd formation; particular soil and root-associated bacteria fix atmospheric nitrogen. Some microbes are sources of antibiotic compounds.
Other bacteria can damage crops or cause illness. For example, cholera is associated with Vibrio cholerae; typhoid fever with Salmonella Typhi; and tetanus with Clostridium tetani. Citrus canker is a bacterial plant disease. These examples show that bacterial impacts range from beneficial to harmful, depending on species and conditions.
| Category | How it functions | Typical significance |
|---|---|---|
| Cyanobacteria | Use light energy to produce organic compounds | Primary production; some species fix nitrogen |
| Chemosynthetic bacteria | Obtain energy from chemical oxidation | Nutrient cycling such as nitrification |
| Decomposers | Break down dead organic material | Release of reusable nutrients |
| Lactic acid bacteria | Ferment available carbohydrates | Curd and fermented food production |
| Pathogenic bacteria | Multiply or produce harmful factors in hosts | Human, animal and plant diseases |
How bacteria multiply and survive
Binary fission
A typical bacterium reproduces asexually through binary fission. First, its DNA is replicated. The copies separate as the cell elongates, and the plasma membrane and cell wall divide the cell into two daughter cells. The cells inherit genetic information from the parent, although mutation and later DNA exchange can introduce variation. Under favourable conditions, some species divide rapidly.
Spores and difficult conditions
Some bacteria form highly resistant dormant structures during adverse conditions. Endospores, produced by certain bacteria, help an individual lineage persist through heat, drying or nutrient shortage. A bacterial endospore is primarily a survival structure, not an extra offspring: one spore later germinates to give one vegetative cell. Do not confuse it with the reproductive spores produced by many fungi.
Exchange of genetic material
Bacteria can acquire DNA through processes such as conjugation, transformation and transduction. Conjugation may involve transfer between contacting cells, transformation the uptake of environmental DNA, and transduction DNA movement mediated by a virus. These mechanisms are not equivalent to fertilisation and do not necessarily increase the number of cells. Their importance lies in genetic variation and the possible acquisition of new traits.
Mycoplasma: a major exception
A group of exceptionally small bacteria that naturally lack a rigid cell wall and instead rely on their plasma membrane and associated adaptations.
Because mycoplasmas lack a cell wall, their shape can vary and antibiotics that target bacterial wall synthesis, such as penicillin, do not act on them through that target. Certain species tolerate oxygen-poor conditions and some cause diseases of animals or plants. The absence of a wall does not make a mycoplasma a virus; it is still a cellular organism with its own ribosomes and metabolism.
Binary fission increases bacterial cell number. A resistant endospore mainly preserves a cell through stress. DNA transfer changes genetic information and is not automatically the same as reproduction.
Problem
How can a cell fix nitrogen if the nitrogen-fixing enzyme is damaged by oxygen?
- 1.Photosynthetic cells release oxygen, creating a challenge for the oxygen-sensitive nitrogenase enzyme.
- 2.A heterocyst differentiates to reduce internal oxygen and support nitrogen fixation.
- 3.Fixed nitrogen supports neighbouring cells, while adjacent photosynthetic cells provide energy-rich carbohydrates.
Quick Quiz
Quiz
Why are cyanobacteria not classified as eukaryotic green algae?
Which special cell is associated with nitrogen fixation in certain cyanobacterial filaments?
Which process directly increases the number of bacterial cells?
What is the main advantage of a bacterial endospore?
Which characteristic best distinguishes mycoplasmas from most typical walled bacteria?
Chemosynthetic autotrophic bacteria obtain useful energy mainly by:
Practice Problems
- Describe the structure and role of heterocysts in selected cyanobacteria.
- Contrast photosynthetic autotrophy and chemosynthetic autotrophy.
- Explain bacterial division step by step and distinguish it from spore formation.
- Give two useful and two harmful effects of bacteria, explaining the biology in each case.
- Why do mycoplasmas respond differently from typical walled bacteria to medicines targeting cell-wall synthesis?
Heterocysts are specialised cells in some cyanobacterial filaments. They limit oxygen exposure to the nitrogenase enzyme and support conversion of atmospheric nitrogen into reduced compounds.
Key Takeaways
True bacteria include autotrophs, decomposers, symbionts and pathogens. Cyanobacteria are prokaryotic photosynthesising organisms; some use heterocysts for nitrogen fixation. Chemosynthetic autotrophs use energy from inorganic chemical reactions. Binary fission produces daughter cells; endospores aid survival; DNA transfer contributes variation. Mycoplasmas are wall-less bacteria, not viruses.