B4.2.2—Oxygen tolerance
Oxygen tolerance shapes microbial niches because some organisms require oxygen, some are poisoned by it and others can switch pathways.
- Syllabus
- First assessment 2025
- Objective
- B4.2.2
- Level
- HL
Oxygen Tolerance Separates Microbial Niches
Oxygen tolerance divides organisms into obligate aerobes, obligate anaerobes and facultative anaerobes according to whether oxygen must be present, must be absent or may vary.
Obligate aerobes require oxygen and cannot grow without it. Obligate anaerobes are harmed or killed by oxygen and occupy anoxic environments. Facultative anaerobes grow with or without oxygen, changing metabolic pathway as conditions change.
Examples: Mycobacterium tuberculosis is an obligate aerobe; many strict anaerobes grow only in oxygen-free sediments or tissues; Escherichia coli is facultative and can tolerate both oxic and anoxic conditions.
Across a sediment gradient, an obligate aerobe is restricted to the oxygenated surface, an obligate anaerobe to deeper anoxic layers, and a facultative anaerobe may occur in both.
Anaerobe does not automatically mean killed by oxygen: that describes an obligate anaerobe. Facultative anaerobes tolerate oxygen absence but often grow differently when oxygen is available.
Oxygen tolerance
Assessment in practice
1 marksHow it is assessed
This objective is assessed through structured response.
What earns marks
Build the answer around this relationship: Obligate aerobes require oxygen for survival.
Representative question
Question 1
[Maximum number: 1]
Seaweeds are obligate aerobes. Describe an environmental condition required for seaweed survival.
oxygen;
Niches, Nutrition, And Competition
A niche is the role of a species in a community, including habitat, abiotic tolerances, nutrition, activity, reproduction, and interactions. The evidence should match the question: nutrition mode, oxygen tolerance, feeding adaptation, predator-prey interaction, plant light strategy, fundamental versus realized niche, or competitive exclusion.
- Definition answers should include role plus habitat/resources/interactions, not only place.
- Nutrition modes explain how organisms obtain carbon, energy, or organic matter.
- Adaptation examples should be linked to feeding, defence, light harvesting, or competition.
- Fundamental versus realized niche and competitive exclusion are tested together because interactions narrow where species actually live.
Concept essentials
- Obligate aerobes require oxygen for survival.
- Obligate anaerobes live where oxygen is absent.
- Facultative anaerobes can switch between aerobic and anaerobic metabolism.
- Methane production is favoured in anaerobic environments.