Explain how the adaptations of a hydrothermal vent organism, such as the tube worm Riftia pachyptila, allow it to obtain energy in the absence of sunlight.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Hydrothermal vents on the ocean floor support dense communities of organisms in complete darkness, where temperatures can exceed 400 °C at the vent opening and hydrogen sulphide is continuously released.
Model answer (4 marks)
Riftia pachyptila has no digestive tract; instead it contains chemoautotrophic bacteria in a specialised organ called the trophosome.
The bacteria oxidise hydrogen sulphide (H₂S) released from the vent to sulphate or elemental sulphur, a reaction that releases energy.
This energy is used by the bacteria to fix CO₂ into organic molecules via the Calvin cycle (chemosynthesis).
The worm’s highly vascularised gill plumes (vestimentum) carry a special haemoglobin that can bind both O₂ and H₂S at the same time, delivering both substrates to the bacteria in the trophosome.
The organic molecules produced by the bacteria are transferred to the worm’s own cells, providing both energy and carbon, so the relationship is a mutualistic symbiosis.
The bacteria oxidise hydrogen sulphide (H₂S) released from the vent to sulphate or elemental sulphur, a reaction that releases energy.
This energy is used by the bacteria to fix CO₂ into organic molecules via the Calvin cycle (chemosynthesis).
The worm’s highly vascularised gill plumes (vestimentum) carry a special haemoglobin that can bind both O₂ and H₂S at the same time, delivering both substrates to the bacteria in the trophosome.
The organic molecules produced by the bacteria are transferred to the worm’s own cells, providing both energy and carbon, so the relationship is a mutualistic symbiosis.
Examiner tips
- Mention the lack of a digestive system and the trophosome first. Explain the H₂S oxidation and energy release. Show the Calvin cycle as the carbon fixation route. Highlight the dual‑binding haemoglobin and the transfer of bacterial products to the worm.
Common mistakes
- Forgetting that Riftia has no digestive system. Confusing the worm’s own metabolism with the bacteria’s chemosynthesis. Omitting the role of the specialised haemoglobin in transporting O₂ and H₂S.
Mark scheme (4 marks)
- Riftia lacks a digestive system and instead houses chemoautotrophic bacteria (endosymbionts) within a specialised organ called the trophosome.
- The endosymbiotic bacteria oxidise hydrogen sulphide (H₂S) to sulphate/sulphur, releasing energy used to fix carbon dioxide via the Calvin cycle (chemosynthesis).
- Riftia has highly vascularised gill plumes (the plume/vestimentum) with a specialised haemoglobin that binds both oxygen and hydrogen sulphide simultaneously, transporting both to the bacteria in the trophosome.
- Organic molecules produced by the bacteria are transferred to and used by the tube worm's own cells as an energy and carbon source, making the association a mutualistic symbiosis.
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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