# 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.

> IB DP Biology Higher Level (2023 syllabus) — B4.1 Adaptation to environment · Explain · 4 marks

> 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.

## Mark scheme (4 marks)

1. Riftia lacks a digestive system and instead houses chemoautotrophic bacteria (endosymbionts) within a specialised organ called the trophosome.
2. The endosymbiotic bacteria oxidise hydrogen sulphide (H₂S) to sulphate/sulphur, releasing energy used to fix carbon dioxide via the Calvin cycle (chemosynthesis).
3. 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.
4. 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

- [Revision notes for IB DP Biology Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-adaptations-of-a-658a0fb5) · Published by Druglandscape Ltd.