Explain how the adaptations of a named deep-sea hydrothermal vent organism allow it to obtain energy in the absence of sunlight.

IB DP Biology Standard Level (2023 syllabus) — B4.1 Adaptation to environment · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Deep-sea hydrothermal vents are found at mid-ocean ridges where superheated, mineral-rich water is released from the ocean floor. No sunlight penetrates to these depths, yet diverse communities of organisms thrive around these vents.

Model answer (4 marks)

Riftia pachyptila (tube worm) – the worm has no mouth or gut; it contains a specialised organ, the trophosome, which houses chemosynthetic bacteria.
The bacteria oxidise hydrogen sulfide released from the vent, using the energy to fix CO₂ into organic molecules.
The organic carbon produced by the bacteria is transferred directly to the worm’s tissues, providing the worm’s nutrition.
The worm’s long, filamentous gills maximise the surface area for absorbing sulfide and oxygen from the vent fluid, ensuring efficient supply of reactants to the bacteria.

Examiner tips

Identify a vent species and its key organ for symbiosis; explain the bacterial energy source; link bacterial metabolism to host nutrition; describe a structural feature that aids uptake.

Common mistakes

Using a non‑vent species; confusing photosynthesis with chemosynthesis; omitting the host’s structural adaptation; not linking the bacteria’s metabolism to the host’s nutrition.

Mark scheme (4 marks)

Named organism correctly identified as one associated with hydrothermal vents (e.g. tube worm / Riftia pachyptila, vent mussel / Bathymodiolus, vent shrimp)
Chemosynthetic bacteria (or Archaea) oxidise inorganic compounds such as hydrogen sulfide to release energy, instead of using light energy
The named multicellular organism (e.g. tube worm) harbours chemosynthetic bacteria as endosymbionts, providing the host with organic carbon / nutrients in return for shelter and access to reactants
Structural adaptation of the named organism that facilitates access to vent chemicals or efficient uptake of nutrients from endosymbionts (e.g. tube worms lack a mouth and digestive tract; rely entirely on trophosome for nutrition / large surface area of gill filaments in vent mussels maximises uptake of dissolved sulfide and oxygen)

Key terms in this question

hydrothermal vent