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← Theoretical A

Q10 — Magnetotactic Bacteria Navigation

Theoretical A Real exam question — full text reproduced under IBO's CC BY-NC-SA 4.0 license
Cell Theory, Prokaryotes & Eukaryotes

Magnetotactic bacteria have magnetosomes (magnetite crystals with a lipid membrane) arranged in a line, helping them find water with suitable oxygen levels in the oxic-anoxic transition zone (OATZ in Figure 1B). These bacteria are classified into two types: polar, which align their body orientation with the magnetic field, and axial, which can have two orientations along the magnetic field axis. In the Earth’s Northern and Southern hemispheres, polar bacteria magnetosomes have opposite polarities. To stay in the OATZ, polar bacteria change the rotation direction of their flagella based on oxygen concentration.

Panel A: a microscope image of a bacterium with a line of labeled magnetosomes, plus a magnetic-field image of the same. Panel B: a water-depth diagram showing Earth’s inclined magnetic field, the OATZ band, and bacteria rotating clockwise below it and counterclockwise above it. Panel C: a water-filled tube with two openings in a magnetic field, with bacteria starting in a red anoxic zone in the middle. Figure 1. A Magnetotactic bacteria with labeled magnetosomes, the lower image shows the magnetic field generated. B Northern hemisphere polar bacteria in water, OATZ - suitable oxygen level. C Experimental setup: water-filled tube with two openings in a magnetic field. Bacteria start in the anoxic zone (red).

On your answer sheet, indicate “T” for true statements and “F” for false ones.

A. Magnetosomes help bacteria find the shortest path toward OATZ.
B. Polar bacteria from the southern hemisphere should have a clockwise flagella rotation when above OATZ.
C. Axial magnetotactic bacteria will form 2 distinct bands in experimental setup.
D. Polar magnetotactic bacteria from the southern hemisphere will start a counterclockwise rotation of flagella when placed into experimental setup.
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Question reproduced from IBO 2024, Theoretical Exam Part A, licensed under CC BY-NC-SA 4.0 — attributed to the International Biology Olympiad. Open the full exam PDF · Community solutions (unofficial)