Q46 — Osmotic and Chemical Stress in a Marine Tardigrade

Theoretical B Real exam question — full text reproduced under IBO's CC BY-NC-SA 4.0 license

Echiniscoides sigismundi, a marine tardigrade, can change its activity level and morphology when exposed to osmotic and chemical stress. To study this, a series of experiments with different environmental conditions was conducted, and the tardigrades’ activity levels were measured. Each experiment consisted of three time periods (24 hours each) during which the environmental changes were applied (Figure 1, panels A–H).

Osmotic conditions used: ultrapure water (~0‰), saturated seawater (~240‰), and natural habitat seawater (Lsw). Chemicals used: DNP — a mitochondrial uncoupling agent. In some cases osmotic and chemical conditions were applied together (e.g. DNP(~240‰) means DNP + saturated seawater).

Four-panel figure (A–D) of tardigrade activity level (%) over 72 hours under different combinations of natural seawater, ultrapure water, and saturated seawater exposure, each panel illustrated with a small tardigrade drawing showing its body shape at that stage. Figure 1 (panels A–D). X — time (h), Y — activity (%). Panel A: natural seawater (Lsw) throughout. Panel B: ~0‰ (ultrapure water) from 0–24h, then Lsw. Panel C: ~240‰ (saturated seawater) from 0–24h, then Lsw. Panel D: DNP(Lsw) from 0–24h, then Lsw. All activity levels at t = 72h in panels B, C, D are not significantly different from panel A.

Four-panel figure (E–H) continuing the tardigrade activity-level time course, testing DNP combined with different osmotic conditions, with significance markers ("*" significant, “ns” not significant) comparing each panel’s 72-hour activity level against the corresponding osmotic-only panels from Figure 1. Figure 1 (panels E–H, continued). X — time (h), Y — activity (%). Panel E: DNP(~0‰) from 0–24h, then Lsw. Panel F: DNP(~240‰) from 0–24h, then Lsw. Panel G: DNP(Lsw) from 0–24h, then ~240‰, then Lsw. Panel H: ~240‰ from 0–24h, then DNP(Lsw), then Lsw. “” — significantly different, “ns” — not significantly different from the corresponding graphs (B, C, D) at t = 72h.*

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

A. Chemical stress has a more pronounced impact on activity level compared to osmotic one.

B. ATP is crucial to tolerate hypoosmotic environments.

C. It is evident that DNP followed by either hypoosmotic or hyperosmotic conditions has the same impact on activity level at t=72h.

D. DNP exposure without osmotic stress is sufficient for a marine tardigrade's morphological changes to happen

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Question reproduced from IBO 2024, Theoretical Exam Part B, licensed under CC BY-NC-SA 4.0 — attributed to the International Biology Olympiad. Open the full exam PDF