Two Sealed Bottles: Why Did Only One Ecosystem Survive?
Two identical 2 L clear bottles are filled with pond water, one sprig of Elodea, and two small snails, then sealed with tight caps. Bottle A is placed on a sunny windowsill. Bottle B is placed inside a closed cabinet. Both bottles sit in the same room at 21–23 °C. After ten days students record:
- Bottle A: water clear, Elodea green and noticeably longer, snails moving on the glass.
- Bottle B: water cloudy, Elodea pale and coated with a fuzzy film, both snails dead on the bottom.
A student writes, "The snails in B starved, because in the dark the plant stopped making food and there was nothing for them to eat."
1. Use photosynthesis and respiration to explain why the two sealed bottles ended up so different.
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In Bottle A, light reaches the Elodea, so photosynthesis runs: the plant uses carbon dioxide and water to make sugar and releases oxygen into the water. The snails and the plant both respire, taking in oxygen and giving off carbon dioxide. In sunlight the plant's oxygen output is larger than the bottle's total oxygen use, so oxygen stays available, snails stay active, and the water stays clear.
In Bottle B there is no light, so photosynthesis stops. Respiration does not stop. The Elodea and both snails keep consuming the dissolved oxygen and releasing carbon dioxide, and nothing replaces that oxygen. Oxygen drops until the snails suffocate. Once organisms die, bacteria and fungi decompose them, which also uses oxygen and clouds the water - that is the fuzz and the cloudiness.
2. Evaluate the student's claim. Is "the snails starved" an accurate explanation? Support your answer.
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It is only partly right, and the main cause is misidentified.
What is right: with no light, the Elodea's photosynthesis shuts down, so the bottle's oxygen supply collapses.
What is wrong: the snails most likely died of low oxygen, not hunger. Respiration kept consuming oxygen in the dark while nothing produced more, so the water ran out of dissolved oxygen long before ten days. Dead snails on the bottom with cloudy water fit suffocation plus decay.
A second mix-up: the sugar a plant makes in photosynthesis is the plant's food, not a food the snails eat. Snails graze on algae and plant material. Losing photosynthesis removes the bottle's oxygen source first; the loss of snail food would be a slower, separate problem.
3. Predict what would happen if Bottle A stayed sealed on the windowsill for another three weeks. Explain the reasoning behind your prediction.
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Most likely Bottle A remains roughly stable, with both organisms alive, because a sealed bottle in light can reach a balance: the oxygen and carbon dioxide produced by the Elodea roughly match what the plant, snails, and bacteria use.
That stability depends on the light staying available and on the organisms staying small relative to the water volume. If the snails reproduce or the Elodea grows enough to shade the water, respiration could overtake photosynthesis and the oxygen would begin to fall. That is the same failure as Bottle B, just reached more slowly.
A reasonable alternative prediction is that the snails eventually run short of food and one dies, after which decay would quickly use up oxygen and cloud the water. Either way, the prediction must be tied to the balance between oxygen produced and oxygen consumed, not to the bottle simply "running out of air."