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Health, Sports & Psychology

Underwater challenge

Updated Monday, 12th June 2006

In the underwater episode of Science Shack, Adam challenged the team to build him a self-contained underwater environment. They succeeded, but what did they have to take into account?

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Adam in invisible car

A human being can survive three months without food, three days without water, but only three minutes without air. So in order of importance we need air, water and food. Of course, to live on the sea floor we would also need power to generate light and heat, but this could be in the form of electricity generated by waves, tide or sun.

As mentioned above, air is the most critical factor. A human uses about half a litre of oxygen (O2) per minute, so in a space of 4m3 there’s enough oxygen for about 4 hours. The critical limiting factor is the carbon dioxide (CO2) that we breathe out at a concentration of about 3.5% (35,000 ppm). This corresponds to just under half a litre per minute (0.4 litre per minute) of pure CO2. Humans are affected by too much CO2 in the air, more quickly than by lack of oxygen. CO2 can be removed from the air chemically, using a CO2 scrubber. A scrubber contains particles of soda lime, which the CO2 reacts with to form chalk. But there’s a more simple way to get rid of CO2 which lasts forever. Plants take in CO2 and give off oxygen in a process called photosynthesis, 12m2 of plants would produce enough oxygen to keep a human being alive, and at the same time they would naturally ‘scrub’ the air clean of CO2.

Next we need water - humans lose water every time they breathe out and by sweating. At depth underwater it is very cold and water would condense on a cold metal wall of the underwater habitat, so all we’d need is a handy little funnel on the bare wall and a bottle to collect the water.

What about food? Well we could build an artificial reef to attract sea creatures, or we could use hydroponics, which is a way of growing plants in a controlled environment. Hydroponics don’t need soil because they are grown on rock-wool, or in water which is full of minerals and nutrients, but they do need light. Unfortunately one person would need about 80m2 of plants to keep them going.

We don’t suggest you try and build your own underwater habitat, but in the programme Adam found out how a closed ecological system works - where plant and animal life are permanently sealed in a balanced, self-sustaining micro-environment. You can make your own simple system using pond water and a bottle.

The idea is that tiny animals in the water take in oxygen from the water and eat algae, whilst giving out carbon dioxide and waste products. Micro-organisms (too small to see by eye) break down this waste, and produce nutrients. The algae (and other plant life) use the carbon dioxide and nutrients to grow and produce oxygen, thus sustaining the animal life. In this way the system recycles itself in a sustainable way - so you should be able to keep your mini ecosystem for months!

Stuff you need materials for making an underwater ecosystem

  • Clean, new mineral water bottle
  • Pond water
  • A bit of sand or pond sediment
  • Pond weed, Elodea is the best
  • Daphnia, water fleas that can be bought from aquarium shops


Adam and the aquashack


What to do

1. Take a previously unopened bottle of mineral water and empty the contents. A mineral water bottle is ideal for this as it’s been sterilised, but don’t drink from the bottle – just tip the contents into a glass.

making an underwater ecosystem - step one 2. Fill it two thirds full of fresh pond or river water.

3. If there aren't any small creatures swimming around, add Daphnia. These are tiny water fleas that you can buy at tropical fish or pet shops. While you’re there you might like to get a small bit of an aquarium plant like pond weed.

4. A bit of sediment is helpful, but don't add too much, making an underwater ecosystem - step 2 because rich organic matter may have too high an oxygen demand.

5. Leave the bottle uncapped for 5 days, and then seal it up with the bottle top.

making an underwater ecosystem - stage 3 6. Put the bottle where it gets light for at least a few hours per day, but not direct sunlight. If you give your system too much light, the algae will use up all the nutrients and the system will die. Remember that you are working with tiny organisms which eat tiny plants! It’s likely that you will have animals swimming around for at least a month, and possibly several months.





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