麻豆传媒

IF YOU fancy a trip to deep space but not the freeze-dried fare, you might be
surprised to learn you could be eating fresh fish.

The further you want to travel in space, the more supplies you normally have
to take with you. Far better to have a system that continually replenishes your
food and oxygen鈥攁nd which reprocesses your waste鈥攚ithout needing any
further nutrients or fuel. And that is just what Volker Bl眉m and Frank
Paris of the Ruhr University in Bochum, Germany, have created.

It may sound like getting something for nothing, but it鈥檚 not. What Paris has
come up with is nothing less than an artificial ecosystem. It only takes light
to drive it, like the ecosystem here on Earth.

When NASA tried growing cereal plants in space, it ran into problems because
the capillary action that conveys nutrients from the roots to the leaves didn鈥檛
work properly in microgravity. So Bl眉m and Paris decided an aquatic system
would be a better bet. Rootless aquatic plants are very efficient
photosynthesisers, and don鈥檛 rely on capillary action.

After years of trying, Paris and Bl眉m have developed a self-contained
miniature ecosystem in which fish, water snails, plants and microbes live in a
sustainable environment, and a trial version was flown on a space shuttle
mission in 1998. Payload restrictions limited it to 8 litres, which is too small
for it to be completely self-sustaining, so it needed fish food to be added from
time to time, as well as light to keep it going.

A 150-litre tank was recently self-sustaining on Earth for 13 months without
any input other than light. Bl眉m says the experiment could have run for
longer if the tank hadn鈥檛 developed a leak.

The biosphere鈥檚 plants are rootless hornweeds, which are almost entirely
edible. They thrive on light, carbon dioxide and nutrients in the water, and
give off oxygen. In turn, the sea snails, swordtail fish and microbes breathe
this oxygen and exhale carbon dioxide that the plants can use
(see Diagram).

Waste management is handled by microbes that oxidise ammonia excreted by the
animals and ultimately convert it into nitrate ions that are taken up by the
plants.

The fast-breeding fish keep their numbers down by eating some of their young.
The water snails live off the remnants of fish bodies鈥攄ead adults and bits
of their fry. The weed is 鈥渟elf shadowing鈥�: it kills back overgrowth when it
grows too big.

Aboard the shuttle, the creatures showed few problems adjusting to
microgravity. The fish manoeuvred just as well in space as on Earth, though they
did seem to swim 鈥渦pside-down鈥�, with their bellies facing the light source.

The snails only got into trouble if they became detached from the side of the
tank. When this happened they stretched their bodies as they floated through the
water, in a bid to re-anchor themselves. Unfortunately, the first object they
found was sometimes another snail. 鈥淭his often resulted in 鈥渟nail balls鈥�, says
叠濒眉尘.

NASA agrees that aquatic systems will play an important part in deep space
life support. William Knott, a biologist at the Kennedy Space Center in Florida,
says getting them to work for long periods is the challenge. 鈥淚 don鈥檛 think we
are quite smart enough to produce a system that will do the job forever,
though,鈥� he says.

Bl眉m says that picking less obvious species was a factor in the system鈥檚
success. Fortunately, the hornweed is quite palatable, but Bl眉m would like
to find an alternative to the Swordtail fish. 鈥淭hey鈥檙e a bit rubbery,鈥� he says.