Behavior of simple closed ecological systems; lower nutrient concentrations allow longer persistence of grazer...

We expect to develop self-sustaining extraterrestrial colonies, and they will approach being closed ecological systems. Using simple closed ecosystems containing Daphnia magna, three species of algae, and microbes, we tested multiple conditions to study long-term organism survival, which is only possible with adequate nutrient recycling. Closed and open systems behaved differently from one another at high nitrate concentrations; in closed systems, the animals were dead by day 14; in open systems, the Daphnia populations persisted beyond 273 days. Daphnia deaths were associated with increased pH and O₂ caused by greater algal photosynthesis and the lack of exchange with the atmosphere. Replicate variability that used small Daphnia suggested that inadequate grazing capability allowed algae to create conditions unfavorable to Daphnia survival. Over months, algal and Daphnia abundance decreased, presumably because of inadequate nutrient recycling; these populations increased temporarily after the addition of nutrients. The addition of natural lake organisms did not increase the nutrient-recycling capabilities of the systems. Understanding the mechanisms of closed systems will be useful in implementing biological processes in managing life support systems.