Long-term life-support in space requires renewable sources of oxygen and food that can survive and thrive in a closed system without any external inputs. In a closed-system experiment, three species of green algae were added to a nutrient mixture together with a grazer species, the common water flea (Daphnia magna). Despite calculations of the appropriate level of carbon and nitrogen needed in the mixture, the pH in the closed system rapidly increased to become highly alkaline (pH 10-11), so much so that most forms of life would not be able to survive. Increasing the ratio of carbon to nitrogen from the previously calculated values of 26.4 to much higher ratios (105-845) did reduce pH somewhat (to 9.3-10.3) and prolong the lifetime of the cultures, but not indefinitely. These results suggest that a continued input of carbon is required for life support systems, and that more work is needed to find appropriate cultures that can allow long-term survival of algal and grazer communities. The work by SAFS Professor Emeritus Fried Taub, appears in the journal Life Sciences in Space Research.