Materially closed, energetically open, natural microbial ecosystems, were constructed in flasks and sealed under ambient air. These systems were used to study ecosystem stability, defined as resistance to departure from light energy-driven coupled bioelemental cycling.Since it is closely associated with terrestrial ecology, oxygen concentration was chosen to measure the states of the closed microecosystems. Results indicate that rates of change in pO2 are indicators of the states of closed complex natural microsystems. Such systems endure indefinitely and reach oxygen concentrations greater than normal atmospheric pO2.In addition, a light energy isolated system yielded an oxygen consumption rate which was used to calculate the apparent quantum energy efficiency, Q. The calculated value, 1.3%, compares favorably with the Q of most terrestrial ecosystems.These studies contribute to general ecosystem analysis and indicate that complex microbial ecosystems could be used as gas exchange mechanisms in closed life support systems.