MELISSA: Behavior of the Ecosystem Under Different Light Radiant Energy Inputs

The MELiSSA (Microbial Ecological Life Support System Alternative) project was designed as an early model of a future artificial ecosystem for long-term space missions. It centres on the recovery of edible biomass from waste, CO₂ and minerals, with direct use of light as a source of energy for photosynthesis. MELiSSA is composed of four axenic compartments colonized by microorganisms, and a fifth compartment formed by the crew on board the craft. Simulation of the entire MELiSSA loop had been performed to obtain mass fluxes and concentrations of microorganisms, and liquid and gas components in all the system streams. This first approach helped to define the process conditions necessary to obtain complete recycling of nitrogen or regeneration of the atmosphere. This paper reports on the simulation of the behaviour of the loop when the photosynthetic compartment is working under various light-limiting conditions. As the radiant light energy flux is increased, the carbohydrate composition of the biomass changes radically. The description of the compartments is modified to allow for these changes in biomass composition and to calculate the percentage of closure of the loop (nitrogen recycling, oxygen regeneration and CO₂ reduction) as a function of radiant light energy flux. In addition, the experimental results obtained on the other compartments are taken into account, providing a reviewed assessment of the performance of the MELiSSA loop.