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The MELISSA (Microbial Ecological Life Support System Alternative) project, conceived as a microorganism based ecosystem, is an early simplified model of a future biological life support system for manned space missions. The driving element is 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 colonised by microorganisms and of a fifth compartment that is the crew on board the craft. This paper reports on the solution of mass balances over the entire MELISSA loop. The compartments within MELISSA are first analysed separately, each compartment being described by one or more stoichiometric equations derived from knowledge of the cells metabolic pathways. For each stoichiometric equation a key substrate entering a compartment is assumed to be completely exhausted at the outlet; process kinetics such as rates of biological reactions and mass transfer are ignored.