A bioregenerative system that permits life support on a long duration (18-20 months) space mission also provides the opportunity to develop a conceptual model for sustainability on Earth.  An extraterrestrial ecosystem must include a closed, small-scale agronomic system and a method for recycling nutrients from crop and human byproducts.  Research at NASA has focused on biological processes for leaching of organic byporducts to provide soluble nutrients for hydroponic plant production.  While minimizing microbial risks presents a challenge, a closed system provides the opportunity to control what enters the system.  Although a decontaminated system was initially favored, current research is focusing on evaluating the functional versatility and richness of microbial communities.  The potential for inclusion of microbes in the system has resulted in the evaluation of composting as an alternative for byproduct management.