Passive Experimental Microbial Systems: A Research Platform for the Analysis of Microbial Community Assembly in Spaceflight Ecosystems

Extension of human habitation into space requires that humans carry with them many of the microorganisms with which they coexist on Earth. Whether microbes are present by design as constructed communities in bioregenerative life support systems or by accident as hitchhikers attached to human, plant, and spacecraft surfaces, the microbial ecosystems of Earth will be present in space. But how may the space environment affect the interaction of microbial communities? Given the potential for rapid change in populations of microorganisms through mutation, recombination, and natural selection (processes accelerated under space conditions of variable microgravity and elevated background radiation), it will be necessary to understand both the pattern and process of community assembly and evolution in the space environment. On Earth, the abundance of individuals in microbial populations is so large that dispersal is unlikely to ever be limited by geographical barriers (i.e., “everything is everywhere”). This will not be true for microbial communities in space where local species richness will be relatively low because of sterilization protocols prior to launch and the barriers between Earth and spacecraft after launch. Although seeding bioreactors with a diverse community may be sufficient to sustain process performance at the onset, species richness may decline over time such that biological systems lose either functionality (e.g., bioreactors fail to reduce organic carbon or nitrogen load) or stability (i.e., low diversity communities may be more susceptible to environmental perturbation or invasion by human-associated microorganisms). In order to evaluate bioregenerative systems for long duration human exploration, we must first identify biological questions of concern in the assembly and function of microbial communities in closed ecosystems in space. Toward this end, we describe modular units with low mass, energy, and crew-time requirements for microbial cultivation over multiple life cycles in passive experimental microbial systems, or PEMS, that enable the exploration of microbial community ecology and evolution in space.