Long-term Nutrient Cycling in a Materially Closed Ecosystem

Closed Microbial Ecosystems (CES) are hermetically sealed microbial communities that support self-sustaining nutrient cycles using only light as an input. CESs have been proposed as controlled model systems to understand the principles governing ecosystem organization and persistence. However, we do not yet understand how the nutrient cycling capabilities of a CES depend on its community structure and composition. To address this question, we present a new method for making precision measurements of carbon cycling in CES using low-cost piezoresistive pressure sensors. With these devices, we quantify carbon cycling rates in a set of CESs over periods of months. Our data show that previously studied synthetic CES comprised of model organisms exhibit declining carbon cycling rates on long-timescales. We go on to self-assemble CESs using the phototrophic alga Chlamydomonas reinhardtii combined with complex, soil-derived, bacterial communities. We find that these CESs persistently cycle carbon on timescales of many weeks. Also, we characterize the limiting nutrients in these CESs as well as their community-level metabolic capabilities. Finally, we use next-generation sequencing to characterize the taxonomic and metagenomic composition of these persistent microbial biospheres.