Closed microbial communities self-organize to persistently cycle carbon

Nutrient cycling plays a critical role in determining ecosystem structure at all scales, from microbial communities to the entire biosphere. Therefore, a central problem in ecology is understanding how ecosystems are organized to robustly cycle nutrients. Here we use closed microbial ecosystems (CES), hermetically sealed microbial consortia that sustain nutrient cycles when provided with only light, to address this problem in the context of carbon cycling. We develop a new technique for quantifying carbon cycling in hermetically sealed microbial communities and show that CES comprised of an alga and diverse bacterial consortia self-organize to cycle carbon. Self-organized CES sustain carbon cycles for months. Comparing a library of CES, we find that carbon cycling does not depend strongly on the taxonomy of the bacteria present. Measurements of the carbon utilization capabilities in CES reveals functional redundancy: despite strong taxonomic differences, self-organized CES exhibit a conserved set of metabolic capabilities. Therefore, an emergent carbon cycle enforces metabolic, but not taxonomic constraints on ecosystem organization.