The metabolic diversity, biological activity, and stability of the steady state condition in closed ecosystems

Abstract

It is proposed that the closure of small but diverse collection of microorganisms on a sub-liter scale can result in a model ecosystem useful for the study of natural ecosystems and can eventually lead to the elucidation of general ecological laws. For a closed ecosystem (CES) to be a useful model system some basic questions must be answered. This thesis focuses on three questions: 1. Do Closed Ecosystems (CES) maintain biological activity and diversity on the same level as natural ecosystems? 2. How predictable and reproducible are CES? 3. What are the characteristics of the steady state condition for CES, and are CES stable? A pair of CES which had been sealed for six years was opened and eighteen biological parameters were measured. These were compared to the results of the analysis of samples from the Kaneohe Bay site from which the CES inoculum was originally obtained. Direct measurements of organic carbon, living carbon and daily carbon production showed the CES were as biologically active or slightly more active than the Kaneohe Bay samples. Elective culture was used to enumerate microorganisms capable of different reactions of the carbon and nitrogen cycles from the two sample types. The functional diversity of the two sample types was similar. The predictability and reproducibility of CES were investigated with 20 ml CES, established as 500 replicate ecosystems. Subsets of these were sacrificed over time and biological activity was measured. Three distinct phases were identified over 144 days: development, steady state and decline. A steady state period which persisted over many live carbon turnovers was found to be resistant to perturbation by gas exchange (i.e. removal of the ambient gas phase and replacement by N2 or H2) but susceptible to carbon pool perturbations (addition of glucose or bicarbonate). The variability of the replicate CES was also quantified. The effects of similar but not identical starting conditions for replicate CES were determined to judge the uniqueness of the steady state. No evidence for alternate steady states was detected unless different carbon pool concentrations were used to establish the CES. In summary CES can retain biological activity and functional diversity over long periods of closure. Replicate CES can also be established with a low degree of variability and with predictable patterns of behavior. This behavior includes a steady state period that is stable to perturbation. These results allow CES to used as benchtop models of ecosystems with predictable results to address significant ecological questions.