A mathematical model for microcosms: formation of the colonies and coupled oscillation in population densities of bacteria

A microcosm constructed, in the present study, consisted of bacteria (five or more species), green algae and rotifers and maintained a steady state for more than 100 days. This microcosm formed numerous colonies at the bottom of the flask and demonstrated coupled oscillation in the population densities of bacteria. The population dynamics and the spatial patterns of organisms were simulated using a cellular automata method. From the calculation results, the following facts were inferred: (1) heterogeneous metabolite concentrations due to the diffusion phenomenon as well as the frequencies of interactions among rotifers, bacteria and green algae and the strengths of their interactions are the contributing factors to the colony formation; (2) the system becomes stable when colonies are formed; and (3) coupled oscillation occurres in the population densities of bacteria when certain combinations are selected for the specific growth rate coefficients of the coexisting bacteria, and the distribution of the ratios of specific growth rate coefficients of the bacteria which can coexist in a stable manner is discrete rather than continuous.