Auto-/heterotrophic endosymbiosis evolves in a mature stage of ecosystem development in a microcosm composed of an alga, a bacterium and a ciliate

Abstract

We investigate an ecological mechanism by which endosymbiotic associations evolve, with a particular focus on the relationship between the evolution of endosymbiosis between auto- and heterotrophic organisms, and the stages of ecosystem development. For this purpose we conducted a long-term microcosm culture composed of three species, a green alga ( Chlorella vulgaris), a bacterium ( Escherichia coli), and a ciliated protozoan ( Tetrahymena thermophila) for 3 years. During this culture T. thermophila cells harboring Chlorella cells emerged by phagocytotic uptake, and increased in frequency, reaching ca. 80–90%. This level was maintained in the late stage of ecosystem dynamics. Analysis of the ecosystem dynamics in the microcosm revealed that a complex causal process through direct/indirect interactions among ecosystem components led to reduction in dissolved O 2 and food ( E. coli) available to the T. thermophila, which gave a selective advantage to the organisms in the endosymbiotic association. This result suggests that the endosymbiosis evolves in a mature stage of ecosystem development, where reproduction and survival of prospective partner organisms is highly resource-limited and density-dependent, favoring efficient matter/energy transfers among participating organisms due to physical proximity. Consequently, a complex web of interactions and pathways of matter/energy flow in ecosystem evolves from an initially simple one.