title: 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 O2 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.
authors.affiliation: Department of Biology, Ehime University, Bunkyo-cho 2-5, Matsuyama 790-8577, Japan. nakajima@sci.ehime-u.ac.jp
authors.forename: Toshiyuki
authors.initials: T
authors.lastname: Nakajima
authors.forename: Akiko
authors.initials: A
authors.lastname: Sano
authors.forename: Hideaki
authors.initials: H
authors.lastname: Matsuoka
mesh_terms: D000818:Animals; D001419:Bacteria; D016798:Ciliophora; D017753:Ecosystem; D056890:Eukaryota; D013559:Symbiosis