Adaptive characteristics of a ciliateTetrahymena thermophila in endosymbiotic association with a green algaChlorella vulgaris derived in a long-term microcosm culture

Abstract

We investigated directly an early stage of the development of an endosymbiosis between an alga and a ciliate, beginning from a non-associated stage by conducting a long-term microcosm culture composed of a green alga (Chlorella vulgaris), a bacterium (Escherichia coli) and a ciliate (Tetrahymena thermophila) for three years. During this culture,Chlorella-harboringTetrahymena appeared and increased in frequency. We examined the adaptive characteristics of theT. thermophila in association with theC. vulgaris derived from a well-established microcosm culture maintained for 1164–1400 days, and compared it with the original culture ofT. thermophila. TheT. thermophila with the associated, intracellularC. vulgaris grew at a lower density ofE. coli than the originalT. thermophila with the originalC. vulgaris, and the former survived longer than the latter in the absence ofE. coli. These results suggest that this induced algal-ciliate association confers some fitness advantage at least to the host under the conditions of less or no available food such asE. coli. Although the algal-ciliate association is not completely stable, this quasi-stable association may enable the host to exploit a new niche through the advantages of mixotrophy.