Growth of Paramecium bursaria as Influenced by the Presence of Algal Symbionts

SYMBIOTIC associations are widely distributed throughout the plant and animal kingdoms (Caullery, 1950; Buchner, 1953; Fuller, 1958), but to date relatively little is known about the detailed nature of the interactions between symbiotic partners. It is apparent, however, that symbiotic complexes frequently possess properties, not characteristic of either partner alone, that may at least partially free the co-operating organisms from competition with related free-living forms. The hereditary endosymbiotic complex of Paramecium bursaria-Chlorella sp. affords exceptionally favorable material for extending our knowledge of hostsymbiont relationships to a physiological and biochemical level. A single paramecium normally harbors several hundred algae that are transmitted to both daughter cells at division and retained during conjugation. Since the two organisms may be dissociated, cultured separately, and reassociated in the original or novel combinations (Pringsheim, 1928; Siegel and Karakashian, 1959; Siegel, 1960), their metabolism may be investigated singly as well as in symbiotic association. Moreover, the fact that P. bursaria, syngen 1, has been studied genetically (Jennings and Opitz, 1944; Siegel and Larison, 1960) suggests that genetic analyses of infection specificity are feasible. In order to exploit the ParameciumChlorella system for biochemical and genetic analysis, it is necessary to know the behavior of specific strains in a controlled environment. Unfortunately, previous workers (Le Dantec, 1892; Oehler, 1922; Pringsheim, 1928) did not quantify their observations nor were they able to verify their findings in the absence of for-