Respiratory Studies on the Giant Amoeba Pelomyxa palustris1

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Abstract
The large fresh‐water microaerobic amoeba Pelomyxa palustris does not contain mitochondria, but three types of bacterial endosymbionts are always present. Thus, it is of interest in the discussion of the possible origin of mitochondria from primitive prokaryotes. Gas exchanges (O2, CO2) and concentration of endosymbionts were determined in individual amoebae, in which the life cycle stage was noted. Grey type (stationary phase) amoebae had a lower O2 uptake and lower endosymbiont concentration than light type (growth phase) amoebae, and highest O2 uptake was found in centrifugal pieces of light type Pelomyxa, centrifuged in vivo, which contained nearly all of the endosymbionts. In light type amoebae, the respiratory activity was independent of O2 concentration between 1 and 21%, and, when compared on the basis of dry weight and protein, of the same order as that of other free‐living protozoa. The R.Q. was slightly higher than 1, indicating that glycolysis does not play a significant role in energy metabolism. It is concluded that P. palustris is fully aerobic, and suggestions are presented as to the role of the endosymbionts in its respiratory metabolism.

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