Twenty-five oxidation-reduction indicators were injected in oxidized or reduced form into Amoeba dubia and Amœba proteus under controlled conditions of oxygen access. (1) Under anaerobiosis the ameba was able to reduce completely all the reversible oxidation-reduction indicators down to and including indigo disulfonate. (2) Under anaerobiosis the ameba was unable to reoxidize six of the most easily oxidizable indicators. (3) Under aerobiosis the ameba was able to reduce completely all the indicators down to and including 1-naphthol-2-sulfonate indo-2, 6-dichlorophenol. Toluylene blue, methylene blue and indigo tetrasulfonate were sometimes completely and sometimes only partly reduced, depending on the quantity of indicator injected and the duration of observation. (4) The time of reduction varied approximately with the size of the injection. Reduction was more rapid under anaerobiosis than under aerobiosis, more rapid in active than in sluggish cells and was retarded by toxic compounds. (5) Sulfonated compounds were somewhat toxic, as a rule.

In interpreting reduction phenomena of micro injection, it is necessary to take into consideration the intensity, capacity and rate factors. It then becomes apparent that the ameba has a high reducing potential lying on the rH scale below the zone of indigo disulfonate. The reducing capacity of the ameba seems to be relatively great in the region of the simple indophenols and of a progressively diminishing magnitude as the zone of the indigos is approached. Material of high reduction potential appears to be generated within the ameba at a measurable rate. These phenomena, observed in the interior of the cell with the aid of indicators, parallel very closely those found in reduction electrode studies on bacterial cultures.

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