Respiratory inhibition of Sphaerotilus by various iron compounds was investigated in association with the distributional pattern of iron sorbed by the bacterium. The inhibitory effects of the iron compounds were examined respectively in the exogenous and endogenous respirations. Any sorption of iron by the microbe was not observed in the dosage of the non-inhibitory compounds such as K4 [Fe(CN)6], K3[Fe(CN)6] and Fe-citrate. Also, the respiration was scarcely inhibited by FeCl3 in spite of dense deposits of iron observed covering the sheath surface. On addition of Fe-cysteine or Potassium ferrate (K2FeO4), a remarkable effect of respiratory inhibition was observed respectively. But, their inhibitory behaviors were not necessarily similar. While the inhibitory effect by Fe-cysteine was principally marked of the exogenous respiration, K2FeO4 inhibited strongly both the exogenous and endogenous respiration. For examining definite locations of the bacterium in which iron sorption actually occurred, an improved procedure on the method by Romano and Peloquin (1963) was applied in the present study. Iron of Fe -cysteine was hardly detected inside the cells, nevertheless the existence of iron penetrated through the sheaths was observed certainly even in the cells in the case of K2FeO4.

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