A study of the kinetics of Mn2" oxidation catalyzed by cell extracts of two bacterial isolates (El, Pseudomonas III [new isolate] and E4, Citrobacter freundii) isolated from the core of manganese concretions from Greek soils is presented. The reaction velocity of Mn2" oxidation was determined from the rate of consumption of Mn2". The oxidation of Mn2" was followed by measuring changes in Mn2" concentration by activation analysis and by atomic absorption spectrophotometry. The reaction velocity

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... as directly proportional to cell extract concentration when the reaction time was 1 h. At longer reaction times, the relationship deviated from linearity because substrate concentration became limiting. The rate of Mn2" oxidation increased with the Mn2" concentration. Analysis of the results by application of the integrated Michaelis equation for determining Michaelis constants and maximal velocities either in the presence (Km = 3.33 ,umol/ml and Vmax = 1.25 ,Amol/ml.h) or in the absence of maleate buffer (Km = on May 4, 2020 by guest http://aem.asm.org/ Downloaded from PHi,ti,i = pHfi.,i = 6.6. b Taken with growth medium. 'Taken with maleate buffer (pH = 6.6). d A[Mn]%, Percentage of Mn2+ lost from the soluble phase.