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Abstract
SUMMARY: The oxidation of ammonia to nitrite in enrichment cultures of Nitrosomonas spp. was hastened by 6 mg. Fe/1. in the medium. Manganese did not replace iron as a stimulant; it was toxic. N. europaea (Jensen’s strain) and a strain of Nitrobacter winogradskyi had, in pure culture, very small absolute requirements for iron; they oxidized ammonia and nitrite respectively in media purified with 8-hydroxyquinoline, and with no iron added. Small amounts of iron hastened the oxidation of ammonia and nitrite; the minimum concentration giving this stimulating effect was 0·1 mg. Fe/1. for the strain of Nitrosomonas europaea and 0·3 mg. Fe/1. for the strain of Nitrobacter winogradskyi. The optimum amount of iron for oxidation appeared to be about 6 mg./1. for both species. Both species tolerated 112 mg. Fe/1. (c. 0·002m), but oxidation was delayed, markedly in the case of N. winogradskyi, by 560 mg./1. (c. 0·01m).
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