The protective effect of surfaces against inhibition of nitrification
A reduced inhibitory response of both nitrifying bacteria Nitrosomonas europaea and Nitrobacter sp., in soil culture as compared to liquid culture was established. The effect of xanthate on batch culture growth kinetics was analyzed and different responses found for ammonia and nitrite oxidizing bacteria. This was investigated further in continuous culture experiments utilizing a model based on enzyme kinetics. Studies indicated that potassium ethyl xanthate acted as a competitive inhibitor on Nitrobacter but not on Nitrosomonas. A detailed kinetic analysis was made of the response of the nitrifying bacteria to xanthate in continuous culture, rate of substrate oxidation per cell as well as overall substrate conversion by the population being determined. Results from this work were compared with those obtained when Nitrobacter was grown in continuous culture with a synthetic particulate for cell adhesion, and a reduced inhibitory effect of xanthate on attached cells established. Stimulation of growth of Nitrobacter on well defined synthetic surfaces was observed in batch culture experiments using glass slides and anion exchange resins and this was related to the phenomena of nutrient accumulation at interfaces. The preferential adhesion of the nitrifiers to surfaces binding their-substrate for growth was established by studying cell attachment to anion and cation exchange resins and was interpreted to explain the different suseptibility of Nitrosomonas and Nitrobacter to inhibitor in soil incubation studies. From results obtained in continuous culture experiments where elements associated with the synthetic particulate surface were analyzed, and by study of bacterial attachment to glass slides with xanthate, it was deduced that the protective effect of surfaces against inhibition by xanthate resulted from interaction of the inhibitor at surfaces, making it less available or less able to exert an inhibitory effect on the bacteria. Results from this model system were extrapolated to explain the reduced inhibitory effect on the nitrifiers in soil where analysis of such mechanisms is not possible because of the complex nature of the environment. A stimulation of nitrite oxidation and ammonia oxidation by low concentrations of xanthate was observed in continuous culture experiments. An oscillatory cell response was also seen. The importance of this stimulation phenomena was related to the use of potassium ethyl xanthate as an agricultural nitrification inhibitor.