Ion exchange resins and glass microscope slides were used to investigate factors affecting attachment of nitrifying bacteria to solid surfaces and the effect of attachment on inhibition ofNitrobacter by potassium ethyl xanthate. The ammonium oxidizerNitrosomonas attached preferentially to cation exchange resins while the nitrite oxidizerNitrobacter colonized anion exchange resins more extensively. Colonization was always associated with growth, and the site of substrate (NH4 (+) or NO2 (-)) adsorption was the major factor in attachment and colonization. The specific growth rate of cells colonizing either ion exchange resin beads or glass surfaces was greater than that of freely suspended cells, butNitrobacter populations colonizing glass surfaces were more sensitive to the inhibitor potassium ethyl xanthate. The findings indicate that surface growth alone does not protect soil nitrifying bacteria from inhibition by potassium ethyl xanthate and explain different patterns of inhibition for ammonium and nitrite oxidizers in the soil.
Inhibition of autotrophic ammonia and nitrite oxidation by potassium ethyl xanthate (PEX) was studied in ammonium-and nitrite-limited chemostat cultures of Nitrosomonas europaea and Nitrobacter, respectively. Ammonia and nitrite oxidation were both stimulated by addition of low concentrations of PEX (0-1 pg ml-I ), which significantly decreased steady-state substrate concentrations and increased steady-state cell concentrations. Addition of similar concentrations to exponentially growing cultures resulted in a slight stimulation of growth of Nitrobacter but did not affect Nitrosomonas. Inhibition of ammonia and nitrite oxidation by higher concentrations of PEX resulted in establishment of steady states without induction of lag phases, observed in batch culture, which would have resulted in wash-out. Establishment of steady states occurred more quickly and with less variation in substrate concentrations following addition of inhibitory rather than stimulatory concentrations of PEX. Steady-state data indicate that PEX may act as a competitive inhibitor of nitrite oxidation by Nitrobacter.
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