The effects of CO and light on the ammonium oxidizing bacterium Nitrosomonas cryotolerans and the nitrite oxidizing bacterium N~trobacter sp Nb297 were investigated. Ammonium oxldahon was inhibited by C O concentrations ranging from 2 n M to 11.4 FM. CO inhibition was cell density dependent and increased if the cells had been either previously starved or deprived of iron. Ammonium oxidation was inhibited by 63 % at intensities a s low a s 5 W m-2 of artificial light. Nitrobactersp. Nb297, on

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... other hand, was able to tolerate C O concentrations as high as 400 PM. Artificial light (25 W m-*) slightly reduced nitrite oxidation while sunlight (628 W m-') decreased activity to ca 20 % of its original value. In addition, C O and CH, oxidation by N. cryotolerans and natural assemblages of bacteria were inhibited by light. It is apparent from these studies that the mechanisms of light and C O inhibition are different for the NH: and NO; oxidizers examined. Characterization of light and C O responses by marine nitrifying bacteria is critical in any attempt to explain the formation of the subsurface primary nitrite maximum.