BIOSENSOR DETERMINATION OF THE MICROSCALE DISTRIBUTION OF NITRATE, NITRATE ASSIMILATION, NITRIFICATION, AND DENITRIFICATION IN A DIATOM-INHABITED FRESH-WATER SEDIMENT

High-resolution NO3- profiles in freshwater sediment covered,vith bent hic diatoms were obtained with a new microscale NO3- biosensor charact erized by absence of interference from chemical species other than NO2 - and N2O. Analysis of the microprofiles obtained indicated no nitrifi cation during darkness, high rates of nitrification and a tight coupli ng between nitrification and denitrification during illumination, and substantial rates of NO3- assimilation during illumination. Nitrificat ion during darkness could be induced by purging the bulk water with O- 2 gas, indicating that the stimulatory effect on nitrification by illu mination was caused by algal production of O-2. NH4+ addition did not stimulate nitrification during darkness when O-2 was restricted to the upper l-mm layer, and there was thus a low nitrification potential in the permanently oxic top I mm of the sediment.