Biosensor Determination of the Microscale Distribution of Nitrate, Nitrate Assimilation, Nitrification, and Denitrification in a Diatom-Inhabited Freshwater Sediment

High-resolution NO(3)(−) profiles in freshwater sediment covered with benthic diatoms were obtained with a new microscale NO(3)(−) biosensor characterized by absence of interference from chemical species other than NO(2)(−) and N(2)O. Analysis of the microprofiles obtained indicated no nitrification during darkness, high rates of nitrification and a tight coupling between nitrification and denitrification during illumination, and substantial rates of NO(3)(−) assimilation during illumination. Nitrification during darkness could be induced by purging the bulk water with O(2) gas, indicating that the stimulatory effect on nitrification by illumination was caused by algal production of O(2). NH(4)(+) addition did not stimulate nitrification during darkness when O(2) was restricted to the upper 1-mm layer, and there was thus a low nitrification potential in the permanently oxic top 1 mm of the sediment.