Biosensor Determination of the Microscale Distribution of Nitrate, Nitrate Assimilation, Nitrification, and Denitrification in a Diatom-Inhabited Freshwater Sediment
Abstract
Biosensor Determination of the Microscale Distribution of Nitrate, Nitrate Assimilation, Nitrification, and Denitrification in a Diatom-Inhabited Freshwater Sediment Jan Lorenzen , Lars Hauer Larsen , Thomas Kjær , and Niels-Peter Revsbech * Department of Microbial Ecology, University of Aarhus, Ny Munkegade, DK-8000 Aarhus C, Denmark ABSTRACT 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.