Response of Glutamine Synthetase Gene Transcription and Enzyme Activity to External Nitrogen Sources in the Diatom Skeletonema Costatum (Bacillariophyceae)1

To understand the enhanced ability of marine diatoms to assimilate nitrogen (N), we measured changes in the transcript abundance and enzyme activity of glutamine synthetase (GS), one of the key enzymes that link carbon (C) and N metabolism, in the common diatom Skeletonema costatum (Greville) Cleve. Transcript abundance of glnII (the gene that encodes the GSII isoenzyme), measured by quantitative reverse transcriptase‐PCR, and total GS activity increased 2 to 3.5 times above background in the cells taking up nitrate (NO3−) but not the cells taking up ammonium (NH4+). A background level of glnII mRNA was maintained at a steady level up to 15 days of N starvation before decreasing to below detection after 21 days. These results confirm that transcription of glnII is induced to assimilate NH4+ derived from reduction of NO3−. Because of this role of GSII in diatoms assimilating NH4+ derived from NO3− reduction rather than from the environmental NH4+, quantification of glnII mRNA promises to be a useful indication of new production by phytoplankton.