Nitrogen source influences natural abundance ¹⁵N of Escherichia coli

Escherichia coli cells were forced to mineralize or assimilate nitrogen in vitro by manipulating substrate carbon and nitrogen availability. When grown on an organic nitrogen source, E. coli cells released NH₄⁺ and were enriched in ¹⁵N relative to the nitrogen source (1.6-3.1‰). However, when cells were grown on an inorganic nitrogen source, the biomass was depleted (6.1-9.1‰) relative to the source. By measuring ¹⁵N enrichment of microorganisms relative to nitrogen pools, ecosystem ecologists may be able to determine if microorganisms are assimilating or mineralizing nitrogen.