Preferential nitrogen uptake by sediment microbial communities in response to variable nitrogen concentrations

All organisms require nitrogen and the relative abundance of different forms of nitrogen can influence microbial growth and reproduction.  We measured preferential uptake of inorganic forms of nitrogen (nitrate NO₃^- and ammonium, NH₄⁺) in Pseudomonas fluorescens cultures and sediment mixed-microbial communities.  Microcosms were inoculated with microbial communities and exposed to high concentrations of both NO₃^- (>1mg N/L) and NH₄⁺ (> 30 µg N/L), to determine if preferential uptake varied with either concentration or form of available nitrogen.  After a 1 and 3 day incubation period, filtered water extracts were collected for measurement of NH₄⁺ and NO₃^- uptake using standard colorimetric (NH₄⁺) and ion chromotagraphy procedures (NO₃^-).  Increased concentration of NO₃^- in natural mixed-microbial communities decreased NH₄⁺ uptake, likely due to preferential uptake of the more abundant NO₃^-. In contrast, increasing NH₄⁺ concentrations yielded higher NH₄⁺ uptake.  Increased NO₃^- concentration yielded saturation of microbial uptake above 0.75 mg N/L. These data indicate both single-species microbial cultures and natural mixed-microbial communities can preferentially utilize NH₄⁺ or NO₃^- in response to changing availability.