AME 42:227-242 (2006)  -  doi:10.3354/ame042227

ABSTRACT: Uptake of urea was studied in batch cultures of natural bacterial assemblages or in GF/C filtered water from 4 estuarine sites, and was related to utilization of other N compounds and the total bacterial N production. During spring in Roskilde Fjord, Denmark, urea uptake varied 100-fold and sustained 0.6 to 44% of the bacterial N demand, while uptake of dissolved free amino acids (DFAA) met 12 to 53% of the N demand. Enrichment with NH₄⁺ and DFAA reduced the urea uptake by up to 2.8-fold, demonstrating a higher preference for NH₄⁺ and DFAA than urea. In Knebel Vig, Denmark, bacterial uptake of urea varied significantly during a 9 d provoked algal bloom (mesocosms enriched with N, P, or Si). At Day 1, uptake of urea and DFAA was similar, but during development of a diatom bloom, bacterial urea uptake either increased 7-fold (Day 5) or was insignificant (Day 9). In contrast, urea uptake in the control mesocosms remained unchanged. Urea uptake by the bacterial assemblages was on average 1.6-fold higher than the in situ (bacteria and algae) dark uptake, which again was 16-fold lower than the in situ light uptake. Urea sustained 0.2 to 30% (nutrient-enriched mesocosms) and 9 to 41% (control) of the bacterial N demand, but NH₄⁺, NO₃^– and DFAA were more important N sources. Bacterial preference for urea or NH₄⁺ alone, or in combination with glucose, was examined in the Limfjorden estuary, Denmark. Relative to controls, a 2-fold higher biomass was produced whether NH₄⁺ or urea was the N source, but urea sustained <30% of the N biomass production and was less important than the ambient DFAA pool. In contrast to the Limfjorden studies, when enriched with glucose, bacteria in Santa Rosa Sound, Florida, USA, produced a lower biomass with urea than with NH₄⁺. Enrichment with urea stimulated the DFAA assimilation, peptidase activity and utilization of NO₃^–, and urea made up <50% of the N incorporation. The studies demonstrate that urea uptake rates by estuarine bacteria are variable and often unpredictable. Uptake of urea will typically be lower than of NH₄⁺ and DFAA, but occasionally, urea uptake can be of importance similar to NH₄⁺ and DFAA.

KEY WORDS: Urea · Bacterial uptake · DFAA · Ammonium-Bacterial N budgets