Urea provides an important nitrogen source for microbial populations in an ultraoligotrophic sea

Urea is an organic nitrogen (N) compound directly utilized by both heterotrophic prokaryotes and some photoautotrophs. Reports of either ambient urea concentrations or urea uptake rates from the ultraoligotrophic eastern Mediterranean Sea are lacking. We investigated urea distribution and uptake in the upper water column in the Levantine basin during three contrasting periods of N-availability: a thermally stratified, ultraoligotrophic period depleted in dissolved inorganic N; a transient mesotrophic period following a winter storm; and a subsequent period of declining inorganic N availability during renewed stratification. Urea concentrations varied seasonally from 280 ± 82 nM in winter/spring 2022 to 124 ± 61 nM during summer thermal stratification. Dissolved organic nitrogen (DON) accounted for 85–99% of total dissolved nitrogen in the euphotic zone, while urea utilization accounted for 45% of the observed decrease in DON, making it the most bioreactive of possible organic N sources. Under simulated replete nutrient conditions, ammonium was the dominant N-species utilized by the microbial community displaying higher potential uptake rates compared to urea and nitrate. Yet, under ambient conditions, the highest in situ uptake rates we observed at all seasons were for urea followed by ammonium and then nitrate. Urea uptake consistently predominated, comprising 37–78% out of the total N uptake rates. The high in situ urea uptake rates throughout the year emphasize its importance as the most bioavailable fraction of DON and N source in the Levantine basin and probably in other oligotrophic marine environments that are predicted to expand under climate change.