Raw data for the GOM urea cycling manuscript

Assessing nitrogen (N) dynamics is key to understanding microbial food web N processes and limitation in coastal ecosystems under various redox conditions, but biogeochemical roles of labile organic N (e.g., urea) vs. ammonium (NH₄⁺) and light effects are not clear. A preliminary isotope dilution study of urea and NH₄⁺ dynamics at three sites (Mississippi River Mouth, a well-studied hypoxic site [C6], and a normoxic control site) in the northern Gulf of Mexico, revealed that urea and NH₄⁺ dynamics were comparable in magnitude. NH₄⁺ and urea-demand patterns, reflecting the degree of N-limitation with depth, were statistically indistinguishable from each other as bioavailable N sources at the oxic control site. In contrast, at the two nutrient-enriched sites, mean potential uptake rates for NH₄⁺ exceeded those for urea in near-surface waters. However, ¹⁵N-urea uptake provided N directly (no detectable regeneration as ¹⁵NH₄⁺) to phototrophic organisms rather than to heterotrophic bacteria during lighted surface water incubations. In contrast, ¹⁵NH₄⁺ production from ¹⁵N-urea additions in the dark incubations of subsurface waters indicated significant external hydrolysis of urea to NH₄⁺in the dark. These preliminary data suggest that depth-dependent, light-dark food web interactions with urea vs. NH₄⁺ are important biogeochemical mechanisms affecting internal microbial N dynamics and fate in hypoxic regions. We speculate that direct incorporation of urea (or other labile N compounds) by phototrophic organisms, without the energetic cost of heterotrophic organic-N decomposition in lighted surface waters, may increase ecological nitrogen/carbon transport efficiency to bottom waters and thereby exacerbate the effects of N pollution on hypoxia development.