The biological transformation of ammonium and urea in a eutrophic estuarine system in Southern China

Abstract

Estuaries channel large amounts of anthropogenic nitrogen (N) from continents to the offshore where productivity is widely limited by N and phosphorus. Ammonium and urea, two reduced forms of anthropogenic N commonly observed, are the preferred substrates for various microorganisms (e.g., uptake by phytoplankton or bacteria and oxidation by nitrifier). Yet, it remains underexplored how they transform and their concentrations vary during transport in the estuary which may influence the offshore microbial community. We applied 15N isotope tracer incubation techniques to determine the two main bio-consumption processes, i.e., uptake and oxidation, of ammonium and urea, in the dark for the Jiulong River Estuary, a eutrophic estuary in southeastern China. Results showed that light penetration depth ranged from 0.8-3.3 m leaving 76% of estuary water bodies to stay in dark situations throughout a day. Ammonia oxidation, which favors dark conditions, dominates the estuarine regenerated-N cycle, showing the rank: ammonia oxidation > ammonium uptake >> urea uptake ≈ urea oxidation. By compiling the reported case studies, we found the relatively low ratio of urea oxidation to ammonia oxidation was accompanied by a relatively high ammonium concentration. Microorganisms’ high preference for ammonium over urea may lead to an inhibitory-like phenomenon. An analogous effect was evident by the increased urea uptake at downstream when ammonium was down to a few µM. The obstructed urea utilization resulted in 10 times longer lifetime for urea relative to ammonium (surface: 19 ± 9 days; bottom 12 ± 7 days). Such an inhibitory-like effect allows urea to be preserved in the estuary and allows urea to be transported farther offshore to stimulate microorganisms capable of utilizing urea, which may have significant impacts on offshore ecology.

Significance of urea oxidation to nitrite production in the oligotrophic ocean

“…To investigate the spatial pattern of urea oxidation and ammonia oxidation rates and examine the potential environmental controls on the relationship between urea and ammonia oxidation rates, manuscript submitted to Global Biogeochemical Cycles 10 we compiled the available published data for simultaneous marine urea and ammonia oxidation rate measurements. A total of 187 measurements were collected for analysis, from study areas extending from the coastal ocean (i.e., estuaries, shelf) (Damashek et al, 2019;J. M. Tang et al, 2022;Kitzinger et al, 2019;Tolar et al, 2017;Xu et al, 2019) to the subtropical oligotrophic ocean (Wan et al, 2021;Xu et al, 2019) and the mid-and high-latitude oceans (Damashek et al, 2019;Tolar et al, 2017;Shiozaki et al, 2021).…”

“…Although urea has long been known as an important nitrogen source for phytoplankton in the oligotrophic ocean (Sipler and Bronk, 2015), its role as an alternative NH4 + source for energy production in marine ammonia oxidizers has only been recently appreciated, and quantitative comparisons between urea-derived nitrogen oxidation (hereafter referred to as urea oxidation) and ammonia oxidation in the ocean remain sparse. The relationship between the rates of urea and ammonia oxidation is highly variable in both coastal (Damashek et al, 2019;Kitzinger et al, 2019;J. M. Tang et al, 2022; and open ocean systems (Laperriere et al, 2021;Shiozaki et al, 2021;Tolar et al, 2017;Xu et al, 2019) and is poorly understood.…”

Seasonal and anthropogenic influences on bacterioplankton communities: ecological impacts in the coastal waters of Qinhuangdao, Northern China

“…As key participants in nearshore biogeochemical cycles, microorganisms play a crucial role in maintaining the balance of marine ecosystems and degrading high-molecular hydrocarbons and nitrogen- or sulfur-containing organic compounds ( Tang et al, 2022 ; Wang et al, 2022 ; Glibert et al, 2023 ; Yu et al, 2023 ). Microbial diversity, community composition and distribution are significantly influenced by their natural habitat.…”