Ammonia oxidizers are among the most ubiquitous and abundant microorganisms in aquatic environments. Recent research has indicated that many ammonia oxidizers are metabolically versatile and can utilize alternative substrates to ammonium, such as urea and cyanate. While marine ammonia oxidizing archaea have been shown to use urea and cyanate, it is unknown whether ammonia oxidizers in estuaries also display metabolic versatility. Here, we measured ammonium, urea and cyanate oxidation rates along a salinity gradient from freshwater to marine waters in a heavily human-perturbed coastal area, the Pearl River Estuary, China, by using a set of comprehensive isotope labeling incubations. Urea and cyanate oxidation rates were observed only occasionally, with minor contribution compared to ammonia oxidation, indicating the presence of ammonia oxidizers that are capable of utilizing urea and cyanate as their substrates. However, urea and cyanate oxidation rates were largely suppressed by high ammonium addition. Our results show that ammonia oxidizers are potentially capable of utilizing urea and cyanate as substrates even under ammonium replete conditions, yet, ammonium is still the preferred and main substrate for nitrification in this eutrophic estuarine environment.

nitrification, ammonia oxidation, urea oxidation, cyanate oxidation, Pearl River Estuary, dissolved organic nitrogen