Comammox bacterial preference for urea influences its interactions with aerobic nitrifiers

While the co-existence of comammox bacteria with canonical nitrifiers is well documented in diverse ecosystems, there is still a dearth of knowledge about the mechanisms underpinning their interactions. Understanding these interaction mechanisms is important as they may play a critical role in governing nitrogen biotransformation in natural and engineered ecosystems. In this study, we tested the ability of two environmentally relevant factors (nitrogen source and availability) to shape interactions between strict ammonia and nitrite-oxidizing bacteria and comammox bacteria in continuous flow column reactors. The composition of inorganic nitrogen species in reactors fed either ammonia or urea was similar during the lowest nitrogen loading condition (1 mg-N/L), but higher loadings (2 and 4 mg-N/L) promoted significant differences in nitrogen species composition and nitrifier abundances. The abundance and diversity of comammox bacteria were dependent on both nitrogen source and loading conditions as multiple comammox bacterial populations were preferentially enriched in the urea-fed system. In contrast, their abundance was reduced in response to higher nitrogen loadings in the ammonia-fed system likely due to ammonia-based inhibition. The preferential enrichment of comammox bacteria in the urea-fed system could be associated with their ureolytic activity calibrated to their ammonia oxidation rates thus minimizing ammonia accumulation to inhibitory levels. However, an increased abundance of comammox bacteria was not associated with a reduced abundance of nitrite oxidizers in the urea-fed system while a negative correlation was found between them in the ammonia-fed system; the latter dynamic likely emerging from reduced availability of nitrite to strict nitrite oxidizers at low ammonia loading conditions. Importance Nitrification is an essential biological process in drinking water and wastewater treatment systems for managing nitrogen and protecting downstream water quality. The discovery of comammox bacteria and their detection alongside canonical nitrifiers in these engineered ecosystems has made it necessary to understand the environmental conditions that regulate their abundance and activity relative to other better-studied nitrifiers. This study aimed to evaluate two important factors that could potentially influence the behavior of nitrifying bacteria, and therefore impact nitrification processes. Colum reactors fed with either ammonia or urea were systematically monitored to capture changes in nitrogen biotransformation and the nitrifying community as a function of influent nitrogen concentration, nitrogen source, and reactor depth. Our findings show that comammox bacterial abundance decreased and that of nitrite oxidizers increased with increased ammonia availability, while their abundance and diversity increased with increasing urea availability without driving a reduction in the abundance of canonical nitrifiers. ### Competing Interest Statement The authors have declared no competing interest.