Ammonia nitrogen posed a significant threat to aquatic animals in aquaculture environments, and the substantial potential of microorganisms in removing ammonia nitrogen had garnered considerable attention. This study identified a marine yeast, Pichia kudriavzevii HJ2, which effectively removed ammonia nitrogen. By combining transcriptomics and metabolomics, the ammonia nitrogen transformation mechanism of HJ2 was elucidated. HJ2 achieved 100% ammonia nitrogen removal efficiency within 1 day of fermentation at 35°C with 300 mg/L ammonia nitrogen and 73.56% removal efficiency within 36 h with 600 mg/L ammonia nitrogen. Transcriptomics revealed that exposure to 600 mg/L ammonia nitrogen resulted in 541 up-regulated genes and 567 down-regulated genes in the HJ2 strain. Differentially expressed genes (DEGs) were primarily involved in the tricarboxylic acid (TCA) cycle and amino acid metabolism. Metabolomics revealed that HJ2 facilitated the production of 383 up-regulated metabolites and suppressed 137 down-regulated metabolites when exposed to 600 mg/L ammonia nitrogen. Integrating transcriptomics and metabolomics analyses showed that HJ2 removed ammonia nitrogen by sensing its presence in the extracellular environment, activating the TCA cycle, enhancing amino acid metabolism and nucleotide metabolism, and promoting its robust growth and reproduction. Amino acid metabolism played an important role in the ammonia transformation mechanism of HJ2. The result was confirmed by the increased activity of glutamate dehydrogenase (GDH) and aspartate aminotransferase (GOT). Up-regulated nitrogen metabolites such as L-glutamate, L-aspartic acid, spermidine, and trigonelline were produced. The results of enzyme activity tests, construction of overexpressing strains, and adding exogenous amino acid experiments demonstrated that HJ2 could utilize GDH and GOT ammonia assimilation pathways. IMPORTANCE: Ammonia nitrogen removal ability was a universal characteristic among the ammonia-oxidizing bacteria or archaea. Recently, yeast strains from the genus Pichia were found to have ammonia nitrogen removal ability. However, the mechanism of ammonia nitrogen removal in Pichia had not been reported. In the study, the ammonia nitrogen removal efficiency of Pichia kudriavzevii HJ2 was identified, and the mechanisms by which HJ2 transformed ammonia nitrogen into non-toxic organic nitrogen were elucidated, offering potential solutions to pollution challenges in aquaculture and helping minimize resource waste. The study offered new insights into the transformation mechanism of microbial ammonia nitrogen removal and its environmentally friendly application.

The emerging pollutants antibiotic resistance genes (ARGs) are prevalent in aquatic environments such as estuary. Coastal mangrove ecosystems always serve as natural wetlands for receiving sewage which always carry ARGs. Currently, the research considering ARG distribution in mangrove ecosystems gains more interest. In this work, we investigated the diversity of ARGs in an urban estuary containing mangrove and nonmangrove areas of the South China Sea. A total of 163 ARGs that classified into 22 resistance types and six resistance mechanisms were found. ARG abundance of the samples in the estuary is between 0.144 and 0.203. This is within the general range of Chinese estuaries. The difference analysis showed that abundances of total ARGs, six most abundant ARGs (mtrA, rpoB, rpoC, rpsL, ef‐Tu, and parY), the most abundant resistance types (elfamycin, multidrug, and peptide), and the most abundant resistance mechanism (target alteration) were significantly lower in mangrove sediment than that in nonmangrove sediment (p < 0.05). Network and partial redundancy analysis showed that sediment properties and mobile genetic elements were the most influential factors impacting ARG distribution rather than microbial community. The two factors collectively explain 51.22% of the differences of ARG distribution. Our study indicated that mangrove sediments have the capacity to remove ARGs. This work provides a research paradigm for analysis of ARG prevalence and proliferation in the subtropical marine coastal mangrove ecosystem.