Abundance and Transcriptional Activity of Thaumarchaeal amoA and ureC genes in North Pacific Ocean

Ammonia oxidation, a key step in nitrification, is performed by both ammonia oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA). Recently, the ubiquity of AOA in mesophilic environments has been increasingly recognized; nevertheless, the transcriptional activity of amoA gene (the functional gene encoding for ammonia monooxygenase, AMO) has yet been rarely reported. Moreover, the putative metabolic pathway of AOA remains to be fully understood. Some researchers hypothesized that some AOA may be mixotrophic as they can use urea as alternative energy source under special conditions. However, this urelytic pathway for AOA still lacks distinct evidence. In this research, we compared the abundance and transcriptional activity of thaumarchaeal amoA gene and thaumarchaeal ureC gene (the functional gene encoding for urease) in five stations in North Pacific Ocean. As results, considerable copies of amoA gene, ureC gene, and amoA transcripts were detected in all samples, while the abundance of ureC transcripts were relatively low or under detection in the North Pacific Ocean. The distribution of amoA gene transcripts and amoA gene copies were highly correlated, while the amoA gene transcripts were generally 1 or 2 orders of magnitude less than amoA gene copies. amoA gene transcripts were recorded with higher copies near surface (at 100 m or 200 m depth), reached 1.6×10³ copies per mL, and still detectable down to 5319 m depth, suggesting archaeal nitrification occurs throughout the water column in North Pacific Ocean. In contrast, ureC gene transcripts only ranged from undetectable to 3.2 ×10⁰ copies per mL. The rare ureC gene expression indicate that only few thaumarchaeota may use urea to fuel nitrification.