Temporal and vertical variability of ammonia oxidizing archaea in the subtropical north Pacific Ocean

Abstract

We sought to investigate environmental controls on two phylogenetically distinct Marine Group I (MGI) ammonia-oxidizing Thaumarchaea at Station ALOHA, the field site for the Hawaii Ocean Time-series (HOT) program. Samples were collected throughout the water column (0-4000 m) on near-monthly cruises between 2006 and 2012. We utilized quantitative PCR amplification of ammonia monooxygenase subunit A (amoA) genes and gene transcripts to evaluate temporal variability in the abundances, transcriptional activities, and vertical distributions of ammonia-oxidizing MGI at Station ALOHA. We observed three to four orders of magnitude differences in the depth distributions of these organisms, with peak abundances occurring between 125 and 300 m. Our results revealed amoA gene abundances underwent moderate increases in the epipelagic waters (0-100 m) during periods of deeper winter mixing. MGI amoA genes near the base of the euphotic zone (100-200 m) were elevated in the fall and winter, suggesting seasonal decreases in irradiance in this region of the water column could partly regulate abundances. Examination of MGI amoA mRNA revealed peak transcriptional activity near the base of the euphotic zone (125-175 m). Our results provide new information on processes influencing the distributions and physiological activities of microorganisms mediating nitrification.