Distinctive physiological response of shallow and deep ecotypes of ammonia-oxidizing marine archaea in seawater cultures

Abstract

Ammonia-oxidizing marine archaea (mAOA) have been divided into two groups based on ammonia monooxygenase subunit A gene (amoA) phylogeny and they link to preferred habitat (depth in the water column); one is the Shallow Marine Clade (SMC) and the other is the Deep Marine Clade (DMC). Several determinative factors have been proposed to account for their depth-dependent distributions, including light intensity, ammonia concentration, water temperature, and dissolved oxygen concentration. Here, we report the change in abundance of SMC and DMC mAOA in response to nitrogen source and water temperature in a long-term incubation experiment using natural seawaters obtained from mesopelagic and bathypelagic layers. The abundance of each type of mAOA was determined by Q-PCR assay targeting amoA. We found that the abundance of SMC after 266 days of incubation increased at all three incubation temperatures (4, 10, 20°C), whereas the abundance of DMC was stable at low temperatures (4°C and sometimes 10°C) or decreased to below the detection limit at high temperatures (20°C and sometimes 10°C). The SMC abundance was higher in seawater cultures supplied with ammonium than in those supplied with nitrite, although DMC abundance was not responsive to ammonium concentration. These results imply that water temperature and ammonia concentration are significant factors in determining the vertical distribution of the SMC and DMC ecotypes of mAOA in the water column.