Assimilation and oxidation of urea-derived nitrogen in the summer Arctic Ocean

Urea sinks are mainly associated with assimilation by phytoplankton. However, recent studies have shown that there is a process by which nitrifiers convert urea-derived nitrogen (urea-N) into nitrate. We examined these two processes in the shelf and off-shelf regions of the Arctic Ocean. Urea concentration was high near the bottom in the shelf region, while it was depleted throughout the water column in the off-shelf region. Urea-N assimilation was generally higher in the upper euphotic zone than the lower euphotic zone. By contrast, urea-N oxidation was low in the upper euphotic zone and increased with depth. These results indicate that urea sinks consist of a two-layer system. We further examined the organisms involved in urea-N oxidation and found a dominance of shallow clade ammonia-oxidizing archaea, whose abundance was low in the upper euphotic zone and increased with depth. The abundances of archaeal ureC and amoA genes of shallow clade ammonia-oxidizing archaea were well correlated (ρ = 0.96, Spearman’s correlation), suggesting that most of shallow clade ammonia-oxidizing archaea could use urea as a source of ammonia oxidation. However, we found that the urea-N oxidation rate often exceeded the ammonia oxidation rate while kinetics experiments suggested that ammonia oxidizers use urea less actively than ammonia. Network analyses indicated that ammonia oxidizers were closely related to other prokaryotes with the ability to decompose urea. These results suggested that ammonia-oxidizing archaea may not necessarily use urea-N directly.