Nitrification (microbial oxidation of ammonia to nitrite and nitrate) controls nitrogen speciation and is the main source of nitrous oxide (N 2 O) in the ocean. It was recently shown that the most abundant marine ammonia oxidizers, the ammonia‐oxidising archaea (AOA), are also capable of oxidising urea, providing a previously ignored source of nitrite. Here, we show that the relative magnitude of urea and ammonia oxidation rat...

Nitrogen (N) bioavailability affects phytoplankton growth and primary production in the aquatic environment. N bioavailability is partly determined by biological N cycling processes that either transform N species or remove fixed N. Reliable estimates of their kinetic parameters can help understand the distribution of N cycling processes. However, available estimates of kinetic parameters are often derived from microbial isola...