Quantification of ammonia oxidation rates and ammonia-oxidizing archaea and bacteria at high resolution in the Gulf of California and eastern tropical North Pacific Ocean

Ammonia-oxidizing microorganisms compete with phytoplankton for reduced nitrogen in the euphotic zone and provide oxidized nitrogen to other microbes present in the sea. We report 15NH⁺₄ oxidation rate measurements made at 5–20-m resolution using an in situ array and quantification of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) in corresponding samples from the upper water column and oxygen minimum zone (OMZ) of the Gulf of California (GOC) and eastern tropical North Pacific Ocean (ETNP). 15NH⁺₄ oxidation rates varied substantially with depth and between stations: they were greatest at the base of the euphotic zone, and maximum rates were up to 28-fold greater than rates measured within 5–10 m. Pyrosequencing and quantitative polymerase chain reactions (QPCR) indicated that AOA were present throughout the water column at all latitudes and always outnumbered AOB. AOB constituted only 39 of 432,240 16S ribosomal ribonucleic acid gene sequences produced via pyrosequencing but were more abundant at greater depths and higher latitudes. 15NH⁺₄ oxidation rates were correlated with AOA abundance at some stations and were detectable in 96% of samples, including depths where oxygen concentrations were < 5 μmol kg−1 and depths within the euphotic zone, where up to 42% of ammonia oxidation occurred. Ammonia is rapidly oxidized within discrete depth intervals in the GOC and ETNP; while pyrosequencing and QPCR demonstrate that AOB are confined to deeper portions of the water column, AOA appear to be active within the euphotic zone, where they may quickly respond to nitrogen inputs.