Depth distribution of ammonia oxidation rates and ammonia‐oxidizer community composition in the Sargasso Sea

Abstract

Ammonia oxidation rates and ammonia‐oxidizer community structure were examined in a depth profile (20–2000 m) at the Bermuda Atlantic Time‐Series Study site in December 2009. Ammonia oxidation rates, measured from trace additions of 15 (12–18 nmol L−1), ranged from undetectable at the surface and 2000 m to 2.0 ± 0.1 nmol L−1 d−1 at 120 m, the depth of the primary nitrite maximum (PNM). Nitrification was not detectable in the photic zone in December, perhaps in part due to the density structure of the upper water column at this time. Ammonium oxidation rates varied with ammonium concentration and yielded an estimate for the half‐saturation constant of 65 ± 41 nmol L−1 for the assemblage at 100 m. This value is similar to that reported for the cultivated marine ammonia‐oxidizing archeon Nitrosopumilus maritimus (134 nmol L−1), confirming the high affinity for ammonium of the in situ community in the Sargasso Sea. Ammonia‐oxidizing archaeal (AOA) amoA gene copy numbers were two orders of magnitude higher than ammonia‐oxidizing bacterial (AOB) amoA gene copy numbers at the PNM depth, suggesting that AOA were responsible for most of the ammonium oxidation, which was in turn responsible for the formation of the PNM. AOB abundance exceeded AOA at depths below 140 m, where both groups were much less abundant. Application of an AOA amoA functional gene microarray showed a diverse and even community distribution. A single archetype (AOA24, representing sequences originally from a coral reef) had the highest fluorescence ratio at depths of 0–1000 m.