Ammonium was injected from the subseafloor hydrothermal system at the Endeavour Segment, Juan de Fuca Ridge, into the deep-sea water column resulting in an
-rich (
177 nM) neutrally buoyant hydrothermal plume. This
d02c8c"> was quickly removed by both autotrophic ammonia oxidation and assimilation. The former accounted for at least 93%of total net
removal, with its maximum rate in the neutrally buoyant plume (
53 nM d
−1) up to 10-fold that in background deep water. Ammonia oxidation in this plume potentially added 26–130 mg
into the deep-sea water column. This oxidation process was heavily influenced by the presence of organic-rich particles, with which ammonia-oxidizing bacteria (AOB) were often associated (40–68%). AOB contributed up to 10.8%of the total microbial communities within the plume, and might constitute a novel lineage of β-proteobacterial AOB based on 16S rRNA and
amoA phylogenetic analyses. Meanwhile,
assimilation rates were also substantially enhanced within the neutrally buoyant plume (
26.4 nM d
−1) and accounted for at least 47%of total net
removal rates. The combined
oxidation and assimilation rates always exceeded total net removal rates, suggesting active
in situ 13d2">regeneration rates of at least an order of magnitude greater than the particulate nitrogen flux from the euphotic zone. Ammonia oxidation is responsible for
turnover of 0.7–13 days and is probably the predominant
in situ organic carbon production process (0.6–13 mg C m
−2 d
−1) at early stages of Endeavour neutrally buoyant plumes.