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Abstract

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 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 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.