Journal
GEOCHIMICA ET COSMOCHIMICA ACTA
Volume 72, Issue 9, Pages 2268-2286Publisher
PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/j.gca.2008.01.033
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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 NH4+-rich (<= 177 nM) neutrally buoyant hydrothermal plume. This NH4+ was quickly removed by both autotrophic ammonia oxidation and assimilation. The former accounted for at least 93% of total net NH4+ 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 NO(3)(-)m(-2) d(-1) 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 beta-proteobacterial AOB based on 16S rRNA and amoA phylogenetic analyses. Meanwhile, NH4+ 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 NH4+ removal rates. The combined NH4+ oxidation and assimilation rates always exceeded total net removal rates, suggesting active in situ NH4+ regeneration rates of at least an order of magnitude greater than the particulate nitrogen flux from the euphotic zone. Ammonia oxidation is responsible for NH4+ 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. (c) 2008 Elsevier Ltd. All rights reserved.
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