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Geochimica et Cosmochimica Acta 72 (2008) 2268–2286 www.elsevier.com/locate/gca
Microbial ammonia oxidation and enhanced nitrogen cycling in the Endeavour hydrothermal plume
Phyllis Lam a,*, James P. Cowen a, Brian N. Popp b, Ronald D. Jones c
b a Department of Oceanography, School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, HI 96822, USA Department of Geology and Geophysics, School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, HI 96822, USA c Department of Biology, Portland State University, Portland, OR 97207, USA
Received 17 April 2007; accepted in revised form 25 January 2008; available online 10 March 2008
Abstract Ammonium was injected from the subseaoor hydrothermal system at the Endeavour Segment, Juan de Fuca Ridge, into the deep-sea water column resulting in an NH4 -rich (6177 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 (653 nM d1) up to 10-fold that in background deep water. Ammonia oxidation in this plume potentially added 26–130 mg NO3 m2 d1 into the deep-sea water column. This oxidation process was heavily inuenced 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 b-proteobacterial AOB based on 16S rRNA and amoA phylogenetic analyses. Meanwhile, NH4 assimilation rates were also substantially enhanced within the neutrally buoyant plume (626.4 nM d1) 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 ux 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 m2 d1) at early stages of Endeavour neutrally buoyant plumes. 2008 Elsevier Ltd. All rights reserved.

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