HR: 1340h
AN: B13A-0186    [Abstracts]
TI: The Fate of Ammonium in the Endeavour Hydrothermal Plume: Microbial Ammonia Oxidation and Ammonium Assimilation
AU: * Lam, P
EM: plam@soest.hawaii.edu
AF: Department of Oceanography SOEST University of Hawaii at Manoa, 1000 Pope Road, Honolulu, HI 96822 United States
AU: Cowen, J P
EM: jcowen@soest.hawaii.edu
AF: Department of Oceanography SOEST University of Hawaii at Manoa, 1000 Pope Road, Honolulu, HI 96822 United States
AU: Popp, B N
EM: popp@soest.hawaii.edu
AF: Department of Oceanography SOEST University of Hawaii at Manoa, 1000 Pope Road, Honolulu, HI 96822 United States
AU: Jones, R D
EM: jonesrd@pdx.edu
AF: Department of Biology , Portland State University, Portland, OR 97207 United States
AB: Highly elevated ammonium concentrations (up to 177$ñ$ 4.5 nM) were observed within the axial valley and the hydrothermal plume along the Endeavour Segment, and were quickly dissipated due to both autotrophic ammonia oxidation and assimilation. Autotrophic ammonia oxidation rates accounted for at least 93 % of the total net ammonium removal rates, and reached a maximum of 53 nM d$^{-1}$ in the neutrally buoyant plume. Ammonia oxidation rates showed a sigmoidal relationship with ammonium concentrations based on data collected over three years. The resulting empirical half-saturation constant (153 nM) was at least an order of magnitude lower than those measured in cultured studies. This high substrate affinity implies that natural oceanic ammonia-oxidizing bacteria are highly adapted to the generally limited ammonium in the deep sea. Ammonia oxidation in this plume potentially added 26-127 mg N m$^{-2}$ d$^{-1}$ of nitrate into the deep-sea water column. This oxidation process was heavily influenced by the presence of organic-rich particles, with which ammonia-oxidizing bacteria were often associated (40-68 %). Ammonia-oxidizing bacteria, including those in both the $\beta$- and $\gamma$- {\it Proteobacteria} subgroups, contributed up to 10.8 % of total microbial abundance within the plume. Ammonium assimilation rates were also substantially enhanced within the neutrally buoyant plume (up to 20 nM d$^{-1}$), and accounted for at least 42 % of total net ammonium removal rates. The two ammonium uptake rates together always exceeded the total net removal rates, suggesting an active {\it in situ} regeneration of ammonium (1.0-23.5 nM d$^{-1}$ or 4.2-99 mg N m$^{-2}$ d$^{-1}$ integrated over plume depths) that was equivalent to 3- to 130-fold of what could be provided by the particulate flux from the euphotic zone reaching these depths. Ammonia oxidation is responsible for ammonium turnover in 0.7-13 days, and is an important {\it in situ} organic carbon production process (3.9-1.8 mg C m$^{-2}$ d$^{-1}$) especially at an early stage of the Endeavour neutrally buoyant plume.
DE: 4805 Biogeochemical cycles (1615)
DE: 4832 Hydrothermal systems
DE: 4840 Microbiology
DE: 4845 Nutrients and nutrient cycling
DE: 4851 Oxidation/reduction reactions
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting