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Montoya,
Joseph P.
Montoya,
Joseph P.
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ItemTravel support for US participants in an international workshop on the nitrogen cycle(Georgia Institute of Technology, 2006-06-30) Montoya, Joseph P.
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ItemPlasticity of N:P ratios in laboratory and field populations of Trichodesmium spp.(Georgia Institute of Technology, 2006-03-29) Krauk, Jamie M. ; Villareal, Tracy A. ; Sohm, Jill A. ; Montoya, Joseph P. ; Capone, Douglas G.We followed changes in N:P ratios in batch cultures of the planktonic marine cyanobacterium Trichodesmium (IMS 101) grown in 2 different media and in field populations from 4 different oceanic regions. Cultures grown on low P media showed a rapid rise in N:P ratio upon depletion of phosphate. Ratios exceeding 125 were reached in 1 experiment before attaining stationary phase. A transect across the North Atlantic Ocean along 32°N showed a monotonic decrease in the N:P ratio of field collected colonies, dropping from about 60:1 on the western side of the basin to about 30:1 on the eastern side. A second cruise sampled colonies and surface slicks in waters along the north coast of Australia, where ratios of N:P were generally lower than in the North Atlantic, ranging from 11:1 to 47:1 with an average of 22:1. A comparison of rising and sinking colonies collected at 8 stations in the Gulf of Mexico shows a higher mean N:P ratio among sinking colonies than floating colonies. Overall, the average N:P in the Gulf of Mexico was about 68:1. N:P ratios of Trichodesmium around the Hawaiian Islands were very consistent between 2 consecutive years of sampling, with an average colony N:P for both years of about 38:1. Our research demonstrates high variability in the cellular N:P in Trichodesmium both in the laboratory and in the field. Trichodesmium N:P ratio may provide an index to the relative severity of P limitation in these diazotrophs. Geochemical and ecological modeling efforts which rely on using the N:P ratio of diazotrophs in deriving nitrogen fixation rates should account for the variability of these ratios in situ.
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ItemGeophysical and geochemical signatures of Gulf of Mexico seafloor brines(Georgia Institute of Technology, 2005-05) Joye, S. B. ; MacDonald, I. R. ; Montoya, Joseph P. ; Peccini, M.Geophysical, temperature, and discrete depth-stratified geochemical data illustrate differences between an actively venting mud volcano and a relatively quiescent brine pool in the Gulf of Mexico along the continental slope. Geophysical data, including laser-line scan mosaics and sub-bottom profiles, document the dynamic nature of both environments. Temperature profiles, obtained by lowering a CTD into the brine fluid, show that the venting brine was at least 10°C warmer than the bottom water. At the brine pool, two thermoclines were observed, one directly below the brine-seawater interface and a second about one meter below the first. At the mud volcano, substantial temperature variability was observed, with the core brine temperature being either slightly (~2°C in 1997) or substantially (19°C in 1998) elevated above bottom water temperature. Geochemical samples were obtained using a device called the "brine trapper" and concentrations of dissolved gases, major ions and nutrients were determined using standard techniques. Both brines contained about four times as much salt as seawater and steep concentration gradients of dissolved ions and nutrients versus brine depth were apparent. Differences in the concentrations of calcium, magnesium and potassium between the two brine fluids suggests that the fluids are derived from different sources or that brine-sediment reactions are more important at the mud volcano than the brine pool. Substantial concentrations of methane and ammonium were observed in both brines, suggesting that fluids expelled from deep ocean brines are important sources of methane and dissolved inorganic nitrogen to the surrounding environment.
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ItemNitrogen fixation by Trichodesmium spp.: An important source of new nitrogen to the tropical North Atlantic Ocean(Georgia Institute of Technology, 2005) Capone, Douglas G. ; Burns, James A. ; Montoya, Joseph P. ; Subramaniam, Ajit ; Mahaffey, ; Gunderson, Troy ; Michaels, Anthony F. ; Carpenter, Edward J.The broad distribution and often high densities of the cyanobacterium Trichodesmium spp. in oligotrophic waters imply a substantial role for this one taxon in the oceanic N cycle of the marine tropics and subtropics. New results from 154 stations on six research cruises in the North Atlantic Ocean show depth-integrated N₂ fixation by Trichodesmium spp. at many stations that equalled or exceeded the estimated vertical flux of NO₃ into the euphotic zone by diapycnal mixing. Areal rates are consistent with those derived from several indirect geochemical analyses. Direct measurements of N₂ fixation rates by Trichodesmium are also congruent with upper water column N budgets derived from parallel determinations of stable isotope distributions, clearly showing that N₂ fixation by Trichodesmium is a major source of new nitrogen in the tropical North Atlantic. We project a conservative estimate of the annual input of new N into the tropical North Atlantic of at least 1.6 X 10 ¹² mol N by Trichodesmium N₂ fixation alone. This input can account for a substantial fraction of the N₂ fixation in the North Atlantic inferred by several of the geochemical approaches.