Person:

Montoya, Joseph P.

Permanent Link

https://hdl.handle.net/1853/71532

Associated Organization(s)

Organizational Unit

School of Biological Sciences

Full item page

Publication Search Results

Now showing 1 - 8 of 8

Biology and ecology of newly discovered diazotrophs in the open ocean

(Georgia Institute of Technology, 2009-10-30) Montoya, Joseph P. ; Zehr, Jon
Travel support for US participants in an international workshop on the nitrogen cycle

(Georgia Institute of Technology, 2006-06-30) Montoya, Joseph P.
Plasticity 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.
Geophysical 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.
Nitrogen 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.
Trophic-level interpretation based on δ¹⁵N values: implications of tissue-specific fractionation and amino acid composition

(Georgia Institute of Technology, 2004-01-30) Schmidt, Katrin ; McClelland, James W. ; Mente, Eleni ; Montoya, Joseph P. ; Atkinson, Angus ; Voss, Maren

Stable nitrogen isotope ratios are routinely used to disentangle trophic relationships. Several authors have discussed factors in addition to diet that might contribute to variability in δ¹⁵N of consumers, but few studies have explored such factors in detail. For a better understanding of tissue-specific differences in δ¹⁵N, we examined postlarval euphausiids across a variety of seasons and regions in the Southern Ocean. The concentration and δ¹⁵N of individual amino acids were analysed to account for both the biochemical and physiological underpinnings of the observed bulk δ¹⁵N. Euphausiids showed consistent d15N differences of 1 to 2 ” between the digestive gland and abdominal segment, and between reproductively active males and females. These differences in bulk δ¹⁵N were accompanied by variations in relative proportions of amino acids (up to 5 mol %) and their δ¹⁵N (up to 11‰). Aspartic acid and glutamic acid had the strongest influence on bulk δ¹⁵N, due to their high abundance and variable δ¹⁵N values. Differences in relative proportions and/or δ¹⁵N of glycine alanine were also important for bulk δ¹⁵N values. Isotopic variations in amino acids between gender and tissues were explained by dominant internal processes such as protein synthesis or degradation for energy supply, and by differences in amino acid pool sizes. Despite the offset in bulk δ¹⁵N between females and males, several lines of evidence suggested that their trophic levels were similar. Thus, specific amino acid composition and metabolism may confound trophic level interpretations of bulk δ¹⁵N values. Micronekton are normally analyzed whole in isotopic studies, and we suggest that their analyses should be restricted to comparable tissues such as muscles.
Use of stable isotopes to investigate individual differences in diets and mercury exposures among common terns Sterna hirundo in breeding and wintering grounds

(Georgia Institute of Technology, 2002-10-22) Nisbet, Ian C. T. ; Montoya, Joseph P. ; Burger, Joanna ; Hatch, Jeremy J.

We measured variations in stable isotope signatures (δ¹³C and δ¹⁵N) and concentrations of mercury (Hg) in breast feathers from pairs of common terns Sterna hirundo and their chicks at a breeding site in Buzzards Bay, northwestern Atlantic Ocean. By collecting 2 sets of feathers from the same adult birds, we compared values of δ¹³C, δ¹⁵N and Hg in feathers grown in the wintering area in the South Atlantic Ocean (‘southern’ feathers) and in the breeding area (‘regrown’ feathers). Regrown feathers had lower δ¹³C, higher δ¹⁵N and higher Hg than southern feathers. Values of δ¹³C , δ¹⁵N and Hg were much more variable in adults than chicks. Within families, δ¹³C and δ¹⁵N were correlated between parents and chicks; Hg was correlated between male and female parents. Among regrown feathers, Hg was positively correlated with δ¹³C and negatively correlated with δ¹⁵N. These findings suggest that high individual exposure of common terns to Hg results from consumption of inshore prey at low trophic levels in restricted parts of Buzzards Bay and that members of pairs have similar diets in the breeding season but not in winter. They demonstrate the power of stable isotope analyses in revealing individual differences in foraging habits, diet and contaminant exposure in generalist predators.
Trophic relationships and the nitrogen isotopic composition of amino acids in plankton

(Georgia Institute of Technology, 2002) McClelland, James W. ; Montoya, Joseph P.

Stable nitrogen isotope ratios of whole organisms and tissues are routinely used in studies of trophic relationships and nitrogen flow through ecosystems, yet changes underlying increases in δ¹⁵ N from food source to consumer are not completely understood. In this study, the δ¹⁵ N of 16 amino acids in marine planktonic consumers and their food sources were examined using gas chromatography/combustion/isotope ratio mass spectrometry of their N-pivaloyl-i-propyl-amino acid ester derivatives. Moderate increases in bulk δ¹⁵ N with trophic position reflect an averaging of large increases in the δ¹⁵ N of some amino acids, and little or no change in others. Amino acids showing consistently large increases (e.g., glutamic acid changes by ~7‰ between food and consumer) provide greater scope for defining trophic position than the smaller isotopic changes in bulk material. In contrast, amino acids like phenylalanine show no change in δ¹⁵ N with trophic position and therefore preserve information about nitrogen sources at the base of the food web. The ability to acquire information about both trophic level and nitrogen sources at the base of the food web from single samples of consumer tissues offers a powerful new tool for elucidating pathways of N transfer through food webs.