Bibliography - K. L. Casciotti

1. Casciotti, K. L., Daniel Sigman, M. G. Hastings, J. K. Böhlke, and A. Hilkert, 2002: Measurement of the oxygen isotopic composition of nitrate in seawater and freshwater using the denitrifier method. Analytical Chemistry, 74(19), doi:10.1021/ac020113w 4905-4912
   [ Abstract ]

   We report a novel method for measurement of the oxygen isotopic composition (¹⁸O/¹⁶O) of nitrate (NO₃ -) from both seawater and freshwater. The denitrifier method, based on the isotope ratio analysis of nitrous oxide generated from sample nitrate by cultured denitrifying bacteria, has been described elsewhere for its use in nitrogen isotope ratio (¹⁵N/¹⁴N) analysis of nitrate.1 Here, we address the additional issues associated with ¹⁸O/¹⁶O analysis of nitrate by this approach, which include (1) the oxygen isotopic difference between the nitrate sample and the N₂O analyte due to isotopic fractionation associated with the loss of oxygen atoms from nitrate and (2) the exchange of oxygen atoms with water during the conversion of nitrate to N₂O. Experiments with ¹⁸O labeled water indicate that water exchange contributes less than 10%, and frequently less than 3%, of the oxygen atoms in the N₂O product for Pseudomonas aureofaciens. In addition, both oxygen isotope fractionation and oxygen atom exchange are consistent within a given batch of analyses. The analysis of appropriate isotopic reference materials can thus be used to correct the measured ¹⁸O/¹⁶O ratios of samples for both effects. This is the first method tested for ¹⁸O/¹⁶O analysis of nitrate in seawater. Benefits of this method, relative to published freshwater methods, include higher sensitivity (tested down to 10 nmol and 1 μM NO₃ -), lack of interference by other solutes, and ease of sample preparation.

2. Sigman, Daniel, and K. L. Casciotti, 2001: Nitrogen isotopes in the ocean. Encyclopedia of Ocean Sciences, London, Academic Press, doi:10.1006/rwos.2001.0172 1884-1894
   [ Abstract ]

   Nitrogen has two stable isotopes, ¹⁴ N and ¹⁵ N (atomic masses of 14 and 15, respectively). ¹⁴ N is the more abundant of the two, comprising 99.63% of the nitrogen found in nature. Physical, chemical, and biological processes discriminate between the two isotopes. This is known as isotopic fractionation, and it leads to subtle but measurable differences in the ratio of ¹⁵ N to ¹⁴ N among different forms of nitrogen found in the marine environment. Nitrogen is a central component of marine biomass and one of the major nutrients required by all phytoplankton. In this sense, biologically available (or ‘fixed’) N is representative of the fundamental patterns of biogeochemical cycling in the ocean. However, N differs from other nutrients in that its oceanic sources and sinks are dominantly internal and biological, with marine N₂ fixation supplying much of the fixed N in the ocean and marine denitrification removing it. The N isotopes provide a means of studying both the internal cycling and input/output budget of oceanic fixed N, yielding information on both its representative and unique aspects. This overview outlines the isotope systematics of N cycle processes and their impacts on the isotopic composition of the major N reservoirs in the ocean. This information provides a starting point for considering the wide range of questions in ocean sciences to which the N isotopes can be applied.

3. Sigman, Daniel, K. L. Casciotti, M. Andreani, C. Barford, M. Galanter, and J. K. Böhlke, 2001: A bacterial method for the nitrogen isotopic analysis of nitrate in seawater and freshwater. Analytical Chemistry, 73(17), doi:10.1021/ac010088e 4145-4153
   [ Abstract ]

   We report a new method for measurement of the isotopic composition of nitrate (NO₃ -) at the natural-abundance level in both seawater and freshwater. The method is based on the isotopic analysis of nitrous oxide (N₂O) generated from nitrate by denitrifying bacteria that lack N₂O-reductase activity. The isotopic composition of both nitrogen and oxygen from nitrate are accessible in this way. In this first of two companion manuscripts, we describe the basic protocol and results for the nitrogen isotopes. The precision of the method is better than 0.2‰ (1 SD) at concentrations of nitrate down to 1 μM, and the nitrogen isotopic differences among various standards and samples are accurately reproduced. For samples with 1 μM nitrate or more, the blank of the method is less than 10% of the signal size, and various approaches may reduce it further.

Direct link to page: http://cmi.princeton.edu/bibliography/results.php?author=4297