Academic Appointments

Administrative Appointments

  • NRC Postdoctoral Fellow, US Geological Survey, Reston VA, US Geological Survey (2002 - 2004)
  • Assistant Scientist, Woods Hole Oceanographic Institution, MC&G Department, Woods Hole Oceanographic Institution (2004 - 2008)
  • Associate Scientist, Woods Hole Oceanographic Institution, MC&G Department, Woods Hole Oceanographic Institution (2008 - 2010)
  • Assistant Professor, EESS Department, Stanford University (2011 - Present)

Honors & Awards

  • Harold W. Dodds Honorific Fellowship, Princeton University (2001)
  • NRC Postdoctoral Research Associateship, National Research Council (2002)
  • WHOI Coastal Ocean Institute Fellow, Woods Hole Oceanographic Institute (WHOI ) (2008-2011)
  • Terman Fellow, Stanford University (2011)

Boards, Advisory Committees, Professional Organizations

  • Chair, Gordon Research Conference in Chemical Oceanography (2017), GRC (2015 - 2017)
  • Vice-Chair, Gordon Research Conference in Chemical Oceanography (2015), GRC (2013 - 2015)
  • Ocean Sciences Meeting Program Committee Co-Chair (2016), American Geophysical Union (2014 - 2016)
  • Associate Editor, Limnology and Oceanography Methods (2006 - 2016)
  • Executive Committee, Ocean Sciences Section, American Geophysical Union (2013 - 2016)
  • Ocean Sciences Meeting Program Committee Member (2014), American Geophysical Union (2013 - 2016)
  • Steering Committee, US GEOTRACES (2012 - 2017)
  • Intercalibration coordinator for GEOTRACES nitrate isotope measurements, GEOTRACES (2007 - Present)
  • Participant, Implementation workshops for the Atlantic Ocean, Pacific Ocean, and Intercalibration workshop, GEOTRACES (2008 - 2008)
  • Participant, GEOTRACES planning workshops for the Pacific, Atlantic, and Indian oceans, GEOTRACES (2007 - 2007)
  • Geobiology Search Committee, Stanford University (2011 - 2012)
  • EESS Department Seminar Organizer Winter 2013, Stanford University (2013 - 2013)
  • Associate Editor, Marine Chemistry (2009 - Present)
  • Reviewer, NSF (OCE CO and BO, MIP/MO, ETBC, Ecosystems, Chemistry, PIRE) (2004 - Present)
  • Reviewer, Global Biogeochemical Cycles (2004 - Present)
  • Reviewer, Proceedings of the National Academy of Sciences (2010 - Present)
  • Reviewer, ISME Journal (2010 - Present)
  • Reviewer, Rapid Communications in Mass Spectrometry (2007 - Present)
  • Reviewer, Methods in Enzymology (2010 - Present)
  • Reviewer, Nature Geoscience (2009 - Present)
  • Reviewer, Nitrogen in the Marine Environment (2006 - Present)
  • Reviewer, Analytical Chemistry (2006 - Present)
  • Reviewer, Progress in Oceanography (2006 - Present)
  • Reviewer, Journal of Geophysical Research (2006 - Present)
  • Reviewer, Geochimica et Cosmochimica Acta (2004 - Present)
  • Reviewer, Geophysical Research Letters (2004 - Present)
  • Reviewer, PLOS Biology (2005 - Present)
  • Reviewer, Limnology and Oceanography (2005 - Present)
  • Reviewer, Limnology and Oceanography: Methods (2004 - Present)
  • Reviewer, Marine Chemistry (2006 - Present)
  • Reviewer, Deep Sea Research (2005 - Present)
  • Reviewer, Applied and Environmental Microbiology (2006 - Present)
  • Reviewer, Environmental Microbiology (2010 - Present)

Professional Education

  • Ph.D., Princeton University, Geosciences (2002)
  • M.S., Princeton University, Geosciences (1999)
  • M.S., UCSD Scripps Institution of Oceanography, Oceanography (1998)
  • B.S., California Institute of Technology, Environmental Engineering Science (1995)

Current Research and Scholarly Interests

My research focuses on nitrogen cycle biogeochemistry, including how nitrate, nitrite, and nitrous oxide (N2O) are produced and consumed in ocean waters. Nitrate and nitrite are important nutrients for marine photosynthesis, and N2O is a climatically important trace gas. I take an interdisciplinary approach to these questions, applying tools from stable isotope geochemistry, geochemical modeling, microbiology and molecular biology.

I teach Marine Chemistry (EESS 152/252), Marine Stable Isotopes (EESS 249), and co-teach Measurements in Earth Systems (EESS 212)

Professional Activities
Executive Committee AGU/Ocean Sciences Section 2013-present; Program Committee for the Ocean Sciences Meeting in 2014 and Co-Chair in 2016; US GEOTRACES Steering Committee January 2012-present; Associate Editor, Limnology and Oceanography: Methods, 2006-present; Associate Editor, Marine Chemistry, 2009-present; Intercalibration coordinator for GEOTRACES nitrate isotope measurements, 2007-present.

Attended GEOTRACES scoping workshops for the Pacific Ocean (June 25-30, 2007) and Indian Ocean (October 24-26, 2007), implementation workshops for the Atlantic Ocean (September 22-24, 2008) and Pacific Ocean (October 1-3, 2008), intercalibration workshops (December 8-9, 2007 and December 13-14, 2008), North Atlantic Section data workshop (March 11-15, 2013), Pacific Section cruise planning meeting (April 24-26, 2013).

Reviewer for NSF (OCE CO and BO, MIP/MO, ETBC, Ecosystems, Chemistry, PIRE), as well as the following journals: Science, Nature, Nature Geoscience, ISME Journal, Environmental Microbiology, PLOS Biology, Limnology and Oceanography, Limnology and Oceanography: Methods, Global Biogeochemical Cycles, Marine Chemistry, Deep Sea Research, Applied and Environmental Microbiology, Analytical Chemistry, Progress in Oceanography, Journal of Geophysical Research, Geophysical Research Letters, Rapid Communications in Mass Spectrometry, Geochimica et Cosmochimica Acta, and two chapters for the new edition of Nitrogen in the Marine Environment.

2017-18 Courses

Stanford Advisees

All Publications

  • Preliminary assessment of stable nitrogen and oxygen isotopic composition of USGS51 and USGS52 nitrous oxide reference gases and perspectives on calibration needs RAPID COMMUNICATIONS IN MASS SPECTROMETRY Ostrom, N. E., Gandhi, H., Coplen, T. B., Toyoda, S., Bohlke, J. K., Brand, W. A., Casciotti, K. L., Dyckmans, J., Giesemann, A., Mohn, J., Well, R., Yu, L., Yoshida, N. 2018; 32 (15): 1207–14


    Despite a long history and growing interest in isotopic analyses of N2 O, there is a lack of isotopically characterized N2 O isotopic reference materials (standards) to enable normalization and reporting of isotope-delta values. Here we report the isotopic characterization of two pure N2 O gas reference materials, USGS51 and USGS52, which are now available for laboratory calibration ( total of 400 sealed borosilicate glass tubes of each N2 O reference gas were prepared from a single gas filling of a high vacuum line. We demonstrated isotopic homogeneity via dual-inlet isotope-ratio mass spectrometry. Isotopic analyses of these reference materials were obtained from eight laboratories to evaluate interlaboratory variation and provide preliminary isotopic characterization of their δ15 N, δ18 O, δ15 Nα , δ15 Nβ and site preference (SP ) values.The isotopic homogeneity of both USGS51 and USGS52 was demonstrated by one-sigma standard deviations associated with the determinations of their δ15 N, δ18 O, δ15 Nα , δ15 Nβ and SP values of 0.12 mUr or better. The one-sigma standard deviations of SP measurements of USGS51 and USGS52 reported by eight laboratories participating in the interlaboratory comparison were 1.27 and 1.78 mUr, respectively.The agreement of isotope-delta values obtained in the interlaboratory comparison was not sufficient to provide reliable accurate isotope measurement values for USGS51 and USGS52. We propose that provisional values for the isotopic composition of USGS51 and USGS52 determined at the Tokyo Institute of Technology can be adopted for normalizing and reporting sample data until further refinements are achieved through additional calibration efforts.

    View details for DOI 10.1002/rcm.8157

    View details for Web of Science ID 000437842100005

    View details for PubMedID 29729051

  • Tropical Dominance of N-2 Fixation in the North Atlantic Ocean GLOBAL BIOGEOCHEMICAL CYCLES Marconi, D., Sigman, D. M., Casciotti, K. L., Campbell, E. C., Weigand, M., Fawcett, S. E., Knapp, A. N., Rafter, P. A., Ward, B. B., Haug, G. H. 2017; 31 (10): 1608–23
  • Paired N and O isotopic analysis of nitrate and nitrite in the Arabian Sea oxygen deficient zone DEEP-SEA RESEARCH PART I-OCEANOGRAPHIC RESEARCH PAPERS Martin, T. S., Casciotti, K. L. 2017; 121: 121-131
  • Multiple metabolisms constrain the anaerobic nitrite budget in the Eastern Tropical South Pacific GLOBAL BIOGEOCHEMICAL CYCLES Babbin, A. R., Peters, B. D., Mordy, C. W., Widner, B., Casciotti, K. L., Ward, B. B. 2017; 31 (2): 258-271
  • Controls of nitrogen cycling evaluated along a well-characterized climate gradient. Ecology von Sperber, C., Chadwick, O. A., Casciotti, K. L., Peay, K. G., Francis, C. A., Kim, A. E., Vitousek, P. M. 2017


    The supply of nitrogen (N) constrains primary productivity in many ecosystems, raising the question "what controls the availability and cycling of N"? As a step toward answering this question, we evaluated N cycling processes and aspects of their regulation on a climate gradient on Kohala Volcano, Hawaii, USA. The gradient extends from sites receiving <300 mm/yr of rain to those receiving >3,000 mm/yr, and the pedology and dynamics of rock-derived nutrients in soils on the gradient are well understood. In particular, there is a soil process domain at intermediate rainfall within which ongoing weathering and biological uplift have enriched total and available pools of rock-derived nutrients substantially; sites at higher rainfall than this domain are acid and infertile as a consequence of depletion of rock-derived nutrients, while sites at lower rainfall are unproductive and subject to wind erosion. We found elevated rates of potential net N mineralization in the domain where rock-derived nutrients are enriched. Higher-rainfall sites have low rates of potential net N mineralization and high rates of microbial N immobilization, despite relatively high rates of gross N mineralization. Lower-rainfall sites have moderately low potential net N mineralization, relatively low rates of gross N mineralization, and rates of microbial N immobilization sufficient to sequester almost all the mineral N produced. Bulk soil δ(15) N also varied along the gradient, from +4‰ at high rainfall sites to +14‰ at low rainfall sites, indicating differences in the sources and dynamics of soil N. Our analysis shows that there is a strong association between N cycling and soil process domains that are defined using soil characteristics independent of N along this gradient, and that short-term controls of N cycling can be understood in terms of the supply of and demand for N.

    View details for DOI 10.1002/ecy.1751

    View details for PubMedID 28130777

  • Hydrothermal impacts on trace element and isotope ocean biogeochemistry PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES German, C. R., Casciotti, K. A., Dutay, J., Heimburger, L. E., Jenkins, W. J., Measures, C. I., Mills, R. A., Obata, H., Schlitzer, R., Tagliabue, A., Turner, D. R., Whitby, H. 2016; 374 (2081)
  • Nitrite isotopes as tracers of marine N cycle processes PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES Casciotti, K. L. 2016; 374 (2081)
  • Vertical modeling of the nitrogen cycle in the eastern tropical South Pacific oxygen deficient zone using high-resolution concentration and isotope measurements GLOBAL BIOGEOCHEMICAL CYCLES Peters, B. D., Babbin, A. R., Lettmann, K. A., Mordy, C. W., Ulloa, O., Ward, B. B., Casciotti, K. L. 2016; 30 (11): 1661-1681
  • Differential N2O dynamics in two oxygen-deficient lake basins revealed by stable isotope and isotopomer distributions LIMNOLOGY AND OCEANOGRAPHY Wenk, C. B., Frame, C. H., Koba, K., Casciotti, K. L., Veronesi, M., Niemann, H., Schubert, C. J., Yoshida, N., Toyoda, S., Makabe, A., Zopfi, J., Lehmann, M. F. 2016; 61 (5): 1735-1749

    View details for DOI 10.1002/lno.10329

    View details for Web of Science ID 000383621800014

  • Variable Nitrification Rates Across Environmental Gradients in Turbid, Nutrient-Rich Estuary Waters of San Francisco Bay ESTUARIES AND COASTS Damashek, J., Casciotti, K. L., Francis, C. A. 2016; 39 (4): 1050-1071
  • Nitrogen and oxygen isotopic fractionation during microbial nitrite reduction LIMNOLOGY AND OCEANOGRAPHY Martin, T. S., Casciotti, K. L. 2016; 61 (3): 1134-1143

    View details for DOI 10.1002/lno.10278

    View details for Web of Science ID 000375748400023

  • Low rates of nitrogen fixation in eastern tropical South Pacific surface waters PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Knapp, A. N., Casciotti, K. L., Berelson, W. M., Prokopenko, M. G., Capone, D. G. 2016; 113 (16): 4398-4403


    An extensive region of the Eastern Tropical South Pacific (ETSP) Ocean has surface waters that are nitrate-poor yet phosphate-rich. It has been proposed that this distribution of surface nutrients provides a geochemical niche favorable for N2fixation, the primary source of nitrogen to the ocean. Here, we present results from two cruises to the ETSP where rates of N2fixation and its contribution to export production were determined with a suite of geochemical and biological measurements. N2fixation was only detectable using nitrogen isotopic mass balances at two of six stations, and rates ranged from 0 to 23 µmol N m(-2)d(-1)based on sediment trap fluxes. Whereas the fractional importance of N2fixation did not change, the N2-fixation rates at these two stations were several-fold higher when scaled to other productivity metrics. Regardless of the choice of productivity metric these N2-fixation rates are low compared with other oligotrophic locations, and the nitrogen isotope budgets indicate that N2fixation supports no more than 20% of export production regionally. Although euphotic zone-integrated short-term N2-fixation rates were higher, up to 100 µmol N m(-2)d(-1), and detected N2fixation at all six stations, studies of nitrogenase gene abundance and expression from the same cruises align with the geochemical data and together indicate that N2fixation is a minor source of new nitrogen to surface waters of the ETSP. This finding is consistent with the hypothesis that, despite a relative abundance of phosphate, iron may limit N2fixation in the ETSP.

    View details for DOI 10.1073/pnas.1515641113

    View details for Web of Science ID 000374393800052

    View details for PubMedID 26976587

    View details for PubMedCentralID PMC4843426

  • Nitrogen and Oxygen Isotopic Studies of the Marine Nitrogen Cycle ANNUAL REVIEW OF MARINE SCIENCE, VOL 8 Casciotti, K. L. 2016; 8: 379-407
  • Nitrate isotope distributions on the US GEOTRACES North Atlantic cross-basin section: Signals of polar nitrate sources and low latitude nitrogen cycling MARINE CHEMISTRY Marconi, D., Weigand, M. A., Rafter, P. A., McIlvin, M. R., Forbes, M., Casciotti, K. L., Sigman, D. M. 2015; 177: 143-156
  • Nitrogen cycling in the secondary nitrite maximum of the eastern tropical North Pacific off Costa Rica GLOBAL BIOGEOCHEMICAL CYCLES Buchwald, C., Santoro, A. E., Stanley, R. H., Casciotti, K. L. 2015; 29 (12): 2061-2081
  • Intense nitrogen cycling in permeable intertidal sediment revealed by a nitrous oxide hot spot GLOBAL BIOGEOCHEMICAL CYCLES Schutte, C. A., Joye, S. B., Wilson, A. M., Evans, T., Moore, W. S., Casciotti, K. 2015; 29 (10): 1584-1598
  • Aspects of the marine nitrogen cycle of the Chukchi Sea shelf and Canada Basin DEEP-SEA RESEARCH PART II-TOPICAL STUDIES IN OCEANOGRAPHY Brown, Z. W., Casciotti, K. L., Pickart, R. S., Swift, J. H., Arrigo, K. R. 2015; 118: 73-87
  • Stable Isotopes and Iron Oxide Mineral Products as Markers of Chemodenitrification ENVIRONMENTAL SCIENCE & TECHNOLOGY Jones, L. C., Peters, B., Pacheco, J. S., Casciotti, K. L., Fendorf, S. 2015; 49 (6): 3444-3452


    When oxygen is limiting in soils and sediments, microorganisms utilize nitrate (NO3(-)) in respiration-through the process of denitrification-leading to the production of dinitrogen (N2) gas and trace amounts of nitrous (N2O) and nitric (NO) oxides. A chemical pathway involving reaction of ferrous iron (Fe(2+)) with nitrite (NO2(-)), an intermediate in the denitrification pathway, can also result in production of N2O. We examine the chemical reduction of NO2(-) by Fe(II)-chemodenitrification-in anoxic batch incubations at neutral pH. Aqueous Fe(2+) and NO2(-) reacted rapidly, producing N2O and generating Fe(III) (hydr)oxide mineral products. Lepidocrotite and goethite, identified by synchrotron X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) spectroscopy, were produced from initially aqueous reactants, with two-line ferrihydrite increasing in abundance later in the reaction sequence. Based on the similarity of apparent rate constants with different mineral catalysts, we propose that the chemodenitrification rate is insensitive to the type of Fe(III) (hydr)oxide. With stable isotope measurements, we reveal a narrow range of isotopic fractionation during NO2(-) reduction to N2O. The location of N isotopes in the linear N2O molecule, known as site preference, was also constrained to a signature range. The coexistence of Fe(III) (hydr)oxide, characteristic (15)N and (18)O fractionation, and N2O site preference may be used in combination to qualitatively distinguish between abiotic and biogenically emitted N2O-a finding important for determining N2O sources in natural systems.

    View details for DOI 10.1021/es504862x

    View details for Web of Science ID 000351324400022

    View details for PubMedID 25683572

  • N2O production in the eastern South Atlantic: Analysis of N2O stable isotopic and concentration data GLOBAL BIOGEOCHEMICAL CYCLES Frame, C. H., Deal, E., Nevison, C. D., Casciotti, K. L. 2014; 28 (11): 1262-1278
  • Interlaboratory assessment of nitrous oxide isotopomer analysis by isotope ratio mass spectrometry and laser spectroscopy: current status and perspectives RAPID COMMUNICATIONS IN MASS SPECTROMETRY Mohn, J., Wolf, B., Toyoda, S., Lin, C., Liang, M., Brueggemann, N., Wissel, H., Steiker, A. E., Dyckmans, J., Szwec, L., Ostrom, N. E., Casciotti, K. L., Forbes, M., Giesemann, A., Well, R., Doucett, R. R., Yarnes, C. T., Ridley, A. R., Kaiser, J., Yoshida, N. 2014; 28 (18): 1995-2007


    In recent years, research and applications of the N2O site-specific nitrogen isotope composition have advanced, reflecting awareness of the contribution of N2O to the anthropogenic greenhouse effect, and leading to significant progress in instrument development. Further dissemination of N2O isotopomer analysis, however, is hampered by a lack of internationally agreed gaseous N2O reference materials and an uncertain compatibility of different laboratories and analytical techniques.In a first comparison approach, eleven laboratories were each provided with N2O at tropospheric mole fractions (target gas T) and two reference gases (REF1 and REF2). The laboratories analysed all gases, applying their specific analytical routines. Compatibility of laboratories was assessed based on N2O isotopocule data for T, REF1 and REF2. Results for T were then standardised using REF1 and REF2 to evaluate the potential of N2O reference materials for improving compatibility between laboratories.Compatibility between laboratories depended on the analytical technique: isotope ratio mass spectrometry (IRMS) results showed better compatibility for δ(15)N values, while the performance of laser spectroscopy was superior with respect to N2O site preference. This comparison, however, is restricted by the small number of participating laboratories applying laser spectroscopy. Offset and two-point calibration correction of the N2O isotopomer data significantly improved the consistency of position-dependent nitrogen isotope data while the effect on δ(15)N values was only minor.The study reveals that for future research on N2O isotopocules, standardisation against N2O reference material is essential to improve interlaboratory compatibility. For atmospheric monitoring activities, we suggest N2O in whole air as a unifying scale anchor.

    View details for DOI 10.1002/rcm.6982

    View details for Web of Science ID 000340452600006

    View details for PubMedID 25132300

  • Placing an upper limit on cryptic marine sulphur cycling NATURE Johnston, D. T., Gill, B. C., Masterson, A., Beirne, E., Casciotti, K. L., Knapp, A. N., Berelson, W. 2014; 513 (7519): 530-?
  • Differential contributions of archaeal ammonia oxidizer ecotypes to nitrification in coastal surface waters. ISME journal Smith, J. M., Casciotti, K. L., Chavez, F. P., Francis, C. A. 2014; 8 (8): 1704-1714


    The occurrence of nitrification in the oceanic water column has implications extending from local effects on the structure and activity of phytoplankton communities to broader impacts on the speciation of nitrogenous nutrients and production of nitrous oxide. The ammonia-oxidizing archaea, responsible for carrying out the majority of nitrification in the sea, are present in the marine water column as two taxonomically distinct groups. Water column group A (WCA) organisms are detected at all depths, whereas Water column group B (WCB) are present primarily below the photic zone. An open question in marine biogeochemistry is whether the taxonomic definition of WCA and WCB organisms and their observed distributions correspond to distinct ecological and biogeochemical niches. We used the natural gradients in physicochemical and biological properties that upwelling establishes in surface waters to study their roles in nitrification, and how their activity-ascertained from quantification of ecotype-specific ammonia monooxygenase (amoA) genes and transcripts-varies in response to environmental fluctuations. Our results indicate a role for both ecotypes in nitrification in Monterey Bay surface waters. However, their respective contributions vary, due to their different sensitivities to surface water conditions. WCA organisms exhibited a remarkably consistent level of activity and their contribution to nitrification appears to be related to community size. WCB activity was less consistent and primarily constrained to colder, high nutrient and low chlorophyll waters. Overall, the results of our characterization yielded a strong, potentially predictive, relationship between archaeal amoA gene abundance and the rate of nitrification.

    View details for DOI 10.1038/ismej.2014.11

    View details for PubMedID 24553472

  • Stable isotope analyses of NO2-, NO3-, and N2O in the hypersaline ponds and soils of the McMurdo Dry Valleys, Antarctica GEOCHIMICA ET COSMOCHIMICA ACTA Peters, B., Casciotti, K. L., Samarkin, V. A., Madigan, M. T., Schutte, C. A., Joye, S. B. 2014; 135: 87-101
  • Stable isotopic analyses of NO2-, NO3-, and N2O in hypersaline ponds and soils of the McMurdo Dry Valleys, Antarctica Geochimica et Cosmochimica Acta Peters, B., Casciotti, K. L., Samarkin, V. A., Madigan, M. T., Schutte, C. A., Joye, S. B. 2014
  • Differential contributions of archaeal ammonia oxidizer ecotypes to nitrification in coastal surface waters ISME Journal Smith, J. M., Casciotti, K. L., Chavez, F. P., Francis, C. A. 2014
  • Aspects of the marine nitrogen cycle on the Chukchi Sea shelf and Canada Basin Deep Sea Research II Browh, Z. W., Casciotti, K. L., Pickart, R. L., Swift, J. H., Arrigo, K. R. 2014
  • Implications of nitrate and nitrite isotopic measurements for the mechanisms of nitrogen cycling in the Peru oxygen deficient zone DEEP-SEA RESEARCH PART I-OCEANOGRAPHIC RESEARCH PAPERS Casciotti, K. L., Buchwald, C., McIlvin, M. 2013; 80: 78-93
  • Stable Isotopes as Tracers of Anthropogenic Nitrogen Sources, Deposition, and Impacts ELEMENTS Hastings, M. G., Casciotti, K. L., Elliott, E. M. 2013; 9 (5): 339-344
  • Excess nitrate loads to coastal waters reduces nitrate removal efficiency: mechanism and implications for coastal eutrophication ENVIRONMENTAL MICROBIOLOGY Lunau, M., Voss, M., Erickson, M., Dziallas, C., Casciotti, K., Ducklow, H. 2013; 15 (5): 1492-1504


    Terrestrial ecosystems are becoming increasingly nitrogen-saturated due to anthropogenic activities, such as agricultural loading with artificial fertilizer. Thus, more and more reactive nitrogen is entering streams and rivers, primarily as nitrate, where it is eventually transported towards the coastal zone. The assimilation of nitrate by coastal phytoplankton and its conversion into organic matter is an important feature of the aquatic nitrogen cycle. Dissolved reactive nitrogen is converted into a particulate form, which eventually undergoes nitrogen removal via microbial denitrification. High and unbalanced nitrate loads to the coastal zone may alter planktonic nitrate assimilation efficiency, due to the narrow stochiometric requirements for nutrients typically shown by these organisms. This implies a cascade of changes for the cycling of other elements, such as carbon, with unknown consequences at the ecosystem level. Here, we report that the nitrate removal efficiency (NRE) of a natural phytoplankton community decreased under high, unbalanced nitrate loads, due to the enhanced recycling of organic nitrogen and subsequent production and microbial transformation of excess ammonium. NRE was inversely correlated with the amount of nitrate present, and mechanistically controlled by dissolved organic nitrogen (DON), and organic carbon (Corg) availability. These findings have important implications for the management of nutrient runoff to coastal zones.

    View details for DOI 10.1111/j.1462-2920.2012.02773.x

    View details for Web of Science ID 000318041800019

    View details for PubMedID 22568592

  • Isotopic ratios of nitrite as tracers of the sources and age of oceanic nitrite NATURE GEOSCIENCE Buchwald, C., Casciotti, K. L. 2013; 6 (4): 308-313

    View details for DOI 10.1038/NGEO1745

    View details for Web of Science ID 000316946500021

  • Measurements of nitrite production in and around the primary nitrite maximum in the central California Current BIOGEOSCIENCES Santoro, A. E., Sakamoto, C. M., Smith, J. M., Plant, J. N., Gehman, A. L., Worden, A. Z., Johnson, K. S., Francis, C. A., Casciotti, K. L. 2013; 10 (11): 7395-7410
  • Insights on the marine microbial nitrogen cycle from isotopic approaches to nitrification. Frontiers in microbiology Casciotti, K. L., Buchwald, C. 2012; 3: 356-?


    The microbial nitrogen (N) cycle involves a variety of redox processes that control the availability and speciation of N in the environment and that are involved with the production of nitrous oxide (N(2)O), a climatically important greenhouse gas. Isotopic measurements of ammonium (NH(+) (4)), nitrite (NO(-) (2)), nitrate (NO(-) (3)), and N(2)O can now be used to track the cycling of these compounds and to infer their sources and sinks, which has lead to new and exciting discoveries. For example, dual isotope measurements of NO(-) (3) and NO(-) (2) have shown that there is NO(-) (3) regeneration in the ocean's euphotic zone, as well as in and around oxygen deficient zones (ODZs), indicating that nitrification may play more roles in the ocean's N cycle than generally thought. Likewise, the inverse isotope effect associated with NO(-) (2) oxidation yields unique information about the role of this process in NO(-) (2) cycling in the primary and secondary NO(-) (2) maxima. Finally, isotopic measurements of N(2)O in the ocean are indicative of an important role for nitrification in its production. These interpretations rely on knowledge of the isotope effects for the underlying microbial processes, in particular ammonia oxidation and nitrite oxidation. Here we review the isotope effects involved with the nitrification process and the insights provided by this information, then provide a prospectus for future work in this area.

    View details for DOI 10.3389/fmicb.2012.00356

    View details for PubMedID 23091468

  • Insights on the marine microbial nitrogen cycle from isotopic approaches to nitrification FRONTIERS IN MICROBIOLOGY Casciotti, K. L., Buchwald, C. 2012; 3
  • Potential importance of physiologically diverse foraminifera in sedimentary nitrate storage and respiration Journal of Geophysical Research: Biogeosciences Bernhard, J. M., Casciotti, K. L., McIlvin, M. R., Beaudoin , D. J., Visscher, P. T., Edgcomb, V. P. 2012; 117

    View details for DOI 10.1029/2012JG001949

  • Distribution of anaerobic ammonia-oxidizing bacteria in a subterranean estuary Marine Chemistry Saenz, J. P., Hopmans, E. C., Rogers, D., Henderson, P., Charette, M. A., Casciotti, K. L., Shouten, S., Sinninghe-Damste, J. S., Eglinton, T. I. 2012; 136-137: 7-13
  • Oxygen isotopic composition of nitrate and nitrite produced by nitrifying co-cultures and natural marine assemblages Limnology and Oceanography Buchwald, C., Santoro, A., McIlvin, M., Casciotti, K. L. 2012; 57: 1361-1375
  • Denitrification likely catalyzed by endobionts in an allogromiid foraminifer Multidisciplinary Journal of Microbial Ecology (ISME Journal) Bernhard, J. M., Edgcomb, V. P., Casciotti, K. L., McIlvin, M. R., Beaudoin, D. J. 2012; 6 (5): 951-960
  • Basin-scale input of cobalt, iron, and manganese from the Benguela-Angola front to the South Atlantic Ocean Limnology and Oceanography Noble, A. E., Lamborg, C. H., Ohnemus, D. C., Lam, P. J., Goepfert, T. J., Measures, C. I., Frame, C. H., Casciotti, K. L., DiTullio, G. R., Jennings, J., Saito, K. A. 2012; 57 (4): 989-1010
  • Isotopic Signature of N2O Produced by Marine Ammonia-Oxidizing Archaea SCIENCE Santoro, A. E., Buchwald, C., McIlvin, M. R., Casciotti, K. L. 2011; 333 (6047): 1282-1285


    The ocean is an important global source of nitrous oxide (N(2)O), a greenhouse gas that contributes to stratospheric ozone destruction. Bacterial nitrification and denitrification are thought to be the primary sources of marine N(2)O, but the isotopic signatures of N(2)O produced by these processes are not consistent with the marine contribution to the global N(2)O budget. Based on enrichment cultures, we report that archaeal ammonia oxidation also produces N(2)O. Natural-abundance stable isotope measurements indicate that the produced N(2)O had bulk δ(15)N and δ(18)O values higher than observed for ammonia-oxidizing bacteria but similar to the δ(15)N and δ(18)O values attributed to the oceanic N(2)O source to the atmosphere. Our results suggest that ammonia-oxidizing archaea may be largely responsible for the oceanic N(2)O source.

    View details for DOI 10.1126/science.1208239

    View details for Web of Science ID 000294406400052

    View details for PubMedID 21798895

  • Technical Updates to the Bacterial Method for Nitrate Isotopic Analyses ANALYTICAL CHEMISTRY McIlvin, M. R., Casciotti, K. L. 2011; 83 (5): 1850-1856


    The bacterial conversion of aqueous nitrate (NO(3)(-)) to nitrous oxide (N(2)O) for isotopic analysis has found widespread use since its introduction (Sigman, D. M.; Casciotti, K. L.; Andreani, M.; Galanter, M.; Böhlke, J. K. Anal. Chem.2001, 73, 4145-4153; Casciotti, K. L.; Sigman, D. M.; Galanter Hastings, M.; Böhlke, J. K.; Hilkert, A. Anal. Chem.2002, 74, 4905-4912). The bacterial strain Pseudomonas aureofaciens (ATTC no. 13985) was shown to convert NO(3)(-) to N(2)O while retaining both N and O isotopic signatures, and automation of the isotopic analysis of N(2)O greatly increased the throughput of the method (Casciotti, K. L.; Sigman, D. M.; Galanter Hastings, M.; Böhlke, J. K.; Hilkert, A. Anal. Chem.2002, 74, 4905-4912). Continued development of the denitrifier method has led to increased precision and throughput of NO(3)(-) isotopic analysis. Presented here are several recent procedural modifications and the demonstration of their effectiveness.

    View details for DOI 10.1021/ac1028984

    View details for Web of Science ID 000287685800052

    View details for PubMedID 21302935

  • Enrichment and characterization of ammonia-oxidizing archaea from the open ocean: phylogeny, physiology, and stable isotope fractionation Multidisciplinary Journal of Microbial Ecology (ISME Journal) Santoro, A. E., Casciotti, K. L. 2011; 5: 1796-1808
  • Nitrous oxide dynamics in a braided river system, New Zealand Journal of Environmental Quality Clough, T. J., Buckthought, L. E., Casciotti, K. L., Keliher, F. M., Jones, P. K. 2011; 40: 1532-1541
  • Activity, abundance and diversity of nitrifying archaea and bacteria in the central California Current ENVIRONMENTAL MICROBIOLOGY Santoro, A. E., Casciotti, K. L., Francis, C. A. 2010; 12 (7): 1989-2006


    A combination of stable isotope and molecular biological approaches was used to determine the activity, abundance and diversity of nitrifying organisms in the central California Current. Using (15)NH(4)(+) incubations, nitrification was detectable in the upper water column down to 500 m; maximal rates were observed just below the euphotic zone. Crenarchaeal and betaproteobacterial ammonia monooxygenase subunit A genes (amoA), and 16S ribosomal RNA (rRNA) genes of Marine Group I Crenarchaeota and a putative nitrite-oxidizing genus, Nitrospina, were quantified using quantitative PCR. Crenarchaeal amoA abundance ranged from three to six genes ml(-1) in oligotrophic surface waters to > 8.7 x 10(4) genes ml(-1) just below the core of the California Current at 200 m depth. Bacterial amoA abundance was lower than archaeal amoA and ranged from below detection levels to 400 genes ml(-1). Nitrification rates were not directly correlated to bacterial or archaeal amoA abundance. Archaeal amoA and Marine Group I crenarchaeal 16S rRNA gene abundances were correlated with Nitrospina 16S rRNA gene abundance at all stations, indicating that similar factors may control the distribution of these two groups. Putatively shallow water-associated archaeal amoA types ('Cluster A') decreased in relative abundance with depth, while a deep water-associated amoA type ('Cluster B') increased with depth. Although some Cluster B amoA sequences were found in surface waters, expressed amoA gene sequences were predominantly from Cluster A. Cluster B amoA transcripts were detected between 100 and 500 m depths, suggesting an active role in ammonia oxidation in the mesopelagic. Expression of marine Nitrosospira-like bacterial amoA genes was detected throughout the euphotic zone down to 200 m. Natural abundance stable isotope ratios (delta(15)N and delta(18)O) in nitrate (NO(3)(-)) and nitrous oxide (N(2)O) were used to evaluate the importance of nitrification over longer time scales. Using an isotope mass balance model, we calculate that nitrification could produce between 0.45 and 2.93 micromol m(-2) day(-1) N(2)O in the central California Current, or approximately 1.5-4 times the local N(2)O flux from deep water.

    View details for DOI 10.1111/j.1462-2920.2010.02205.x

    View details for Web of Science ID 000280101200016

    View details for PubMedID 20345944

  • Abundance and diversity of archaeal ammonia oxidizers in a coastal groundwater aquifer Applied and Environmental Microbiology Rogers, D., Casciotti, K. L. 2010; 76: 7938-7948
  • Oxygen isotopic exchange and fractionation during bacterial nitrite oxidation Limnology and Oceanography Buchwald, C., Casciotti, K. L. 2010; 55: 1064-1074
  • Novel strains isolated from a coastal aquifer suggest a predatory role for flavobacteria Federation of European Microbiological Societies (FEMS) Microbiology Ecology Banning, E. C., Casciotti, K. L., Kujawinski, E. B. 2010; 73: 254-270
  • Assessment of nitrogen and oxygen isotopic fractionation during nitrification and its expression in the marine environment Methods in Enzymology Casciotti, K. L., Buchwald, C., Santoro, A. E., Frame, C. H. 2010; 486: 253-280
  • Biogeochemical controls and isotopic signatures of nitrous oxide production by a marine ammonia-oxidizing bacterium Biogeosciences Frame, C. H., Casciotti, K. L. 2010; 7: 2695-2709
  • Abiotic nitrous oxide emission from the hypersaline Don Juan Pond in Antarctica Nature Geoscience Samarkin, V. A., Madigan, M. T., Bowles, M. W., Casciotti, K. L., Priscu, J. C., McKay, C. P., Joye, S. B. 2010; 3: 341-344
  • Fully automated system for stable isotopic analyses of dissolved nitrous oxide at natural abundance levels Limnology and Oceanography: Methods McIlvin , M. R., Casciotti, K. L. 2010; 8: 54-66
  • Inverse Kinetic Isotope Fractionation During Bacterial Nitrite Oxidation Geochimica et Cosmochimica Acta Casciotti, K. L. 2009; 73: 2061-2076
  • Determining the nitrogen isotopic composition of porphyrins by the denitrifier method Analytical Chemistry Higgins, M. B., Robinson, R. S., Casciotti, K. L., McIlvin, M., Pearson, A. 2009; 81: 184-192
  • Dual isotope analyses indicate efficient processing of atmospheric nitrate by forested watersheds in the northeastern U.S. Biogeochemistry Barnes, R. T., Raymond, P. A., Casciotti, K. L. 2008; 90: 15-27
  • Nitrogen Isotopes in the Ocean Encyclopedia of Ocean Sciences Sigman, D. M., Karsh, K. L., Casciotti, K. L. edited by Steele, J. H., Thorpe, S. A., Turekian, K. K. Academic Press, New York. 2008
  • Supersaturated N2O in a perennially ice-covered Antarctic lake: Molecular and stable isotopic evidence for a biogeochemical relict Limnology and Oceanography Priscu, J. C., Christner, B. C., Dore, J. E., Westley, M. B., Popp, B. N., Casciotti, K. L., Lyons, W. B. 2008; 53: 2438-2450
  • Insights into nutrient assimilation and export in naturally iron-fertilized waters of the Southern Ocean from nitrogen, carbon, and oxygen isotopes Deep Sea Research II Trull, T. W., Davies, D., Casciotti, K. L. 2008; 55: 820-840
  • Barium in Twilight Zone Suspended Matter as a Proxy for Particulate Organic Carbon Remineralization: Results from the North Pacific Deep Sea Research II Dehairs, F., Jacquet, S., Savoye, N., Bishop, J., van Mooy, B., Buesseler, K., Lamborg, C., Elskens, M., Boyd, P., Casciotti, K., Baeyens, W. 2008; 55: 1673-1683
  • Constraints on Nitrogen Cycling at the Subtropical North Pacific Station ALOHA from Isotopic Measurements of Nitrate and Particulate Nitrogen Deep Sea Research II Casciotti, K. L., Glover, D. M., Trull, T., Davies, D. 2008; 55: 1661-1672
  • Isotopic analyses of nitrate and nitrite from reference mixtures and application to Eastern Tropical North Pacific waters Marine Chemistry Casciotti, K. L., McIlvin, M. R. 2007; 107: 184-201
  • Oxygen isotopes in nitrite: analysis, calibration, and equilibration Analytical Chemistry Casciotti, K. L., Böhlke, J. K., McIlvin, M. R., Mroczkowski, S. J., Hannon, J. E. 2007; 79: 2427-2436
  • Revisiting Carbon Flux Through the Ocean's Twilight Zone Science Buesseler, K. O., Lamborg, C. H., Boyd, P. W., Lam, P. J., Trull, T. W., Bidigare, R. R., Bishop, J. K., Casciotti, K. L., Dehairs, F., Elskens, M., Honda, M., Karl, D. M., Siegel, D. A., Silver, M. W., Steinberg, D. K., Valdes, J., Van Mooy, B., Wilson, S. 2007; 316: 567-570
  • Method for the analysis of d18O in water Analytical Chemistry McIlvin, M., Casciotti, K. L. 2006; 78: 22377-2381
  • Denitrification in the Hypolimnion of Permanently Ice-Covered Lake Bonney, Antarctica Aquatic Microbial Ecology Ward , B., Granger, B. J., Maldonado, M. T., Casciotti, K. L., Harris, S., Wells, M. L. 2005; 38: 295-307
  • Phylogenetic analysis of nitric oxide reductase gene homologues from aerobic ammonia-oxidizing bacteria Federation of European Microbiological Societies (FEMS) Microbiology Ecology Casciotti, K. L., Ward, B. B. 2005; 52: 197-205
  • Using dual bacterial denitrification to improve d15N determinations of nitrates containing mass independent 17O Rapid Communications in Mass Spectrometry Coplen, T. B., Böhlke, J. K., Casciotti, K. L. 2004; 18: 245-250
  • Preparation and Analysis of Nitrogen-bearing Compounds in Water for Stable Isotope Ratio Measurement Handbook of Stable Isotope Analytical Techniques Chang, C. Y., Silva, S. R., Kendall, C., Michalski, G., Casciotti, K. l., Wankel, S. edited by deGroot, P. A. 2004: 305–354
  • Linking Diversity and Stable Isotope Fractionation in Ammonia-Oxidizing Bacteria Geomicrobiology Journal Casciotti, K. L., Sigman, D. M., Ward, B. B. 2003; 20: 335-353
  • Localization of Mn(II)-oxidizing activity and the putative multicopper oxidase, MnxG, to the exosporium of the marine Bacillus sp. strain SG-1 Archives of Microbiology Francis, C. A., Casciotti, K. L., Tebo, B. B. 2002; 178: 450-456
  • Measurement of the Oxygen Isotopic Composition of Nitrate in Marine and Fresh Waters Using the Denitrifier Method Analytical Chemistry Casciotti, K. L., Sigman, D. M., Hastings, M. G., Böhlke, J. K., Hilkert, A. 2002; 74: 4905-4912
  • A Bacterial Method for the Nitrogen Isotopic Analysis of Nitrate in Marine and Fresh Waters Analytical Chemistry Sigman, D. M., Casciotti, K. L., Andreani, M., Barford, C., Galanter, M., Böhlke, J. K. 2001; 73: 4145-4153
  • Dissimilatory Nitrite Reductase Genes in Autotrophic Ammonia-Oxidizing Bacteria Applied and Environmental Microbiology Casciotti, K. L., Ward, B. B. 2001; 67: 2213-2221
  • Initiation of the Spring Phytoplankton Increase in the Antarctic Polar Front Zone at 170º W Journal of Geophysical Research-Oceans Landry, M. R., Brown, S. L., Selph, K. E., Abbott, M. R., Letelier, R. M., Christensen, S., Bidigare, R. R., Casciotti, K. L. 2001; 106: 13903-13915
  • Nitrogen Isotopes in the Ocean Encyclopedia of Ocean Sciences Sigman, D. M., Casciotti, K. L., Steele, J. H., Thorpe, S. A. edited by Turekian, K. K. Academic Press, New York. 2001: 1884–1894