title: Exploration of inorganic C and N assimilation by soil microbes with time-of-flight secondary ion mass spectrometry.
abstract: Stable C and N isotopes have long been used to examine properties of various C and N cycling processes in soils. Unfortunately, relatively large sample sizes are needed for accurate gas phase isotope ratio mass spectrometric analysis. This limitation has prevented researchers from addressing C and N cycling issues on microbially meaningful scales. Here we explored the use of time-of-flight secondary ion mass spectrometry (TOF-SIMS) to detect 13C and 15N assimilation by individual bacterial cells and to quantify N isotope ratios in bacterial samples and individual fungal hyphae. This was accomplished by measuring the relative abundances of mass 26 (12C14N-) and mass 27 (13C14N- and 12C15N-) ions sputtered with a Ga+ probe from cells adhered to an Si contact slide. TOF-SIMS was successfully used to locate and quantify the relative 15N contents of individual hyphae that grew onto Si contact slides in intimate contact with a model organomineral porous matrix composed of kaolin, straw fragments, and freshly deposited manure that was supplemented with 15NO3-. We observed that the 15N content of fungal hyphae grown on the slides was significantly lower in regions where the hyphae were influenced by N-rich manure than in regions influenced by N-deficient straw. This effect occurred over distances of tens to hundreds of microns. Our data illustrate that TOF-SIMS has the potential to locate N-assimilating microorganisms in soil and to quantify the 15N content of cells that have assimilated 15N-labeled mineral N and shows promise as a tool with which to explore the factors controlling microsite heterogeneities in soil.
authors.affiliation: Department of Crop and Soil Science, Oregon State University, Corvallis, Oregon 97331-7306, USA. John.Cliff@orst.edu
authors.forename: John B
authors.initials: JB
authors.lastname: Cliff
authors.forename: Daniel J
authors.initials: DJ
authors.lastname: Gaspar
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Bottomley
authors.forename: David D
authors.initials: DD
authors.lastname: Myrold
mesh_terms: D002244:Carbon / Q000378:metabolism; D002247:Carbon Isotopes / Q000378:metabolism; D003470:Culture Media; D005658:Fungi / Q000378:metabolism; D007287:Inorganic Chemicals / Q000378:metabolism; D009584:Nitrogen / Q000378:metabolism; D009587:Nitrogen Isotopes / Q000378:metabolism; D009606:Nitrosomonas / Q000378:metabolism*; D011551:Pseudomonas fluorescens / Q000378:metabolism*; D012988:Soil Microbiology; D018629:Spectrometry, Mass, Secondary Ion / Q000379:methods
chemical_list: D002247:Carbon Isotopes; D003470:Culture Media; D007287:Inorganic Chemicals; D009587:Nitrogen Isotopes; D002244:Carbon; D009584:Nitrogen