title: Fractionation of nitrogen isotopes by mixed ruminal bacteria.
abstract: Mixed ruminal bacteria were cultured with glucose, cellulose or no carbohydrate, and ammonium bicarbonate or casein hydrolysate. Changes in amounts of bacterial ammonia and non-ammonia N were measured. Ratios of N isotopes expressed as delta 15N (delta 15N) were measured by isotope ratio mass spectrometry. When bacteria were cultured with glucose and ammonium bicarbonate, bacterial delta 15N decreased from .9 to -5.8/1000 and residual ammonia delta 15N increased from -1.4 to 12.7/1000. Fractionation of N isotope occurred during ammonia incorporation because the difference between delta 15N of ammonia and delta 15N of bacteria (delta 15N) was 18.8/1000 (P < .01). However, when casein hydrolysate was the N source, delta 15N between non-ammonia and bacteria averaged only 1.3/1000 (P > .1), indicating no fractionation of N isotopes occurred during utilization of amino acids. The amount of bacterial N was highest at 24 h of incubation when cellulose was the carbohydrate source. At that time, delta 15N between ammonia and bacteria was 8.9/1000 when ammonia was the N source, but delta 15N between non-ammonia and bacteria was 1.7/1000 when casein hydrolysate was the N source. Bacterial N decreased after 24 h when cellulose was the source of carbohydrate. Results indicate that fractionation of N isotopes occurred during ammonia incorporation, but not during incorporation of N from amino acids, deamination, and release of ammonia. Fractionation of N isotopes during incorporation of ammonia N may be used as a marker to study N metabolism by ruminal bacteria.
authors.affiliation: Department of Dairy Sciences, University of Wisconsin-Madison 53706.
authors.forename: M A
authors.initials: MA
authors.lastname: Wattiaux
authors.forename: J D
authors.initials: JD
authors.lastname: Reed
mesh_terms: D000596:Amino Acids / Q000032:analysis / Q000378:metabolism; D000641:Ammonia / Q000032:analysis / Q000378:metabolism; D000818:Animals; D001421:Bacteria, Anaerobic / Q000254:growth & development / Q000378:metabolism*; D050260:Carbohydrate Metabolism; D002241:Carbohydrates / Q000032:analysis; D002417:Cattle / Q000382:microbiology*; D005260:Female; D005285:Fermentation; D005947:Glucose / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D009584:Nitrogen / Q000032:analysis / Q000378:metabolism*; D009587:Nitrogen Isotopes; D012417:Rumen / Q000378:metabolism / Q000382:microbiology*
chemical_list: D000596:Amino Acids; D002241:Carbohydrates; D009587:Nitrogen Isotopes; D000641:Ammonia; D005947:Glucose; D009584:Nitrogen