FRACTIONATION OF NITROGEN ISOTOPES BY MIXED RUMINAL BACTERIA

Mixed ruminal bacteria were cultured with glucose, cellulose or no car bohydrate, and ammonium bicarbonate or casein hydrolysate. Changes in amounts of bacterial ammonia and non-ammonia N were measured. Ratios o f N isotopes expressed as delta N-15 (delta(15)N) were measured by iso tope ratio mass spectrometry. When bacteria were cultured with glucose and ammonium bicarbonate, bacterial delta N-15 decreased from .9 to - 5.8 parts per thousand and residual ammonia delta N-15 increased from -1.4 to 12.7 parts per thousand. Fractionation of N isotope occurred d uring ammonia incorporation because the difference between delta(15)N of ammonia and delta N-15 of bacteria (Delta(15)N) was 18.8 parts per thousand (P < .01). However, when casein hydrolysate was the N source, Delta(15)N between non-ammonia and bacteria averaged only 1.3 parts p er thousand (P > .1), indicating no fractionation of N isotopes occurr ed during utilization of amino acids. The amount of bacterial N was hi ghest at 24 h of incubation when cellulose was the carbohydrate source . At that time, Delta(15)N between ammonia and bacteria was 8.9 parts per thousand when ammonia was the N source, but Delta(15)N between non -ammonia and bacteria was 1.7 parts per thousand when casein hydrolysa te was the N source. Bacterial N decreased after 24 h when cellulose w as the source of carbohydrate. Results indicate that fractionation of N isotopes occurred during ammonia incorporation, but not during incor poration of N from amino acids, deamination, and release of ammonia. F ractionation of N isotopes during incorporation of ammonia N may be us ed as a marker to study N metabolism by ruminal bacteria.