Rumen bacteria and 15N natural abundance

Nitrogen use is an important index of efficiency in ruminants and can be indirectly evaluated through the N isotopic discrimination between the animal and its diet (?15Nanimal-diet). Evidence also exists that the concentration and source of N may determine the extent of the N isotopic discrimination in bacteria. We hypothesised that the uptake and release of ammonia by rumen bacteria will have an impact on the natural 15N enrichment of the bacterial biomass over their substrates (?15Nbacteriasubstrate) and thereby further impacting ?15Nanimal-diet. To test this hypothesis, two independent in vitro experiments were conducted using two contrasting N sources (organic vs inorganic) at different levels either in pure rumen bacteria culture incubations (Experiment #1) or in mixed rumen cultures (Experiment #2). In experiment #1, ?15Nbacteria-substrate was lowest when the ammonia-consumer bacterium F. succinogenes was grown on ammonium chloride, and highest when the proteolytic bacterial strain X. ruminicola was grown on tryptone. In experiment #2, ?15Nbacteriasubstrate was lower with inorganic (ammonium chloride) vs organic (peptone) N source. A strong negative correlation between ?15Nbacteria-substrate and Rikenellaceae_RC9_gut_group, a potential fibrolytic rumen bacterium, was detected. Together, our results showed that ?15Nbacteria-substrate may change according to the balance between synthesis of microbial protein from ammonia versus nonammonia N sources and confirm the key role of rumen bacteria as modulators of ?15Nanimal-diet.