Growth and fermentation responses of Selenomonas ruminantium to limiting and non-limiting concentrations of ammonium chloride

Abstract

 The objective of this study was to assess fermentation product, growth rate and growth yield responses of Selenomonas ruminantium HD₄ to limiting and non-limiting ammonia concentrations. The ammonia half-inhibition constant for S. ruminantium in batch culture was 296 mM. Cells were grown in continuous culture with a defined ascorbate-reduced basal medium containing either 0.5, 5, 25, 50, 100 or 200 mM NH₄Cl and dilution rates were 0.07, 0.14, 0.24 or 0.40 h^-1. Ammonia was the growth-limiting nutrient when 0.5 mM NH₄Cl was provided and the half-saturation constant was 72 μM. Specific rates of glucose utilization and fermentation acid carbon formation were highest for 0.5 mM NH₄Cl. Lactate production (moles per mole of glucose disappearing) increased at the fastest dilution rate (0.40 h^-1) for 5.0 mM NH₄Cl while acetate and propionate decreased when compared to slower dilutions (0.07 and 0.14 h^-1). Lactate production remained low while acetate and propionate remained high for all dilution rates when NH₄Cl concentrations were 25 mM or greater. Yield (Y _Glc and Y _ATP) were nearly doubled when NH₄Cl was increased from 0.5 mM (25.1 g cells/mol glucose used and 13.9 g cells/mol ATP produced respectively) to the higher concentrations. Y _Glc was highest at 25 mM and 50 mM NH₄Cl (48.2 cells/mol and 43.1 cells/mol respectively) as was Y _ATP (23.2 cells/mol and 20.8 cells/mol respectively). Y _NH3 was highest at the lowest NH₄Cl concentration. The maximal fermentation product formation rate occurred at a growth-limiting ammonia concentration, while maximal glucose and ATP bacterial yields occurred at non-growth-limiting ammonia concentrations. Given the growth response of this ruminal bacterium, it is possible that maximization of ruminal bacterial yield may necessitate sacrificing the substrate degradation rate and vice versa.