Kinetics of Denitrifying Growth by Fast-Growing Cowpea Rhizobia

Abstract

Two fast-growing strains of cowpea rhizobia (A26 and A28) were found to grow anaerobically at the expense of NO 3 − , NO 2 − , and N 2 O as terminal electron acceptors. The two major differences between aerobic and denitrifying growth were lower yield coefficients ( Y ) and higher saturation constants ( K s ) with nitrogenous oxides as electron acceptors. When grown aerobically, A26 and A28 adhered to Monod kinetics, respectively, as follows: K s , 3.4 and 3.8 μM; Y , 16.0 and 14.0 g · cells eq −1 ; μ max , 0.41 and 0.33 h −1 . Yield coefficients for denitrifying growth ranged from 40 to 70% of those for aerobic growth. Only A26 adhered to Monod kinetics with respect to growth on all three nitrogenous oxides. The apparent K s values were 41, 270, and 460 μM for nitrous oxide, nitrate, and nitrite, respectively; the K s for A28 grown on nitrate was 250 μM. The results are kinetically and thermodynamically consistent in explaining why O 2 is the preferred electron acceptor. Although no definitive conclusions could be drawn regarding preferential utilization of nitrogenous oxides, nitrite was inhibitory to both strains and effected slower growth. However, growth rates were identical (μ max , 0.41 h −1 ) when A26 was grown with either O 2 or NO 3 − as an electron acceptor and were only slightly reduced when A28 was grown with NO 3 − (0.25 h −1 ) as opposed to O 2 (0.33 h −1 ).