Kinetics ofDenitrifying GrowthbyFast-Growing CowpeaRhizobia

Twofast-growing strains ofcowpearhizobia (A26andA28) werefound togrowanaerobically attheexpense ofN03-,NO2-, andN20asterminal electron acceptors. Thetwomajordifferences between aerobic and denitrifying growth were loweryield coefficients (Y)andhigher saturation constants (K5) withnitrogenous oxides aselectron acceptors. Whengrownaerobically, A26andA28adhered toMonodkinetics, respectively, asfollows: K5,3.4and3.8,uM;Y,16.0 and14.0 g- cells eq-1; lLmax 0.41and0.33h-'.Yield coefficients for denitrifying growth ranged from40to70%ofthose foraerobic growth. OnlyA26adhered toMonodkinetics withrespect togrowth onallthree nitrogenous oxides. Theapparent K,values were41,270,and460,uMfor nitrous oxide, nitrate, andnitrite, respectively; theXKforA28grown onnitrate was250,uM.Theresults are kinetically andthermodynamically consistent inexplaining why02 isthepreferred electron acceptor. Although no definitive conclusions could bedrawnregarding preferential utilization ofnitrogenous oxides, nitrite was inhibitory tobothstrains andeffected slower growth. However, growth rates were identical (F.Lmx, 0.41h-1) whenA26was grownwitheither 02 orN03-asanelectron acceptor andwereonlyslightly reduced whenA28 was grown withN03-(0.25 h-1) asopposed to02 (0.33 h-1). Manymisleading concepts regarding thegrowth ofcowpea rhizobia andRhizobiumjaponicum havearisen asaresult of Norris' (19) judgement thattheserhizobia areinherently slowgrowers.Although thisconcepthasbeenrecently discredited (9), theapriori consideration ofcowpearhizobia as slowgrowers ledDaniel etal.(6)toconclude that denitrification infast-growing rhizobia israre,since most rhizobial strains that theyfound tobedenitrifiers were ofthe cowpea-R. japonicum group. Denitrification byRhizobium spp.was first noticed bythe disappearance ofnitrate andthesubsequent production of gasinDurhamtubes in1938(22). Others (16, 23)havesince observed that some strains ofR.japonicum grow anaerobically withnitrate asanelectron acceptor andproduce nitrite as an endproduct. Zablotowicz etal.(28) confirmed that some strains ofcowpea rhizobia andR.japonicum are denitrifiers, thatothers arenitrate respirers(those thatdo notreduce nitrite), andthatothers lacktheability togrow anaerobically withnitrate. Nealetal.(17,18)haveshown that R.japonicum can grow as a chemolithotrophic denitrifier withH2andNO3 astheelectron donorandacceptor,