Further considerations of the potential of nitrate as a high affinity electron acceptor to lower enteric methane production in ruminants

摘要

Rumenfermentationresultsintheproductionofapoolofreducedcofactorsinthepathwaysofcatabolismoforganicmatterwithinrumenmicrobes.Thesereducedcofactorsareregeneratedby1)synthesisofmicrobialcells,2)productionofmorereducedendproductssuchaspropionateand3)saturationofunsaturatedlongchainfattyacidsbutbyfarthelargestproportionofreducedcofactorappeartoberegeneratedby4)formationofhydrogen.Theconcentrationofhydrogeninrumenfluidnegativelyfeedsbackontherateoffermentationandmicrobialgrowth.TherumenecosystemhasevolvedtoremovethishydrogenthroughgrowthofArchaethatobtainenergyfortheirgrowthbyreducingcarbondioxidetomethaneandwaterintherumen.nbsp;Ruminalmethanogenesisrepresentsalossofdietaryenergytotheanimalanditisasignificantgreenhousegas.Ruminantsarecreditedwithalargeproportionofthemethaneaccumulatingintheworld#8217;satmosphereandahighproportionoftheradiativeheatforcingofgreenhousegasesthathaveaccumulatedintheatmosphere.Thesefactorshaveledtoaglobalsearchforstrategies(includingnutritionalintervention)tomitigatemethaneemissionfromruminants.nbsp;Theuseofnitrateasahydrogensinkhasbeendownplayed,duetothepossibletoxiceffectsofnitritethatundersomecircumstancesisformedasanintermediateduringthereductionofnitratetoammoniaintherumen.Afewreportshaveexaminedthepotentialofnitrateinvitroasamethanereducingfeedadditive,whichappearstolowermethanogenesisconsistently.nbsp;TherumenecosystemandtheanimalneedtimetoadapttoanyfermentableNsourceincludingureaandnitrate.TherequirementsformineralsindifferentdietaryconditionsappeartobeimportantindeterminingthemicrobialconsortiumsthatusethesefermentableNsources.nbsp;Thepotentialofnitrateconversionammoniatoactasahydrogensinkintherumenwasreviewed(Leng2008)withaclearindicationemergingthatitisentirelyfeasiblenbsp;thatnitratecouldbeusedasafermentableNsourcebyruminantsprovidedtherumenecosystemwasallowedtoadaptoverasufficientlylongperiodandprovidedcertainnutritionalconditionsweremet.Inparticulartheavailabilityofsulphurnbsp;appearsnbsp;tobeacrucialissue.nbsp;Informationderivedfromothermicrobial,anaerobicecosystemsshowedthatinthepresenceoffermentableorganicmatterandasourceofsulphurandnitrate,nitratereducingorganismsdevelopedthatcanbothreducenitratetoammoniaandoxidizesulphidetosulphate(NRSOB)withoutreleaseofnitrite.Atleastoneprominentrumenorganismhasthiscapacity(Wollinellasuccinogenese)andmaybetermedanNRSOB.nbsp;StudiesfromtheMEKARNgrouphavedemonstratedthatruminantsfedlowproteinagroindustrialbyproductscanutilizenitrateasafermentablenitrogensource.Thesegroupshavebeenthefirsttoshowthatnitratecanbefedtoruminantssafelyunderpracticalconditionsandmaintainorincreaseproduction.Twoothergroups,withthefacilitiestomeasuremethaneproduction,haveconcentratedontheextenttowhichreplacingureawithnitratesaltslowersmethaneproduction.Dependingondietandinclusionrateofnitrate,thereductioninmethaneproductionhasvariedfrom16-50%.nbsp;nbsp;Approximately1moleofnitrateinadietreducesmethaneproductionby10%andthereisevidenceofaninteractionwithdietarysulphurlevelsnbsp;nbsp;ThemajorlimitationstoprogressinusingthesealternativefermentableNsourcesthatalsoactashighaffinityelectronacceptorhasbeentheproductionofmethaemoglobinaemiathatresultsfromnitritegeneratedintherumen.nbsp;Thespecializednatureandexpensiveequipmentformeasuringrespiredgasesfromruminantshasalsobecomeamajorlimitationtoprogress.Considerableadvancementappearspossiblebyanewapproachwheretheratioofmethanetocarbondioxideinairreceivingtheruminantsbreathareusedtocalculatethepercentageloweringofmethanereleasefromtheanimal.Thisapproachispossibleasitiswelldemonstratedthatwheretheonlyinterventionistoreplaceureawithnitrateinadiettheanimal#8217;senergymetabolism(carbondioxideproduction)nbsp;isaconstantinbothcontrol(ureafed)andtreatment(nitratefed).Inthepresentstudiesamodificationofpublishedmethodshavebeenusedtoassessthepotentialreductionsinmethanethatcanbeachievedwhenprovidingalternativeelectronacceptorvianitrateinthedietandmanipulationofothermineralstopromoteinparticularnitratereducingsulphideoxidationsystemsnbsp;Thesimplisticandinexpensiverequirementsmakethisausefulapproachwithpotentialtoincreasetherateofprogressatlowcost.nbsp;nbsp;