The Rumen - a fermentation vat or a series of organized structured microbial consortia: implications for the mitigation of enteric methane production by feed additives

摘要

Therumenhasbeenviewedasafermentationvatwithfreefloating(planktonic)microbes,particleassociatedmicrobesandmicrobesthatattachtotheepithelialcellsoftherumenwall.Therumenisdescribed,ingeneral,asa,anaerobicmicrobialecosystemcontainingamilieuofbacteria,archaea,protozoa,bacteriophageandphycomycetousfungithatdegradeorganicmatterwiththeultimateproductionofvolatilefattyacids(VFA),theenergythatbecomesavailableisusedformaintenanceandgrowthofcells.TheVFAareabsorbedfromtheupperpartofthedigestivetractandthemicrobialcellsprovideessentialnutrientssuchasaminoacids,whentheyaredigestedbyintestinalenzymesonwashoutfromtherumen.Sinceabout1980therehasbeenanemergingrealizationthatpossiblyashighas80-90%oftherumenmicrobialbiomassispresentasassociatedandinteractivecoloniesinbiofilmsonbothinertandinsolubleorganicmatterintherumenandthattheplanktoniccellsplaylittleroleinfeeddigestion.Biofilmsconsistofmicrobialcellsandawiderangeofself-generatedextra-cellularpolymericsubstances.nbsp;Biofilmformationisadynamicprocess,whichiscoordinatedbytheinteractionsofdifferentmicrobialspecies.Thesebiofilmsthatformwithinafewminutesoffeedbeingimmersedinrumenfluid,providespatiallyseparatedmicrobialcoloniesheldinanextracellularpolymericsubstance(EPS),includingpolysaccharides,nucleicacids,andproteinsthatisprogressivelyproducedbythemicrobesastheypositionthemselvesinthebiofilm.Thesequentialdepositionandgrowthofsessilemicrobeswithinthebiofilmmatrixallowsthenbsp;nutrientsproducedasendproductsofonespeciestobeefficientlytransferredtootherspecieswhichusetheseassubstrateforenergymetabolism.TheoverallconversionofplantorganicmattertoVFAinnbsp;thebiofilmresultsintheproductionofreducedcofactorswhichreverttotheiroxidizedformnbsp;byreleasingtheelectronashydrogen.Thisdependsonthemaintenanceofalowconcentrationofhydrogenandrequiresrapiduptakeofhydrogenbymicrobesthatcanusehighaffinityelectronacceptorssuchasnitrate,sulphateandcarbondioxide.MethanogenicArchaewhichobtainenergybyreducingcarbondioxidearemainlyresponsible,butsulphurreducingbacteriaalsoplayaminorroleintherumen.Therequirementsforminimalconcentrationsofhydrogenintheculturemediumdemandthatthesyntrophicmicrobesarecloselyassociated.nbsp;Syntrophismisaspecialcaseofsymbioticcooperationbetweentwometabolicallydifferenttypesofbacteriawhichdependoneachotherfordegradationofacertainsubstrate,typicallyforenergeticreasons.Biofilmformationonfeedparticulatematterisessentialforefficientfermentativedigestionintherumen,asitprovidestheecologicalnichethatensuresthatthesyntrophicorganismsinvolvedarespatiallyalignedtoensureefficientcaptureofhydrogentopreventfeedbackinhibitionofthereactionsinvolvedinconvertingpolymericfeedmaterialstotheirendproducts.Theoverallconclusionpertinenttothissymposiumisthatbecauseoftheneedforsymbioticassociationsinbiofilmmicrobialcoloniesonfeedparticlesintherumen,itisnotlogicalnbsp;toattempttolowermethaneproductionbyincludingchemicalsinadietthatspecificallytargettheinhibitionofmethanogens,unlessalternativeelectronsinksareprovided.Wherecompoundsthatcanbeusedbymicrobesaselectronsinksareincludedintheculturemedia,theorganismusingtheseassourcesofenergymustalsobespatiallydistributedinthebiofilmtomaintainhydrogenconcentrationsataminimumfortheefficientoxidationofreducedcofactorsgeneratedinthereactionsoccurringinfermentation.Nitratereducingandsulphurreducingbacteriaappeartofullfillthisrolehighlyefficiently.