Effects of glucose overloading on microbial community structure and biogas production in a laboratory-scale anaerobic digester IngvarSundh a, * ,HelenaCarlsson a ,  AkeNordberg b ,MikaelHansson b ,BeritMathisen b a Department of Microbiology, Swedish University of Agricultural Sciences, P.O. Box 7025, SE-750 07 Uppsala, Sweden b Swedish Institute of Agricultural and Environmental Engineering, P.O. Box 7033, SE-750 07 Uppsala, Sweden Received 8 October 2002; received in revised form 10 February 2003; accepted 25 February 2003 Abstract This study characterizes the response of the microbial communities of a laboratory-scale mesophilic biogas process, fed with a syntheticsubstratebasedoncelluloseandeggalbumin,tosinglepulsesofglucoseoverloading(15or25timesthedailyfeedbasedon VS). The microbial biomass and community structure were determined from analyses of membrane phospholipids. The ratio be- tween phospholipid fatty acids (PLFAs; eubacteria and eucaryotes) and di-ethers (PLEL; archaea) suggested that methanogens constituted 4–8% of the microbial biomass. The glucose addition resulted in transient increases in the total biomass of eubacteria while there were only small changes in community structure. The total gas production rate increased, while the relative methane contentofthebiogasandthealkalinitydecreased.However,thebiomassofmethanogenswasnotaffectedbytheglucoseaddition. Theresultsshowthatthemicrobialcommunitiesofbiogasprocessescanrespondquicklytochangesinthefeedingrate.Theglucose overloadresultedinatransientgeneralstimulationofdegradationratesandalmostadoublingofeubacterialbiomass,althoughthe biomass increase corresponded to only 7% of the glucose C added. Ó 2003 Elsevier Science Ltd. All rights reserved. Keywords: Biogas; Microbial community structure; PLFA; Substrate overload; Methanogenesis; Di-ether lipid 1. Introduction In biogas production from organic materials, the in- tegrated action of several types of microorganisms, whichperformdifferentdegradationsteps,resultsinthe sequential degradation of polymeric carbohydrates, proteins and fats (Gujer and Zehnder, 1983; Schink, 1988). Functionally, the organisms can be broadly classified into hydrolytic, fermentative organic acid- producing, acetate-producing and, in the terminal step, methane-producing, organisms. Due to low energy yields, many anaerobic microor- ganisms grow slowly, the methanogens in particular. Therefore, in completely mixed systems with relatively short retention times, disturbance due to, for example, poor composition or overloading of the substrate, ac- cumulation of inhibitory substances, or presence of an- thropogenic contaminants, may inhibit the active organisms and result in serious malfunctioning of the process. However, knowledge of the response of the microbial communities to different kinds of disturbance is still meagre, particularly regarding the communitiesÕ structural characteristics. In the present study, we investigated the effects of substrateoverload,withasingledoseofglucose,onthe size and structure of the microbial community and on the biogas production in a laboratory scale biogas pro- cess fed with a synthetic substrate. The microbial bio- mass and community structure were determined from analysis of membrane phospholipids. In addition, the fate of the added glucose was determined by construc- tionofaCbudget. 2. Methods 2.1. Reactor system The reactor was a continuously mixed glass tank of 8 l active volume, operated under mesophilic condi- tions (37 °C).Itwasstartedwithdigestercontentsfrom a larger reactor which had operated with the same * Corresponding author. Tel.: +46-18-673210; fax: +46-18-673392. E-mail address: ingvar.sundh@mikrob.slu.se (I. Sundh). 0960-8524/03/$ - see front matter Ó 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0960-8524(03)00075-0 Bioresource Technology 89 (2003) 237–243