Microbial Biogeochemistry of Isoprenoid Hydrocarbon Cycling Jenn Macalady, Department of Geosciences, The Pennsylvania State University University Park, PA 16802 Isoprenoid compounds are polymers of isoprene and include many lipid biomarkers. Methyl branching infers additional stability as compared to n-alkanes, on geologic time scales. Anaerobic microbial biodegradation of this compound class is limited to two studies of 2,6,10,14-tetramethylpentadecane (pristane). Here we aim to combine degradation experiments with full cycle rRNA analysis of the microbial community. We now have microcosms capable of degrading both pristane and archaeal diether lipids with concomitant loss of nitrate that is not observed in microcosms lacking a carbon substrate or containing an archaeal glycerol diether glycerol tetraethers (GDGT’s). 16s rDNA clone libraries from two stages of enrichment show a decrease in overall microbial diversity and suggest β- or γ-proteobacterial clones play a major role in the degradation of isoprenoids. Our next goal is to propose a stoichiometry for the degradation reaction and to identify metabolites associated with the degradation mechanism.