Substrate amendments composed of crab shell (CS) waste materials have been shown to significantly improve the longevity and performance of acid mine drainage (AMD) treatment systems containing spent mushroom compost (SMC), yet the development of key microbial populations within these systems has not been investigated. To better understand the effects of CS on microbial dynamics in these systems, clone libraries and real-time quantitative PCR (qPCR) were performed on materials from a laboratory-scale AMD treatment system containing SMC and 0 to 100% CS substrate after receiving a continuous flow of AMD for 148 days (428 pore volumes). The proportion of CS in the substrate positively correlated with the diversity of sulfate-reducing bacteria (SRB) and archaeal clones, but negatively correlated with fungal diversity. CS also impacted microbial community structure, as revealed in Unifrac significance and principal coordinate analysis tests. The column containing 100% CS substrate supported 7 different genera of SRB-the most ever observed in an AMD treatment system. Moreover, the copy numbers of functional genes representing fermenters, sulfate reducers, and chitin degraders increased with increasing proportions of CS. These observations agree well with the chemical performance data, further validating that by supporting more abundant key microbial groups, chitinous substrates may provide benefits for improving both the longevity and performance of AMD treatment systems, and may provide similar benefits for the treatment of other environmental contaminants that are amenable to anaerobic bioremediation.