472224 Switchgrass Solubilization By Mixed Methanogenic Enrichments with Comparison to Pure Cultures of Clostridium Thermocellum
Inoculum was collected from a two-stage anaerobic digester (Vermont Technical College) fed with manure and food waste. Triplicate semi-continuous, anaerobic digesters were operated at 55oC and 30g/L mid-season switchgrass concentration for 214 days, during which two of them were allowed to come to steady-state sequentially at residence times (RT) of 20 days, 10 days, 5 days and 3.3 days, with the other one running as control at RT = 20 days. This feed rate was maintained by regular replacement of 1/10 of reactor contents to achieve the nominal residence times. Whereas we expected a shift from methanogenesis to acidogenesis as RT decreased to 5 days, surprisingly, even at RT=3.3 days stable methane production was still seen and acids inhibition was not observed, which indicates that the digesters were still healthy and balanced. Total carbohydrate solubilization at RT = 20, 10, 5 and 3.3 days was 69.1%, 62.8%, 54.9% and 50.0% respectively. In addition to these continuous trials, batch studies compared the performance of mixed enrichment cultures to pure cultures of Clostridium thermocellum, where solubilization was similar despite the more limited enzyme diversity of the pure culture species.
16S rRNA analysis was used to characterize microbial populations dynamics. It was found that Firmicutes dominated each of the reactors, representing between 54-96% of the sequence reads, and these populations increased with reduced residence time. In addition, Synergistetes, Euryarchaeota etc. taxa were also well represented and relatively stable. By using bioinformatics tools, species present in the consortia were also matched to closely related cultured species including Clostridium clariflavum that have cellulolytic activities. Plate culture isolation further revealed that Clostridium clariflavum was one of the major cellulolytic species existing in the mixed culture consortia. Data at feed concentrations ≥ 75 g/L, approaching industrial conditions, will also be presented.
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