Mixed-culture biotechnologies producing storage compounds as biofuels and biomaterials: engineering waste treatment communities

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Improving the sustainability of wastewater treatment systems passes in part by the recovery of value added products and energy such as bioplastics (e.g., polyhydroxyalkanoates, PHA) and biodiesel (from lipids). Microbial communities in activated sludge treatment systems naturally produce storage compounds because of the rapidly changing conditions of their environment. Since carbon storage compounds include PHA, triacyglycerol lipids, and glycogen, wastewater treatment systems are potential powerhouses for the production of industrially relevant biomaterials. In the case of PHA, the current pure culture production costs are 4-9 times higher than the cost of chemically synthesized plastics. It is clear that production from a waste stream where substrate and sterilization would not be necessary has the potential to reduce tremendously this cost. A fundamental understanding of microbial selection leading to the accumulation of given storage compounds is missing to sustain a rational development approach to these processes. Why glycogen or PHA are accumulated when glucose or acetate are fed to the mixed activated sludge culture as single carbon sources? How can we select for organisms with capacities to accumulate different kinds of PHA (short-chains vs long-chains). A (meta)genomics approach to these problems is proposed by first using the genome-scale metabolic model of Rhodococcus jostii RHA1 that we constructed and determining the objective functions that are most relevant for growth in activated sludge situations. Second, these finding will be expanded by asking similar questions to a selection of genome-scale models of heterotrophs to predict the outcome of microbial competitions in activated sludge. Finally, gene and protein expression profiles will be determined in populations with different storage capacities in full-scale activated sludge systems to link the ecological function of specific organism to their accumulation of storage compounds. These data will provide directions for the rational design of activated sludge treatments optimized for the production of specific storage compounds.