Microbial contamination of bioreactors is a major obstacle to the scale-up of microalgae-based biofuel production. Whether the reactors are open or closed to the environment, sterilization of large systems will likely be cost- and energy-prohibitive. Therefore, we studied whether bioreactor conditions could be engineered to maintain the predominance of oleaginous microalgal concentrations in mixed-culture. Scenedesmus dimorphus, an oleaginous and CO2-tolerant microalga, was combined with a microbial community collected from an algal biofilm growing in a trickling filter of a wastewater treatment plant. This mixed culture was grown in sequencing batch photobioreactors for multiple cycles, where we tested the impact of CO2 concentration (ambient air or 20% v/v) on S. dimorphous growth. DNA of the microbial community was collected at the end of growth cycles for quantitative PCR and 16S/18S ribosomal DNA pyrosequencing analysis to monitor the S. dimorphus concentration and microbial community structure over time. In the first four growth cycles, the mixed-culture reactors were very similar in terms of growth and microbial community structure, regardless of CO2 concentration. By growth cycle 5, however, the ambient-air reactors had larger decreases in cell density and growth rate, and increases in species richness and non-algal microorganisms. By the end of growth cycle 7, the concentration of S. dimorphus cells was 4.4 times higher, on average, in the CO2 reactors than in the ambient-air reactors. These results suggest that elevated CO2 concentrations may be used with S. dimorphus to help control microbial contamination of growth reactors. Finally, this study prompts the investigation of the ability of other reactor conditions, such as pH and salinity, to help limit microbial contamination in microalgae biofuel reactors.

Using carbon dioxide to maintain the abundance of the oleaginous microalgae Scenedesmus dimorphus in mixed-culture growth reactors

249th National Meeting (2015)

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