Hybrid thermochemical/biological processing of biomass for the production of polyhydroxyalkanoates and hydrogen gas from <i>Rhodospirillum rubrum</i> cultured on synthesis gas

Chipman, David
Major Professor
Robert C. Brown
Committee Member
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Mechanical Engineering
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Mechanical Engineering

The goal of this research is to optimize PHA production in the syngas fermenting organism Rhodospirillum rubrum. Syngas fermentation is the hybrid thermochemical/ biological approach to processing biomass into valuable fuels and chemicals. The process begins with the gasification of biomass to produce syngas, a flammable gas mixture consisting primarily of carbon monoxide (CO), hydrogen (H2), and carbon dioxide (CO2). Microorganisms are then used to ferment the syngas into biofuels and chemicals.

Rhodospirillum rubrum, a non-sulfur purple bacterium, utilizes the CO in syngas to produce H2 and polyhydroxyalkanoates (PHAs), biobased-biodegradable polymers. Information is limited pertaining to the optimization of growth media as well as the optimal harvest point for R. rubrum in terms of H2 and PHA production.

RRNCO growth media was optimized on the small scale (40 ml). The resulting media recipe was scaled up to 10L for the evaluation of the optimal harvest point. A syngas fermentation facility and control system was designed and developed for fermentation scale (10L) experiments. The results of the small scale experiments did not agree with literature in terms of the effect of the carbon to nitrogen balance on cellular PHA content. Yeast extract may have been acting as an additional source of available carbon and nitrogen in the media.

The results of the harvest point optimization indicated that cell density and media acetate levels may serve as good indicators of maximum PHA production. PHA production appeared to maximize when cell growth reached late exponential phase or early stationary phase. Maximum PHA production corresponded well with the point of media acetate exhaustion.