Growth of Rhodospirillum rubrum on synthesis gas: Conversion of CO to H2 and poly-eta-hydroxyalkanoate

Abstract To examine the potential use of synthesis gas as a carbon and energy source in fermentation processes, Rhodospirillum rubrum was cultured on synthesis gas generated from discarded seed corn. The growth rates, growth and poly-hydroxyalkanoates (PHA) yields, and CO oxidation/H2 evolution rates were evaluated in comparison to the rates observed with an artificial synthesis gas mixture. Depending on the gas conditioning system used, synthesis gas either stimulated or inhibited CO-oxidation rates compared to the observations with the artificial synthesis gas mixture. Inhibitory and stimulatory compounds in synthesis gas could be removed by the addition of activated charcoal, char-tar, or char-ash filters (char, tar, and ash are gasification residues). In batch fermentations, approximately 1.4 mol CO was oxidized per day per g cell protein with the production of 0.75 mol H2 and 340 mg PHA per day per g cell protein. The PHA produced from R. rubrum grown on synthesis gas was composed of 86% -hydroxybutyrate and 14% -hydroxyvalerate. Mass transfer of CO into the liquid phase was determined as the rate-limiting step in the fermentation. Biotechnol. Bioeng. 2007;97: 279-286.