Biodiesel Glycerol Byproduct as a Carbon Source for Microorganisms

Graves, James, Isaac Miller, and Vincent Vuljaj

In making biodiesel from plant and animal oils, glycerol is a byproduct. 

Selected microbes of interest to bioenergy research which were likely to utilize pure glycerol were assessed for ability to utilize glycerol byproduct as a carbon source. In contrast to pure glycerol, byproduct from manufacture of biodiesel by transesterification was pale yellow and pasty. Samples of unsterilized byproduct which were inoculated on Brain Heart Infusion (BHI) agar and incubated at 25oC for 7 days failed to produce evidence of any resident microbial flora. Byproduct did not produce any bacterial growth inhibition zones when incubated at 37oC for 24 h on a lawn of Escherichia coli inoculated on BHI agar. Before pH adjustment for use in media a 20% stock solution of byproduct made from transesterification showed high alkaline pH, in contrast to that for pure glycerol which was slightly acidic.  In phenol red broth, which is a differential medium that assesses the ability of an organism to ferment an added carbon source, E. coli, and Klebsiella pneumonaie demonstrated a positive reaction in presence of byproduct, while Acinetobacter calcoaceticus, Alcaligenes faecalis, Pseudomonas species, Saccharomyces cerevisiae, and Streptomyces albus did not produce acid. On minimal agar, containing only the essential nutrients for wild type strains and byproduct as a potential carbon source, E. coli, K. pneumonaie, Pseudomonas species, and S. albus produced growth, but not A. calcoaceticus, A. faecalis, and S. cerevisiae. Inability of microorganisms which do not use pure glycerol to grow on byproduct suggested that it did not contain a variety of carbon sources to support growth. This work may help to identify microorganisms that can utilize glycerol byproduct as an industrial feedstock and develop a laboratory exercise dealing with biodiesel for undergraduate microbiology.