The cell suspension used for this photomicrograph is out of a bioreactor culture of the filamentous fungus Mucor circinelloides during bioconversion of a fatty acid ethyl ester to the corresponding hydroxy acid. This intermediate is used for the production of biotechnologically derived lactones which are widely used as natural flavoring compounds.
The ethyl ester is intracellularly uptaken by M. circinelloides for hydrolysis and regioselective hydroxylation (visible as large intracellular vacuoles). This rather unphysiologic, solvent-like bioconversion precursor highly affects both viability and morphology of the biomass. A submerged culture of M. circinelloides grown under unlimited and noninhibited conditions consists mainly of branched mycelium. If it gets exposed to the fatty acid ester, the mycelium segmentation becomes more and more narrow and yeast-like cells are formed (see photomicrograph). This development ends up with the formation of apical sporangiophores, sporulation (release of sporangiospores), and almost complete loss of biocatalytic activity.
This photo was previously published on the cover of ASM News, October 1997, 63:10.