A variety of cultural conditions were examined to determine the relationship between respiratory capacity and the growth of Mucor racemosus in the yeast and mycelial form. The results show that both yeasts and hyphae can develop when the respiratory capacity is low (e.g., in N2). In addition, the yeast form of the fungus could be grown in air in the presence of cyclic adenosine 5'-monophosphate with high respiratory rates characteristic of air-grown mycelia. These results indicate that their is not an obligatory relationship between respiratory capacity and morphogenesis in M. racemosus. Low intracellular levels of cyclic adenosine 5'-monophosphate, however, were correlated with aerobic mycelial development, whereas yeast development under CO2 was characterized by higher cyclic adenosine 5'-monophosphate levels.

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Asexual sporangiospores of Mucor racemosus produced on a minimal sporulation medium (M spores) germinated only if glucose, mannose or a complex substrate such as peptone, yeast extract or Casamino acids was present. Once germinated, growth was supported by a wide range of substrates including amino acids, carbohydrates or organic acids. Sporangiospores produced on a nutritionally complex sporulation medium (C-spores) germinated on a wide range of carbon sources. C-spore phenotype was pleiotropic in that sporangiospores capable of germinating on cellobiose could always germinate on glutamate or xylose; but C-spores capable of germinating on xylose or glutamate did not always germinate on cellobiose. There was a hierarchy of substrates capable of initiating germination with glucose = mannose greater than xylose greater than glutamate greater than cellobiose. C-spores also differed from M-spores by initiating germination in the presence of the non-metabolizable glucose analogue 3-O-methylglucose. These results suggest that at least two sporangiospore phenotypes are produced depending upon the concentration and type of ingredients present in the sporulation medium.