Ultrastructural changes in Rhizopus stolonifer sporangiospores in response to anaerobiosis.

Rhizopus stolonifer sporangiospores developed prominent ultrastructural changes under anoxia. Mitochondria were scattered randomly throughout the cytoplasm in spores incubated aerobically for 3 hr, but moved to the periphery of the cytoplasm when the spores were exposed to anoxia. Endoplasmic reticulum, inconspicuous in control spores, appeared near the plasmalemma and was frequently found connected to a nuclear membrane. The cytoplasm, otherwise usually uniformly dense and containing abundant ribosomes, appeared mottled under anoxia. The absence of ribosomes could account for the mottled appearance. Lipid inclusions were distinctively separated from the electron-transparent areas of the mottled cytoplasm. The increasing thickness of the inner wall indicated that the metabolic activity of spores continued under anoxia. When aerobic conditions succeeded anoxia, germ-tube protrusion occurred.