Composition and ultrastructure of the sporangiospore wall of Rhizopus stolonifer

Abstract

The chemical composition and ultrastructure of the cell wall of Rhizopus stolonifer sporangiospores were determined. Spores were examined by transmission electron microscopy using both thin sections and surface replicas, and by scanning electron microscopy. The spore wall was found to be composed of three layers: (i) a ridged electron-opaque outer layer (10–240 nm thick) occasionally covered by a very thin extra layer; (ii) an electron-transparent layer containing electron-dense areas (160–245 nm thick); and (iii) an innermost layer of relatively high electron density overlaying the plasma membrane (15–40 nm thick). The spore wall had a rough amorphous surface without rodlet fascicles. Chemical analysis showed that the major components were protein, glucan, chitosan, and melanin, followed by smaller amounts of uronic acids, lipids, chitin, and mannose. The protein(s) contained high levels of aspartic acid and glutamic acid, followed by glycine, alanine, lysine, histidine, serine, and by smaller amounts of other amino acids. Melanin was intimately associated with protein and glucosamine.