Influence of Inorganic Ions on Color of Lime in the Myxomycetes

Abstract

Energy dispersive X-ray spectroscopy combined with scanning electron microscopy was employed to examine several species of Myxomycetes to determine whether the presence of specific inorganic ions correlated with particular colors in peridial lime deposits, stalk lime deposits, inherent peridial color, or other colored structures in sporangia. Species examined were: Physarum bilgramii, P. globuliferum, P. galbeum, P. melleum, P. polycephalum, P. pulcherripes, P. roseum, P. tenerum, Craterium leucocephalum, C. paraguayense, Didymium ovoideum, Arcyria cinerea, A. denudata, Comatricha typhoides, Cribraria violacea, Dictydium cancellatum, Hemitrichia stipitata, and Metatrichia vesparium. In Physarum and Craterium, the presence of calcium, manganese, barium, and zinc ions appears to correlate well with colored lime deposits on the peridium and stalk. No such correlation exists in the other species examined. The color of various parts of the sporangium has long been an important key character employed in taxonomy of Myxomycetes (Martin and Alexopoulos, 1969). Color in particular collections varies somewhat with age, and is also variable among fresh collections from different localities. About 15 years ago, Physarum bilgramii, an unusual bright blue species, was collected in Austin, Texas, by Mary Henney and me. Dr. Henney cultured the isolate in the laboratory and obtained a white plasmodium and white sporangia typical of P. globuliferum, a species morphologically similar to P. bilgramii except that it lacks the blue color. She concluded that this and other species of Physarum might owe their bright coloration to inorganic elements present in the fruiting environment but made no further attempts to test this hypothesis (Henney, 1968). More recently, several workers have utilized energy dispersive X-ray spectrometers on scanning electron microscopes to determine elemental composition of myxomycete sporangial structures. Nelson et al. (1977) showed that high levels of calcium were present in Metatrichia vesparium sporangia, a species long considered limeless. They also pointed out that considerable variation existed between collections with regard to other trace elements present and concluded that the calcium might not necessarily be present as the carbonate. Schoknecht (1975) found calcium present in dictydine granules of Cribraria, another genus not usually expected to have calcium deposits. Schoknecht and Keller (1977) located calcium in members of the Trichiales. Gustafson and Thurston (1974) found that in Didymium squamulosum the mitochondria accumulate calcium phosphate prior to sporulation, and it disappears as lime is deposited on the sporangial surface. The principle of energy dispersive X-ray analysis is that X-rays characteristic of particular elements are produced when an electron beam interacts with a specimen. A detector collects the X-rays and displays them according to energy value on a mini-computer-controlled display. The different spectral peaks can then be identified as coming from particular elements in the specimen. Many scanning microscopes can be manipulated to map the locations of particular elements by