Light-induced changes in the ultrastructure of a plasmodial myxomycete

Light, required to induce sexual sporulation (fruiting) of the plasmodial myxomycete Physaruin polycephalum, causes the formation of electron-dense deposits in the intermembrane spaces of mitochondria and in cytoplasmic vacuoles. Such mitochondrial deposits are also present but to a much smaller extent in the dark. This deposition is nutritionally dependent on calcium. The deposits are rich in calcium as determined by EGTA treatment of sections and by X-ray spectroscopy. Light also induces the formation of these deposits in young plasmodia not yet competent to sporulate upon exposure to light. In addition, growing microplasmodia which are severely inhibited by light also form these characteristic deposits when illuminated. A basic part of the photometabolism appears to be an enhanced movement of calcium and possibly other ions into and within the plasmodium. The physiological response evoked by light, sporulation or growth inhibition, depends on the nutritional history and immediate environment of the plasmodium.