The morphology and life history of Characiosiphon rivularis Iyengar (Chlorophyta: Characiosiphonaceae): a light and electron-microscopic study

Abstract

Light and electron-microscopic studies of Characiosiphon rivularis Iyengar reveal its structure to be a sac-like coenocyte, with a large central vacuole and a thin peripheral layer of cytoplasm. Scattered through the cytoplasm are large, reticulately arranged, disc-shaped chloroplasts, each with a large pyrenoid surrounded by starch plates. Immediately internal to each plastid is a nucleus. Golgi apparatus, mitochondria, and ribosomes are dispersed through the cytoplasm in the area of the chloroplast, as are conspicuous contractile vacuoles. The thallus wall, which is fibrillar in nature and which, upon analysis, is shown to consist only partly of cellulose, increases in thickness as the thallus grows. At maturity the sacs undergo zoosporogenesis, which usually takes place in young cultures, or gametogenesis, which occurs mainly in older cultures. Discrete protoplasts, formed by invagination of the plasma membrane around each chloroplast and its associated organelles, cleave to form four to eight motile cells each. During this process a cytoplasmic layer between the motile cells and the central vacuole persists until broken by the activity of the motile cells. Unreleased zoospores function as aplanospores. The organism is heterothallic and isogamous, gametic union resulting in a thick-walled, quiescent zygote. Meiosis occurs upon germination of the zygote into zoospores or aplanospores. Observations of meiotic and mitotic figures indicate the chromosome number to be between 10 and 12. The morphology, life history, and pigment, composition of the alga, as revealed by this investigation, confirm its earlier assignment by (Iyengar 1936, 1954) to a new family, Characiosiphonaceae, of the Chlorococcales.