LIGHT AND ELECTRON MICROSCOPIC CHARACTERIZATION OF TWO TYPES OF PYRENOIDS INGONIUM(GONIACEAE, CHLOROPHYTA)1

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Abstract
The single, basal pyrenoids ofGonium quadratumPringsheim ex Nozaki andG. pectoraleMüller (Goniaceae, Chlorophyta) differed in appearance when vegetative colonies were cultured photoheterotrophically in medium containing sodium acetate. Chloroplasts ofG. quadratumhad distinct pyrenoids when grown in medium without major carbon compounds. However, the pyrenoids degenerated and were markedly reduced in size when such cells were inoculated into a medium containing 400 mg·L−1of sodium acetate. No pyrenoids were visible under the light microscope; however, with electron microscopy small pyrenoids and electron‐dense bodies were visible within the degenerating chloroplasts, which had only single layers of thylakoid lamellae at the periphery. The chloroplasts subsequently developed distinct pyrenoids and several layers of thylakoid lamellae as the culture aged. In contrast, vegetative cells ofG. pectoralealways showed distinct pyrenoids when cells were inoculated into medium containing sodium acetate, sodium pyruvic acid, sodium lactate, and/or yeast extract. Therefore, we propose two terms, “unstable pyrenoids” and “stable pyrenoids,” for pyrenoids ofG. quadratumandG. pectorale, respectively. Chloroplasts of the colonial green flagellates should thus be examined under various culture conditions in order to determine whether their pyrenoids are unstable or stable when pyrenoids are used as taxonomic indicators. Immunogold electron microscopy showed that the ratios of gold particle density of ribulose‐1,5‐biphosphate carboxylase/oxygenase (RuBisCO) between pyrenoid matrix and chloroplast stroma inG. quadratumgrown in medium with or without sodium acetate were lower than those ofG. pectorale.Heavy labeling by anti‐RuBisCO was observed in both the electron‐dense bodies and pyrenoid matrix ofG. quadratum.This is the first electron microscopic demonstration of degeneration and development of both pyrenoids and thylakoid lamellae in the chloroplast as a function of culture condition in green algae.