PRELIMINARY FLUORESCENCE AND ULTRASTRUCTURAL OBSERVATIONS IN DICTYOSPHAERIUM PULCHELLUM (CHLOROCOCCALES, CHLOROPHYCEAE)

Dictyosphaerium pulchellum is a common freshwater phytoplankter. After cell division, autospores remain attached to each other in a characteristic fashion through the remnants of the partially hydrolyzed mother cell walls (tracts), originating up to 64‐celled colonies immersed in abundant mucilage. Staining with ruthenium red and toluidine blue revealed mucilage acid properties, probably due to uronic acids in polysaccharides. Tracts and cell walls were Periodic Acid‐Schiff positive and showed fluorescence with calcofluor white, indicating the presence of fibrillar polysaccharides of the type cellulose and beta‐mannans. Intense fluorescence with berberine suggested some hydrophobic coating of fibrillar polysaccharides. Though cell walls proved to be acetolysis resistant, no secondary fluorescence was observed with primuline, suggesting that the hydrophobic polymer in the cell wall differed from algaenan. Ultrastructural observations revealed no trilaminar structure in the cell wall, but ruthenium red stained sections exhibited a “zipper‐like” band in the outermost cell wall layer both in autospores and mother walls. The cells have a large cup‐shaped chloroplast containing a single pyrenoid with the matrix traversed by double thylakoids and circled by starch plates. A lobulated nucleus occupied the concavity of the chloroplast. Dictyosomes appeared closely related with the nuclear envelope. Several vacuoles scattered in the cytoplasm and close to the nucleus showed electron dense content, which could also be observed in the periplasmalemmal space. Fluorescent dye quinacrine also showed active vesicle traffic.