NEW PYRENOID FORMATION IN THE BROWN ALGA,SCYTOSIPHON LOMENTARIA (SCYTOSIPHONALES,PHAEOPHYCEAE)1

The Scytosiphon lomentaria (Lyngbye) Link cell characteristically has only one chloroplast with a prominent protruding pyrenoid. We observed the appearance of a new pyrenoid in each chloroplast during first mitosis in zygotes of S. lomentaria, using the freeze substitution technique. At first, a pyrenoid matrix appeared within the outermost stroma, in which thylakoid triplets and ribosomes were absent. At this time, the surface of this part remained smooth. The old pyrenoid was covered with a pyrenoid cap on the cytoplasmic side, whereas there was no pyrenoid cap on the new pyrenoid before protrusion. Irregularly shaped membranous sacs containing fine granular materials associated with the cytoplasmic side of the new pyrenoid. The sacs fused with each other and changed conformation and finally transformed into the pyrenoid cap. The new pyrenoid gradually protruded toward the cytoplasm, and the new pyrenoid cap became curved along the surface of pyrenoid. Cytokinesis occurred, and each chloroplast had two prominent protruding pyrenoids in two‐celled zygotes. We examined immunolocalization of β‐1,3‐glucans within the pyrenoid cap with a monoclonal antibody, using EM. Gold particles indicating localization of β‐1,3‐glucans were detected in vacuoles but never in the pyrenoid cap. This observation suggests that the pyrenoid cap in brown algae contains no photosynthetic products such as polysaccharide.