Ultrastructural Aspects of the Cell Cycle in Planktosphaeria gelatinosa (Chlorophyceae), with Emphasis on the Flagellar Apparatus of the Zoospore

Abstract

Summary The cell cycle of the multinucleate coccoid green alga, Planktosphaeria gelatinosa was studied with transmission electron microscopy. Young, uninucleate cells have a more or less cup-shaped, parietal chloroplast containing a pyrenoid. The nucleus is surrounded by the chloroplast, and penetrates into the pyrenoid by a finger-like lobe that is branched at its end. In maturing multinucleate cells, the parietal chloroplasts are lens-shaped or conical, each containing a pyrenoid associated with a nucleus. In vegetative cells, bundles of microtubules are present between the nuclei, radiating from the centre of the cell towards the plasma membrane. The processes of mitosis and cytokinesis are similar to those known from other chlorophycean algae, including a closed, centric spindle, a perinuclear envelope and centrifugal cleavage mediated by a phycoplast that probably is formed by the microtubular arrays present in the vegetative cell. Mitosis and cytokinesis may show overlap in time, so that Planktosphaeria is not considered as a true coenocyte. The flagellar apparatus of the zoospore is characterized by two basal bodies that essentially are directly opposed, oriented at a variable angle and connected by a distal fibre and two striated proximal fibres. The central part of the distal fibre is differentiated into a ribbed structure. The flagellar apparatus has a cruciate microtubular root-system, with the left root consisting of 4 microtubules in a 3/1 configuration. The 2-stranded right roots are covered by a system I fibre or SMAC (striated microtubule-associated component), which is continuous between the two opposite roots and connected to the ribbed structure. At their proximal end the basal bodies are covered by a partial terminal cap. The ultrastructure of the flagellar apparatus indicates a close affinity to the unicellular genera Neochloris, Tetraedron and Chlorotetraedron , and also to the coenobial genera Pediastrum and Hydrodictyon .