Motility in the siphonous green alga Bryopsis. I. Spatial organization of the cytoskeleton and organelle movements.

In the giant marine green alga Bryopsis the chloroplasts are attached with their flattened, ventral sides to the inner surface of the cortical cytoplasm. They move at speeds up to 60 microns/min in the direction of the long axis of the cell either in a coordinated fashion or independently of each other. Intracellular sedimentation of chloroplasts by centrifugation leaves an intact cell cortex in which the movement of mitochondria and nuclei--normally obscured by chloroplasts--can be observed. Mitochondria display a saltatory type of movement alongside an extensive, two-dimensional system of phase-translucent channels. Nuclei appear to be entangled in the channel system and move in an unusual, rolling fashion. With a new technique involving the microsurgical removal of the chemically fixed cytoplasm from the confinement of the cell wall, this unique cell type is made accessible to immunocytochemical procedures. Microtubules (MT) can be visualized using a variety of tubulin antibodies, while actin only reacts with one monoclonal antibody out of several antibodies tested. Microtubules form a dense, two-dimensional palisade of bundles extending longitudinally in the cortical cytoplasm. Parallel arrays of actin fibers closely, but not exclusively, colocalize with the MT bundles. Particularly strong actin staining is observed near converging MT bundles underneath the tip regions of the chloroplasts. Because of the extensive superposition of actin and MTs, both cytoskeletal elements could potentially cooperate in creating the diversity of organelle movements in this alga. The respective roles of MTs and actin in chloroplast movement are experimentally tested in the accompanying paper.