California Sea Grant College Program

Bioluminescence (BL), light produced by organisms, is a diverse and widespread marine phenomenon. yet little studied by researchers. Major contributors to sea surface BL displays are dinoflagellates, which produce rapid BL flashes upon fluid motion; mechanical stimulation triggers a 200-ms flash within 20 ms, representing one of the most rapid sensor-effector transduction systems described. In some dinoflagellate species the sensor-effector link is not constant throughout a 24-hour period. Mechanical agitation during day phase produces no BL, even in cells kept in constant lighting conditions, indicating that BL is controlled by a circadian rhythm. The mechanism of this day-phase BL reduction is the subject of this thesis. Pyrocystis fusifonnis, has circadian­ controlled BL,. punctate intracellular BL sources (microsources), and is a sensitive fluid shear sensor. In this species, BL microsources and chloroplasts appear to migrate bidirectionally twice daily, and up to 500 µm.

An examination of the filamentous actin (F-actin) cytoskeleton was conducted using the stain, rhodamine phalloidin. F-actin radiated from the cell center in thick bundles, and formed a mesh in the cortical cytoplasm. Close associations were found between the F-actin and the chloroplasts. Cytochalasin-D (CD) was found to disrupt F-actin in a dose-dependent manner. Follow-up experiments indicated that the large-scale circadian movement of chloroplasts and BL microsources was dependent upon F-actin, in that disrupting the filaments led to inhibition of normally occurring movement during light cycle changes. Further, inhibition of BL microsource movement affected mechanically stimulable BL. indicating that the intracellular location of the BL microsources is important in determining the stimulability of BL. Because the cytoskeleton is also likely to underlie mechanosensation, the role of F-actin in BL mechanotransduction of flow sensation was investigated. Following treatments with CD, fluid shear­ stimulated BL was measured. A 20% reduction in the BL response was observed in cells treated with CD.

BL control, even in a dinoflagellate, is a remarkably complex process, seeming to involve the cytoskeleton in at least two distinct processes, (1) F-actin mediated organelle movements as a means of controlling the activity of those organelles, and (2) BL mechanotransduction involving F-actin.