Two flash forms in the bioluminescent dinoflagellate,Pyrocystis fusiformis

Summary

• 1. A method was developed for studying bioluminescent activity in single cells of the dinoflagellate,Pyrocystis fusiformis. Individuals were isolated in holding tubes in day phase and held without stimulation until bioluminescence was maximally excitable, between circadian time (CT) 14 and CT 22, where CT 0 designates daybreak. Mechanical stimulation, via a pulse generator controlled solenoid, was applied to individual cells that had received no prior excitation in the night phase tested.

• 2. Two different flash forms were recorded. The first flash (FF) in response to a mechanical stimulus was very bright and had a rise time of 10 ms, and a biphasic decay that was 90% complete approximately 200 ms from flash onset. The form of subsequent flashes in response to further stimuli differed radically from the FF. They were dimmer and longer lasting than the FF with approximately 150 ms rise times and a monotonic decay that was 90% complete as long as 500 ms from flash onset.

• 3. Cells responded with one flash per mechanical stimulus and recordings were made until the response was exhausted. Total mechanically stimulated luminescence (TMSL) was measured with a digital integrator. TMSL and the integral of the FF were functions of cell size.

• 4. The degree of potentiation and number of flashes per cell were functions of stimulus frequency. At higher stimulus frequencies cells produced fewer flashes and more light per flash. The effects of potentiation were long lasting, persisting for stimulus intervals of up to one minute.

• 5. At slow stimulus frequencies (one pulse per 48 s) bioluminescent activity was not totally exhausted during the 8 h night phase test period. With prolonged stimulation a fatigued flash form developed that combined elements of the FF and subsequent flashes.

• 6. Cells that were stimulated to exhaustion recovered some bioluminescent capacity once stimulation ceased. Initial recovery was rapid and cells stimulated after only a 15 min recovery period produced as many flashes in the second stimulus series as in the first even though their TMSL was reduced 82%. Therefore, the number of flashes a cell produced was not simply proportional to the amount of bioluminescent material available.

• 7. The unique FF kinetics recovered with time, requiring 30–60 min in unfatigued cells and more than 6 h in fatigued cells. With a 24 h recovery period FF kinetics were more dependent on the cell receiving a normal 12 h day phase than was TMSL recovery.

• 8. Mechanically triggered bioluminescence inPyrocystis fusiformis appeared to be the result of at least two temporally distinct processes, one of which was dependent on a precharging period.