Stress-Induced Dinoflagellate Bioluminescence at the Single Cell Level

One of the characteristic features of many marine dinoflagellates is their bioluminescence, which lights up nighttime breaking waves or seawater sliced by a ship’s prow. While the internal biochemistry of light production by these microorganisms is well established, the manner by which fluid shear or mechanical forces trigger bioluminescence is still poorly understood. We report controlled measurements of the relation between mechanical stress and light production at the single cell level, using high-speed imaging of micropipette-held cells of the marine dinoflagellate Pyrocystis lunula subjected to localized fluid flows or direct indentation. We find a viscoelastic response in which light intensity depends on both the amplitude and rate of deformation, consistent with the action of stretch-activated ion channels. A phenomenological model captures the experimental observations.

Figure 1

The unicellular marine dinoflagellate Pyrocystis lunula, held on a glass micropipette. Chloroplasts (yellow-brown) are in the cytoplasmic core at night and the crescent-shaped cell wall encloses the cell.

Figure 2

Light production by P. Lunula under fluid and mechanical stimulation. (a) Stimulation by fluid flow; color map in upper half indicates flow speed, lower half is a streak image of tracers. (b) Particle tracking of flow lines near cell surface. (c)–(f) Cell deformation due to flow and consequent light production. (g),(h) Second protocol, in which a cell is deformed under direct contact by a second pipette (${\delta }_f=10$, $\dot{\delta }=76\mu \mathrm{m}/\mathrm{s}$). (i)–(l) Light production triggered by mechanical deformation. Indicated times are with respect to start of light emission.

Figure 3

Dynamics of light production following mechanical stimulation. (a) Response of a cell to repeated deformation with ${\delta }_f=10\mu \mathrm{m}$ and $\dot{\delta }=76\mu \mathrm{m}/\mathrm{s}$. Inset: Loops in $I-dI/dt$ plane for successive flashes (black to yellow). (b) Loops at fixed $\dot{\delta }$ and varying ${\delta }_f$ for the first flash under given parameters. (c) As in (b), but for fixed ${\delta }_f$ and varying $\dot{\delta }$. Standard errors are shown for outermost data. (d) Master plot of data from 25 (out of a total of 164) first flashes, normalized by maximum intensities and rates. Circles (squares) are data in (b) (c). Black curve is result of model in (1) and (3).

Figure 4

Dependence of light production on deformation and rate. (a) Histogram of maximum intensity with standard errors for ${\delta }_f=10\mu \mathrm{m}$. Note nonuniform grid. (b) Variation of signal strength $s_f$ predicted by phenomenological model, as a function of deformation and rate.

Figure 5

Perturbation stress versus perturbation area for three kinds of experiments on P. lunula. Atomic force measurements are from [12], while macroscopic measurements are performed with a Taylor-Couette flow [8].