Bioluminescence of the Dinoflagellate Pyrocystis noctiluca induced by laminar and turbulent couette flow

The excitation of bioluminescence by different flow regimes generated within a Couette chamber was examined using the dinoflagellates Pyrocystis noctiluca. Cultured cells of P. noctiluca were gently transferred into a cylindrical Couette chamber in a dark room. In initial experiments, the velocity of the outer Couette cylinder was then gradually increased. The bioluminescence emissions in response to stationary-laminar and turbulent flows were quantified using a photomultiplier tube. Video images were also recorded in order to identify the location of bioluminescence emissions within the Couette chamber. Reflective flake flow visualizations were used to correlate these locations to the flow regimes in those parts of the chamber. These experiments clearly demonstrated that the strongest bioluminescence emissions were only triggered by the onset of turbulence at high rotation speeds. Below the turbulence threshold, much lower bioluminescence emissions were detected and appeared to be in response to a nonhomogeneity in the stationary-laminar flow (end cap effects and Ekman cells). In a second set of experiments, the excitation of bioluminescence in response to acceleration was studied by abrupt starts of the rotating Couette cylinder. These experiments also triggered massive bioluminescence emissions. We conclude that pure laminar-stationary, homogenous shear flow excites very little bioluminescence in P. noctiluca. The bulk of bioluminescence emissions primarily occurred under nonhomogenous or nonstationary flow conditions, where the cells experience velocity changes as they move through the flow. These findings are discussed in relation to the theory that bioluminescence in dinoflagellates is an antipredation mechanism