Excitation of bioluminescence by laminar fluid shear associated with simple Couette flow

The effect of fluid motion on the excitation of bioluminescence was examined for cultured dinoflagellates and plankton samples subjected to steady state laminar shear associated with simple Couette flow estab-lished in the gap between concentric cylinders with only the outer cylinder rotating. The excitation threshold for the thecate dinoflagellate, Gonyaulax polyedra, occurred at a shear stress of 1 dyn cm-2. At higher shear stresses, light output per cell was proportional to approximately the second power of shear stress. At each maintained shear stress, bioluminescence decreased exponentially at a rate proportional to the magnitude of shear stress. The nonthecate dinoflagellates, Pyrocystis fusijixmis and Pyrocystis noctiluca, were more sensitive to stimulation and exhibited a rate of depletion an order of magnitude higher than for G. polyedra. Plankton samples from the Sargasso Sea and eastern Pacific had similar excitation thresholds but differed in the slope of the intensity vs. shear response, most likely due to different luminescent populations. The excitation threshold obtained from this study is several orders of magnitude greater than oceanic shear stress values in the mixed layer, suggesting that ambient fluid motion, with the exception of surface-breaking waves, does not stimulate bioluminescence. Fluid turbulence affects plankton growth, feeding, and spatial distributions (e.g. Thomas and Gibson 1990; Haury et al. 1990; Saiz et al. 1992) and also excites planktonic biolu-minescence; it has been known since ancient times that agitation of seawater often results in luminescent displays. Turbulent flows have been incorporated into bathyphotometer de-sign for measurement of in situ stimulated bio