EXCITATION OF BIOLUMINESCENCE BY LAMINAR FLUID SHEAR ASSOCIATED WITH SIMPLE COUETTE-FLOW

The effect of fluid motion on the excitation of bioluminescence was ex amined for cultured dinoflagellates and plankton samples subjected to steady state laminar shear associated with simple Couette flow establi shed in the gap between concentric cylinders with only the outer cylin der rotating. The excitation threshold for the thecate dinoflagellate, Gonyaulax polyedra, occurred at a shear stress of 1 dyn cm-2. At high er shear stresses, light output per cell was proportional to approxima tely the second power of shear stress. At each maintained shear stress , bioluminescence decreased exponentially at a rate proportional to th e magnitude of shear stress. The nonthecate dinoflagellates, Pyrocysti s fusiformis and Pyrocystis noctiluca, were more sensitive to stimulat ion and exhibited a rate of depletion an order of magnitude higher tha n 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 lumine scent populations. The excitation threshold obtained from this study i s several orders of magnitude greater than oceanic shear stress values in the mixed layer, suggesting that ambient fluid motion, with the ex ception of surface-breaking waves, does not stimulate bioluminescence.