FLUID SHEAR‐STIMULATED DINOFLAGELLATE BIOLUMINESCENCE

Bioluminescence studies provide insight into the properties of water motion that are stimulatory to flow‐sensitive organisms such as dinoflagellates, the most common sources of near‐surface oceanic bioluminescence. Previous laboratory studies employing steady flows have characterized the luminescent response of dinoflagellates in terms of shear stress. In the present study, computational and experimental approaches were used to investigate the contributions of shear and acceleration to cells responding in a laminar converging flow field, where regions of high acceleration and shear are spatially separated. Flow‐stimulated flashes by the dinoflagellates Lingulodinium polyedrum and Ceratocorys horrida were used as a near‐instantaneous monitor of cell response. By combining video analysis of flash trajectories with computational methods, the location of each stimulated cell was determined and flow parameters at that location were calculated. Based on several criteria, shear stress was considered the flow parameter most stimulatory to cells. For both dinoflagellates species and for all flow rates, essentially all cells responded downstream near the wall where shear stress levels were maximal, and levels of acceleration and extensional stress were as much as two orders of magnitude less than locations away from the wall. Minimum shear stress levels at the cell positions were consistent with response thresholds based on previous studies. Bioluminescence is an excellent tool for examining how organisms respond to flow at the small temporal and spatial scales relevant to planktonic organisms.