Bacteria bioluminesce in response to fluid shear

Bioluminescence by marine microorganisms is generally characterized by either pulse-like or continuous light production. Large eukaryotic organisms such as dinoflagellates exhibit a brief burst of light when mechanically stimulated, while bacteria can produce light continuously for long periods of time. An interesting open question is whether, like dinoflagellates which respond to fluid shear, bacteria exhibit any direct response to such stimulation. We have developed a method of investigating this possibility using an optical rheometer, and have studied the response of 5 separate species of marine bacteria under a range of shear rates. For all species, we find that the intensity of bioluminescence is a sigmoidal function of the applied shear rate and that the light production exhibits hysteresis under cycles of increasing and decreasing shear. Since oxygen consumption is a necessity for bacterial bioluminescence, I will discuss how the interpretation of such experimental observations requires a careful analysis of the possible competing effects of direct response to fluid dynamical shear and the effects of shear-induced mixing on oxygen availability.