Future Directions in Bioluminescence Research

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ABSTRACT

Spontaneous and Stimulated Bioluminescence

Michael Latz¹ and Jim Rohr²

1. Scripps Institution of oceanography, UCSD, CA. 92093-0209
2. SPAWARSYSCEN San Diego CA. 92152-5000

Bioluminescence is a ubiquitous and conspicuous nighttime phenomenon in near-surface littoral waters. Background levels of bioluminescence can originate from the glowing of bacteria (milky seas), predator-prey interactions, swarming displays, and spontaneous flashing in dinoflagellates, as well as from water motion caused by waves, surge, and animal swimming. In general, flow-stimulated bioluminescence produces a greater signal than background luminescent sources. Dinoflagellates, the most common sources of near-surface bioluminescence in littoral waters, are excellent model organisms for examining the flow conditions that stimulate bioluminescence. Because they can be cultured, dinoflagellates are suitable for laboratory study using well-characterized flow fields. It is hypothesized that fluid shear stress, rather than pressure or acceleration, is the most stimulatory component of fluid motion, because of the association of bioluminescence with boundary layer and wake conditions. Using simple laboratory shear flows, it has been possible to characterize the flow-induced stimulation of bioluminescence in terms of quantified fluid forces.

There is a well-defined response threshold in laminar flow at a shear stress level of 0.1-0.3 Newton m-2. This level of shear stress is greater than that occurring in typical mixed layer flow, but is consistent with an anti-predation function of bioluminescence in dinoflagellates. Increases in mean bioluminescence are due primarily to more organisms responding and not to an increase in flash intensity. The response threshold and shear dependence of bioluminescence is similar for cultured organisms and mixed plankton samples, and consistent between pipe flow and Couette flow.

Extensive studies with laminar and turbulent pipe flows have verified that the response is dependent not on the flow rate but on the shear stress; there is no difference between the effects of laminar and turbulent flows at equivalent shear stress levels. Therefore the luminescent response is correlated with the shear stress levels in the flow, not its laminar or turbulent nature.

Based on this understanding from laboratory studies, it is predicted that the amount of flow-stimulated bioluminescence associated with a moving object of interest is related to the thickness of the boundary (shear) layer and the volume of the wake. This total volume will determine how many organisms are entrained in the high shear regions.

Laboratory tests with models verify that a thicker boundary produces more bioluminescence from either cultured organisms or natural plankton assemblages. Tests with free-swimming dolphins show that forced flow separation results in a conspicuous increase in bioluminescence. For predictions of flow-stimulated bioluminescence signatures associated with a moving object, the amount of bioluminescence is a function of boundary layer thickness, the degree of flow separation, wake volume, and bioluminescence potential (related to the abundance of luminescent plankton). In general, flow-stimulated bioluminescence associated with a moving object will be much greater than background levels of bioluminescence.

Figure Legend: Increase in bioluminescence due to thickened boundary layer associated with (A,B) a 2.54 cm diameter sphere and flow speed of 26 cm s-1, with 10 cells ml-1 of the dinoflagellate Lingulodinium polyedrum, and (C,D) the head of a bottlenose dolphin moving at 2 m s-1 through natural plankton. In (B) an O-ring added to the leading edge of the sphere thickens the boundary layer and results in more bioluminescence. In (D) a cup in the mouth of the dolphin makes the boundary layer turbulent, which causes flow separation and a conspicuous increase in bioluminescence.

Relevant Literature on Flow-Stimulated Bioluminescence

Anderson, D.M., D.M. Nosenchuck, G.T. Reynolds, and A.J. Walton 1988. Mechanical stimulation of bioluminescence in the dinoflagellate Gonyaulax polyedra Stein. J. Exp. Mar. Biol. Ecol. 122: 277-288.

Donaldson, T.Q., S.P. Tucker, and R.V. Lynch 1983. Stimulation of bioluminescence in dinoflagellates by controlled pressure changes. Naval Res.Lab. Report 8772.

Gooch, V.D. and W. Vidaver 1980. Kinetic analysis of the influence of hydrostatic pressure on bioluminescence of Gonyaulax polyedra. Photochem. Photobiol. 31: 397-402.

Latz, M.I., J.F. Case, and R.L. Gran 1994. Excitation of bioluminescence by laminar fluid shear associated with simple Couette flow. Limnol. Oceanogr. 39: 1424-1439.

Latz, M.I. and J. Rohr. 1999. Luminescent response of the red tide dinoflagellate Lingulodinium polyedrum to laminar and turbulent flow. Limnol. Oceanogr. 44: 1423-1435.

Rohr, J., J. Allen, J. Losee, and M.I. Latz 1997. The use of bioluminescence as a flow diagnostic. Physics Letters A 228: 408-416.

Rohr, J., M.I. Latz, S. Fallon, J.C. Nauen, and E. Hendricks 1998. Experimental approaches towards interpreting dolphin-stimulated bioluminescence. J. Exp. Biol. 201: 1447-1460.

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