The cover of Biophysical Journal (Volume 108, Issue 6) shows autofluorescence of a cell of the bioluminescent dinoflagellate Pyrocystis lunula. In this laser scanning confocal microscope image, blue is the fluorescence of luciferin, the substrate molecule for the bioluminescence reaction that originates from vesicles called scintillons, while red is chlorophyll fluorescence originating from the plastids. Dinoflagellate bioluminescence, which in nature functions in predator defense, here serves as a rapid whole-cell reporter of mechanosensitivity. Bioluminescence emission is mediated by a poorly understood but complex signaling pathway that involves activity at the plasma membrane, release of calcium from intracellular stores, depolarization of the vacuolar membrane, and acidification of the scintillons to activate the oxidation of luciferin. With a delay from mechanical stimulation to response of only 15-20 ms, dinoflagellate bioluminescence is one of the fastest known mechanosensitive cell systems. Thus dinoflagellate bioluminescence serves as an extremely rapid, whole cell noninvasive reporter of mechanosensitivity.

In our study, we used atomic force microscopy and a spherical probe to stimulate individual cells and to examine the relationship between cell mechanical properties and mechanosensitivity as assessed by intrinsic bioluminescence. The dinoflagellate flash, in this species lasting about 400 ms, is an all-or-nothing phenomenon that served as an indicator of cell response. By varying the parameters of the applied stimulation, we were able to determine a threshold force and velocity that was necessary to stimulate the cell. We observed that cells did not respond to a low indentation velocity. To explain this phenomenon we carried out stress relaxation experiments to measure the viscoeleastic properties of the cell. We formulated a simple viscoelastic model involving dashpots and springs to explain the velocity-dependent responses in terms of mechanosensor activation. At high rates of stimulation, stress accumulates in the cell membrane leading to a conformational change in mechanosensors, while at low stimulation rates the energy is dissipated due to relaxation. We are excited to develop our studies using dinoflagellate bioluminescence as a tool to investigate cellular mechanisms of rapid mechanosensing.

In nature, dinoflagellate bioluminescence is responsible for spectacular nighttime light displays when stimulated by mechanical stress associated with swimming animals, boat wakes, and breaking waves. In the laboratory, dinoflagellate bioluminescence is demonstrating its value to the physical sciences as a flow visualization tool for regions of increased mechanical stress, especially in applications not amenable to conventional measurement methods, such as shear stress within bioreactors, in breaking waves, above a rippled seabed, and associated with a moving dolphin.

Our use of dinoflagellate bioluminescence as a flow visualization tool was inspired by Leonardo da Vinci, who more than 500 years ago used grass seeds as particles to visualize flow patterns. Bioluminescence is a beautiful expression of nature, and it has been inspiring to collaborate with artists to express that beauty in photographs and video, for example http://www.erikablumenfeld.com/artworks/gallery/bioluminescence/. Dinoflagellate bioluminescence also serves as a tool in education and public outreach, that, along with its artistic value, is valuable in bringing science to the public. For more information about our research and dinoflagellate bioluminescence, visit http://siobiolum.ucsd.edu

– Benoit Tesson, Michael Latz