Now and then our shorelines are graced with billions of single-celled organisms that produce beautiful light when disturbed. The shimmering blue visible at night from waves breaking at the shoreline, or even a rock thrown into the waters, is one of the wonders of nature. Shakespeare and Darwin sang the phenomenon's praises, and in recent years photographers have allowed all of us to witness it. Nevertheless, the causes, both evolutionary and physical, are poorly understood.
Professor Raymond Goldstein and Dr Maziyar Jalaal of the University of Cambridge are seeking to explain not why the dinoflagellates commonly known as “sea sparkles” produce their light, but the forces that trigger it.
Goldstein and Jalaal pushed on the cell walls of Pyrocystis lunula, one bioluminescent dinoflagellate species, using a variety of tools, from microscopic cantilevers to fluid jets. In Physical Review Letters, they report deformation of the cell membrane leads P. lunula to emit a pulse of light. Moreover, the more its cell wall is pushed out of shape, and the faster this happens, the brighter the light. This probably avoids the organisms expending too much energy making light in response to small disturbances.
Curiously, the light curve has the same shape even when the pressure is applied more quickly, although it generates a brighter flash. The light is also produced as the pressure eases off, restoring the cell's shape. However, each organism can only produce so much light without time to recover – repeated application of pressure leads to decreasing brightness.
The authors think when stress is applied to the membrane the cells' ion channels open, allowing calcium to move between different parts of the cell in what is known as a viscoelastic effect. This in turn triggers chemical reactions that lead to the release of energy at a wavelength we can see and appreciate. “The production of a given amount of light...can be achieved through the action of many channels weakly recruited, or a small number strongly recruited,” the paper notes.
"Despite decades of scientific research, primarily within the field of biochemistry, the physical mechanism by which fluid flow triggers light production has remained unclear," Goldstein said in a statement. Jalaal added, "Our findings reveal the physical mechanism by which the fluid flow triggers light production and show how elegant decision-making can be on a single-cell level."
For some animals bioluminescence serves a clear purpose, usually to attract mates or lure prey. The benefits are less clear for the asexual, photosynthesizing dinoflagellates. It is thought most likely they use their light to attract animals higher up the food chain that feed on things that eat dinoflagellates, like grass that could call lions when zebras consume it, possibly scaring away threats in the process.