From ancient times, people observed a mysterious night-time glow from the sea. In some cases, it looked like milky light flowing through the water, in other cases it left a trail behind a ship's stern, and sometimes it appeared as flashes and sparks in a water column. Scientists at first were stymied by the phenomenon. They offered various hypotheses, until finally in the XVII^th century ACE, its biological basis was discovered: it turned out that the reason for the sea’s enigmatic glow was bioluminescence, i.e. the ability of living organisms to give off light. The biological glow can be caused by plankton organisms of various sizes, as well as by microscopic bacteria. The best-studied - and the most common representative of bioluminescent organisms in the Black Sea - is the sea sparkle (Noctiluca scincillans). It is usually referred to as a phytoplankton, along with other species of dinoflagellates (Dynophyta).

But these large spherical algae, sized between 0.5-2mm (occasionally reaching 3 mm), have lost the ability to photosynthesise, and became predators. At the same time, the inclusions of fat in their cytoplasm helps sea sparkles to stay afloat in water columns. Thus, the now-useless chlorophyll produced an interesting derivative, which released a ray of light when oxidised, and this causes the glowing of the sea sparkle when physically or chemically disturbed. As the sea glow was originally associated with the infernal, the chlorophyll derivative responsible for it was wittily called ‘luciferine’, and its particular oxidative fermentation product – ‘luciferase’.

At the end of summer, the glowing of the Black Sea surf, or from wake trails left by passing motor boats, can often be observed. Sparkling lines, left by swimming fish, are seen throughout the water column. There is an old Crimean legend about the glowing waves foamed with ships' oars, which prevented the Greek fleet from coming unnoticed to the shores of Taurida. If this legend has a historical basis, then, most likely, it was the sea sparkle that saved Tauri from an unexpected assault. It is not the only case of bioluminescence helping humans. Before sonar was invented, bioluminescence could save ships by showing navigators the proximity of coastlines, shallows, or reefs. Fishermen often discovered schools of fish at night, thanks to the glow from the depth.

Sea sparkle can also be found far from the Black Sea, and continues to spread further afield. Scientists believe this distribution is a function global warming, which affects ocean currents, and in turn influences the spread of plankton.

The publication was prepared with the financial support of the EU – UNDP project ‘Improving Environmental Monitoring in the Black Sea: Selected measures’ (EMBLAS-Plus). This publication was produced with the financial support of the European Union and UNDP. Its contents are the sole responsibility of the authors and do not necessarily reflect the views of the European Union or UNDP.

Authors: B. Aleksandrov, O. Adrianova, N. Atamas, V. Bolshakov, O. Bondarenko, I. Chernichko, V. Demchenko, S. Dyatlov, Y. Dykhanov, E. Dykyi, O. Garkusha, P. Gol’din, S. Hutornoy, V. Komorin, Y. Kvach, V. Mamaev, O. Manturova, O. Marushevska, A. Mikelyan, Yu. Mikhalev, G. Minicheva, I. Sinegub, T. Shiganova, J. Slobodnik, A. Snigiryova, M. Son, K. Vishnyakova, A. Zotov. Illustrator: I. Pustovar.