Lingulodinium polyedrum, like other algae and plants, converts light energy into organic matter through photosynthesis, and lives in the well-lit upper layers of temperate to warm oceans. Like all dinoflagellates, Lingulodinium possesses two flagella enabling movement within the water column. An undulating flagellum causes a rotation. This feature is known as ‘dineo’ which means ‘to rotate’ or ‘to curl’. At night dinoflagellates migrate several metres down into deeper water where they take up nutrients such as nitrate. If there are enough nutrients in the sea and temperatures are also favourable for development, flagellates can produce large algal blooms. As with many other dinoflagellates, Lingulodinium is protected by outer, rigid plates. In our latitudes, dinoflagellates form, together with diatoms, the bulk of the phytoplankton. When blooms occur under optimum conditions they may colour the sea red, orange or brown, depending on their pigments.
How the dinoflagellate colours the sea
Lingulodinium polyedrum is one of few species that are able to induce an internal blue light called bio-luminescence which is induced via biochemical reactions. These reactions take place in tiny organelle-like compartments of the cells, the so-called ‘scintillons’. These compartments contain a specific enzyme and a binding protein for the substrate. Lingulodinium produces flashes of blue light when cells are either damaged or simply by shear movement of the cells. The phenomenon is particularly pronounced at the end of the night. When million of cells are moved at the same time, for example by water motion caused by boats or breaking waves, flashes of individual cells merge into a continuous glow. Although a mass occurrence of Lingulodinium is unlikely to occur along Northern European coastlines as water temperatures are too low, in our latitudes a similar phenomenon may be observed off the island of Helgoland where it is caused by another dinoflagellate, Noctiluca.
Glowing to attract predators?
It is not fully understood why Lingulodinium polyedrum bio-luminesce. One hypothesis suggests that the glow attracts predatory fish which, in turn, attack the enemies of the dinoflagellate, the grazers. These fish feed more effectively during night when dinoflagellates glow. This is described as the ‘burglar alarm’ hypothesis” explains Dr Hoppenrath who is an expert on the taxonomy of dinoflagellates. She has already described 21 new dinoflagellate species. World-wide, approximately 2000 to 2500 dinoflagellate species exist. ”It is important to scientifically describe and name each single species as such detailed knowledge forms the foundation for many other scientific disciplines such as biodiversity, ecosystem, fisheries or evolutionary studies”, concludes biologist Hoppenrath.
More interdisciplinary research is needed
Some dinoflagellates, including Lingulodinium polyedrum, are associated with the production of substances such as yessotoxin and saxitoxin which are extremely important for food safety. These compounds become toxic to humans if they are concentrated in the food chain. Shellfish such as mussels or crustaceans are able to concentrate these substances as they filter seawater through food intake. To protect humans from dangerous concentrations of dinoflagellate toxins, the European Food Safety Authority (EFSA) has defined critical limits for the consumption of shellfish. Currently, the quantities of such substances produced by Lingulodinium polyedrum are unknown. As it is never possible to ascertain exactly which organisms are present in the water when swimming in a ‘glowing sea‘, people should always be careful in such a situation. Other dinoflagellates species may be able to produce relevant amounts of toxins which may be harmful to people, even without actual consumption. This is another reason why it is so very important to carefully describe and classify these tiny unicellular organisms, emphasizes Hoppenrath. For further collaborative research between toxicologists and nutritive scientists is required, and specialised biologists and taxonomists also need to contribute.
The night life of Lingulodinium
The species is also of interest to other scientists as its activity follows a distinct diurnal cycle. The glowing occurs mostly during night. ”At night more scintillons are present than during the day. These compartments contain specific proteins which are involved in the bio- luminescent reaction and these proteins increase in concentration at night” explains Prof. Maria Mittag. Prof. Mittag used to work on the regulatory mechanisms of these proteins of the diurnal cycle when Lingulodinium was still named Gonyaulax polyedra. The migration from the shallow areas of the ocean into greater depths is also regulated by its endogenous clock. ”Interestingly the period of the bio-luminescent rhythm of Lingulodinium is a just under 24 h. This is in contrast to that of humans who have a sleep-wake rhythm of 25 h if it is not regulated by the daily adjustment to daylength” says Prof. Mittag.
Lingulodinium polyedrum fascinates biologists since it is an organism which still presents some genetic mysteries. For example, so far it is not known why this small organism possesses so much more genetic material than humans. It is also unclear why some genes coding certain proteins have thousands of DNA copies, such as specific binding proteins or the pigment peridinin which is responsible for the brown-orange colour of Lingulodinium. The high number of DNA copies currently hampers the investigation of molecular biological processes.
Application as a sensor
Due to its ability to glow, the dinoflagellate L. polyedrum can also be employed to detect toxins and/or other new active components in water bodies. For example, the onset of Lingulodinium cell death can be determined as dying cells glow more strongly. On the other hand, Lingulodinium cells can be used to detect cell stress which is again measurable via its bio-luminescencent behaviour. ”Lingulodiunium is particularly suitable as a test organism as it is relatively well researched, easy to cultivate, and its bio-luminescence can be measured via automated processes” explains Hoppenrath. ”The number of animal experiments which are normally used for such experiments can be significantly reduced by utilising this dinoflagellate”.