Noctiluca scintillans is a marine species of dinoflagellate that can exist in a green or red form, depending on the pigmentation in its vacuoles. It can be found worldwide, but its geographical distribution varies depending on whether it is green or red. This unicellular microorganism is known for its ability to bioluminesce, giving the water a bright blue glow seen at night. However, blooms of this species can be responsible for environmental hazards, such as toxic red tides. They may also be an indicator of anthropogenic eutrophication.
Etymology
The name Noctiluca scintillans comes from the Latin Noctiluca, meaning "light at night" and scintillans, meaning "shining, throwing out flashes of light".
Description
Taxonomy
It was classified with the jellyfish until 1873 when Haeckel then decided to move it to the crystoflagellates with the dinoflagellates. This remained the case until 1920 when Kofoid finally placed it in the order Noctilucales following certain observations. This classification is still subject to discussion today and the relationship of Noctiluca to the dinoflagellates is not yet clearly demonstrated, as the results of analysis are still too variable to assert a single classification.
At present, it is part of the phylum Myzozoa, which are unicellular flagellated organisms. It is then part of the class Dinophyceae, which has two flagella, the order Noctilucales, whose nucleus is not dinocaryonic in the adult, and the family Noctilucaceae, which has a globular shape with a tentacle.
Morphology and anatomy
Noctiluca scintillans is a single-celled spheroid organism, ranging from 400 to 1500 μm in length. It moves with the current and cannot really swim. The fact that it is translucent facilitates the observation. N. scintillans has a long cytoplasmic expansion that hangs at the base of a deep groove, close to which is the nucleus. Another identifying feature are the fine striae that start from the central nucleus and extend towards the periphery of the cell. This species is known by the appearance of blue flashes during night dives. N. scintillans should not be confused with Spatulodinium pseudonoctiluca, which is a similar but smaller species (<200 micrometers).
There are 2 colours of N. scintillans. This depends on the pigment present in the vacuoles. The red form is heterotroph. This form of N. scintillans competes with copepods to feed on phytoplankton. The green form has a photosynthetic symbiont inside called Pedinomonas noctiluca which causes the green colour. It is mainly autotroph or even photoautotrophic if this photosynthetic symbiont is abundant in the cells.
Noctiluca scintillans is a species capable of managing its buoyancy by regulating the intracellular ion concentration. To rise, the concentration of potassium will increase and to fall, it will use heavier elements such as calcium or magnesium.
Place in the food chain
N. scintillans has an important place in the pelagic food chain. N. scintillans is preyed upon by many copepods such as Calanus sp., Temora sp. and Acartia sp., chaetognaths and hydromedusae. Because of their excessive proliferation, they attract many predators due to their very dense aggregations and frequent bioluminescence in this phase of their life.
The diet varies according to the green and red form. The green form is indeed autotrophic if the symbiont Pedinomonas noctiluca is abundant in its vacuole. Otherwise, it is heterotrophic, like the red form. N. scintillans then feeds on diatom aggregates, as well as copepod eggs, naupilar larvae and fish eggs.
N. scintillans can be parasitised by Euduboscquella, an intracellular parasite that infects mainly tintinnids but also dinoflagellates.
Life cycle
Trophonts
Noctiluca scintillans is a heterotrophic dinoflagellate that causes toxic red tides. To explain the life cycle of this species, we need to start with the trophonts. Trophonts are the non-reproductive adult life stage of many ciliated protozoa. They are eggplant-shaped with a crust consisting of two distinct layers; an outer gelatinous layer and a plasma membrane. Like all eukaryotes, the trophont is composed of a nucleus that lies close to the cytostome surrounded by cytoplasm forming the cytoplasmic centre.
Gamonts
It is with the gamonts, which is the name of the cells during gametogenesis that cell division occurs. These gamonts are produced by a small fraction of the trophonts that spontaneously initiate gametogenesis. During this transformation, the cell becomes spherical and loses some organelles including the tentacle and the nucleus moves to just below the cell surface.