Noctiluca is a genus of dinoflagellates in the family Noctilucaceae. Its only species is Noctiluca scintillans, a marine species 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". 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 dinokaryonic 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.
thumb|Close-up of Noctiluca scintillans, revealing its photophores
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. Because of their excessive proliferation, they attract many predators due to their very dense aggregations and frequent bioluminescence in this phase of their life.
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. The life cycle of this species begins as trophonts, which 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 center.
When the concentration of food sources in the environment changes dramatically (the concentration drops below 400 cells/ml), Noctiluca scintillans will transform from trophonts to gametocyte mother cells, increasing the proportion of gametocyte mother cells in the population (from less than 1% to nearly 10%). When the food supply in the environment decreases sharply, Noctiluca scintillans may reproduce sexually and produce a large number of gametes as another way of survival after the algal bloom occurs. and the Red Sea. It is the transformation of chemical energy into light energy by a living being which then emits this light. Bioluminescence differs from fluorescence and phosphorescence because the latter two require contact with light to trigger the phenomenon.
N. scintillans produces luminous flashes, which constitute bioluminescence, during mechanical stress. This phenomenon can therefore be observed in agitated water, i.e. when boats are passing, near the coast at wave level or after water agitation.
Luciferin combines with luciferase and the two react with oxygen to form an oxidized complex. The luciferin then emits a photon. Of course, the reaction itself is not so simple, in fireflies it also requires two additional cofactors, ATP and magnesium. There are also several types of luciferin and each is associated with a specific luciferase giving different chemical reaction systems.
The light is produced by mechanical stimulation due to shear stress. The deformation of the cell membrane causes an action potential across the vacuole membrane caused by Ca<sup>2+</sup> ions released from intracellular stores. And is involved in the activation of GTP-binding protein coupled receptors in the plasma membrane. Most of the Ca<sup>2+</sup> ions are released from intracellular stores, while some are released from extracellular sources. Under mechanical disturbance, this action potential releases an influx of protons from the acidic vacuole to the scintilla, lowering the pH from 8 to 6. This changes the conformation of luciferase making it active. Luciferin contains a binding protein that prevents it from auto-oxidizing in an alkaline pH. It releases it by a conformational change in acidic pH, activating luciferin. This activation then allows the enzyme to oxidize luciferin to oxyluciferin. It is this molecule that leads to the emission of photons by an unknown process.
The eutrophication of the water is therefore not directly related to Noctiluca scintillans, but the fact that the dissolved oxygen concentration is already slightly low during the monsoon period shows a more consistent development of the species which worsens the situation by increasing its oxygen uptake and decreasing the amount of available dissolved oxygen. This decrease in natural dissolved oxygen is actually caused by the presence of phytoplankton brought in by the hypoxic waters of the Southern Ocean during the monsoon period. To date, this is the only explanation for the arrival of the low oxygen waters.
Impact on coral reefs
Coral reefs have been in severe decline in recent decades. According to a study conducted in 2019 in the Gulf of Mannar (South India), hypoxic conditions caused by algal blooms are causing massive mortality of coral reefs.
In this study, it is shown that Noctiluca scintillans causes the death of these corals significantly by overgrowth, as their reproduction causes a decrease in dissolved oxygen of 2 mg/L. This causes lethal hypoxia for corals of the genus Acropora, Montipora and Pocillopora.
There is still a lot of work to be done to find ways to remedy this problem, especially to understand the precise mechanisms of the interaction.
Role in the environment
{| class="wikitable"
|+Summary of the role of N. scintillans in the environment
!Positive effect
!Neutral effect
!Negative effect
|-
|In the food chain
|Bioluminescence (role unknown)
|Euthrophication, impacts on coral reefs, red tides
|}
Calendar
The phenomenon of bioluminescence is very nice to observe, but it is not found everywhere at any time. Attached is a calendar of peak abundance in different regions of the world and in different months of the year.