Noctiluca scintillans

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.[2]

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.

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.[2] 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.[1] N. scintillans should not be confused with Spatulodinium pseudonoctiluca, which is a similar but smaller species (<200 micrometers).[1]

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.[3][4]

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.[1]

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.[2]

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.[2]

This life cycle continues with two consecutive nuclear divisions to obtain 4 nuclei. This division creates bulges above the cell surface. This is followed by a continuum of synchronous nuclear divisions with each 'progenitor' connected to the others by thin filaments. As gametogenesis progresses, there is a condensation of chromosomes within the different nuclear divisions which darkens the colour of the cell. The result is four petal-shaped clusters of progenitors.[2]

The progenitors of the previous stage have transformed into zoospores. At this point they are evenly distributed in one part of the cell. At the same time as the progenitors are maturing, two flagella start to develop and are actively beating. These flagella develop outside the mother cell and the mature gametes are then released into the surrounding environment. When they have all emerged, the mother cell remains ghostly.[2]

The two flagella formed are not of the same length and therefore do not have the same function. The longer of the two is used for direction of movement in the sea water, while the shorter one provides more of a swimming force to activate the movement.[2]

This stage is still highly open to speculation. It seems that Noctiluca scintillans produces isogametes, which are gametes that fuse together to form a zygote. This zygote then has 4 flagella and 2 nuclei. This means that the species is in fact diploid, differentiating it from most dinoflagellates which are haploid.[2]

At the beginning of trophont formation, the number of flagella decreases and the cells become fusiform. During further development they become rounder, and two distinct flagella are formed, one longer and one shorter, and finally only one is left. After this, the outer layer becomes discernible and the crust is formed. The result is a miniature trophont with a tentacle through which it absorbs food to eat by means of viscous materials to which the algae cling.[2]

Thanks to its high specificity, Noctiluca scintillans could increase its biomass up to 100 times in one week.[3]

The environment plays an important role in the proliferation of Noctiluca scintillans. The population varies according to sunlight, current, the presence of nutrients (especially nitrate, ammonium and urea), water salinity, temperature and trophic stress. The amount encountered also varies according to the geography and the ocean concerned, although it is present throughout the world.[3]

Noctiluca scintillans is found in temperate, subtropical and tropical waters. It is found abundantly close to the coast; it is a neritic species.[1] It is also found abundantly near the mouths of rivers after heavy rainfall. They are mostly found during the warm seasons, although they can be found all year round.[1]

Extreme conditions for the species are 2 to 31 °C and 17 to 45 psu (practical salinity unit).[3] However, each form has its own preferences and the temperature and salinity ranges are generally more restricted.

The red form is found over a wide temperature range: between 10 and 25 °C and in salty environments. It is very abundant in euthrophic environments where diatoms dominate as this is its favourite food source. The green form is more restricted, with a temperature range of 25-30 °C.[4]

Noctiluca scintillans ranges from tropical oceans to northern seas.[2] It is a cosmopolitan species, found in all seas of the world.[1]

The green form of N. scintillans is mainly found in the tropical waters of Southeast Asia, the Bay of Bengal, the Arabian Sea, the Gulf of Oman,[6] and the Red Sea.[4] The red form is more widespread, and is found in the seas of Central America, Europe, the Black Sea, East, South and Southeast Asia, and the Tasman Sea. It is also found on the coasts of South America and in the seas of West Africa.[6]

The two forms overlap in the western, eastern and northern Arabian Sea with a seasonal difference in abundance. The green form is found in cold waters, with winter convective mixing, while the red form is found in the warmer summer season.[4]

The proliferation of N. scintillans can be toxic, and has been linked to massive mortality of fish and marine invertebrates. However, this species does not produce toxins, which are often the cause of the harmful effect of these tides when they are caused by other organisms. It is actually because of the accumulation of ammonium in excessive quantities and the reduction of dissolved oxygen in the direct ecosystem during its proliferation that N. scintillans is harmful to other species of fish and invertebrates that experience high mortality.[1]

When the concentration of individuals exceeds one and a half million per litre, the water turns pink or orange, hence the name of the red tide phenomenon. In 1970, concentrations of 2,400,000 N. scintillans per litre were found.[1]

This phenomenon is not always red. The colour depends on the pigment in the vacuole of the organism and can be green. (there is a picture in the morphology section).[1]

Other species can also cause red tides, such as species of dinobionts, which are single-celled organisms with 2 flagella. It is necessary to check under the microscope whether the red tide is indeed caused by Noctiluca scintillans or not.[1]

Noctiluca scintillans was first discovered in the Arabian Sea in the 2000s, according to a recent study, which was also the first time that the sea water was undersaturated with oxygen. Since then, winter dissolved oxygen concentrations in the upper euphotic zone have remained low. It has been shown that the species grows best in an environment with abundant light (for the green genus) and with a lower dissolved oxygen concentration, this increases oxygen uptake in the species and further decreases oxygen levels. This allows the species to grow faster and thus creates waves of green Noctiluca scintillans blooms in the Arabian Sea every winter.[11]

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.[11]

Another interesting detail is that Noctiluca scintillans produces large amounts of phosphorus and nitrogen in its excretions.[3] The bloom of the species has often been linked to mass mortalities of marine invertebrates and fish but in reality it does not produce toxins, it accumulates lethal amounts of ammonium which is then excreted into the environment. It is during toxic red tides, that the red genus excretes these lethal amounts to the animals around it.[4]

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.[12]

This phenomenon will only increase with climate change, which will increase the frequency and intensity of blooms. More and more corals will be affected.

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.[12] Corals are home to 25% of the Earth's marine life. So there is a lot at stake in understanding this.[13]