What is Phototaxis? What is Positive and Negative Phototaxis? Explained in Detail
What is Phototaxis?
Phototaxis is derived from the two words ‘Photo’ meaning light and ‘Taxis’ meaning the movement of an organism in response to an external stimulus.
Therefore, Phototaxis can be simply defined as the locomotory movement of living organisms directionally towards the source of light or away from the source of light.
Simply meaning that a taxis is the movement of an organism in response to a stimulus such as light or the presence of food. Here, in the case of Phototaxis, the organism move towards or away from the direction of light, and not food.
Phototaxis is the innate response of an organism to variation in light intensity and direction. Meaning that the more the intensity of light in a particular direction the more the phototactic stimulation occurs in the organism.
Each and every organism that is phototactic in nature has its own specific biological reason for a phototactic response, many of which are incidental and serve no end purpose.
However, being Phototactic is an advantage to a wide variety of organisms.
The advantages of phototaxis include the regulation of light exposure for photosynthesis, the finding of phototrophic organisms for food, the facilitation of larval dispersal, or the increased likelihood of gamete fusion on the surface.
There are various light-sensitive cells containing photopigments that mediate the Phototactic response in an organism.
These light-sensitive cells are also called photoreceptors and the photopigments they include are retinal (in rhodopsin), flavin (in cryptochrome), bilin (in phytochrome).
What is Positive Phototaxis?
Positive Phototaxis is defined as the response of an organism in a direction towards the source of light.
In simple words, Phototaxis is called positive if the movement is in the direction of increasing light intensity.
The source of light provides many organisms with both energy and information about their surroundings, which is why these organisms commonly display locomotory movement towards the direction of light, which is positive phototaxis.
A positive phototaxis is exhibited by phototrophic organisms. Examples of phototrophic organisms exhibiting phototaxis are the various Phytoflaggellates, Euglena, and photosynthetic bacteria.
Positive phototaxis also shows the natural response of chlorophyll-containing green colored plants to move towards the direction of sunlight to prepare their own food by photosynthesis.
Positive phototaxis can be found in many flying insects such as moths, grasshoppers, and flies.
Drosophila melanogaster (fruit fly) has been studied extensively for its innate positive phototactic response towards the light sources under various laboratory conditions.
The relation between Phototropism and Phototaxis is that Phototropism is the movement of a plant towards or away from light. While Phototaxis is the movement of an organism either towards the light (positive phototaxis), or away from a source of light (negative phototaxis).
What is Negative Phototaxis?
Negative Phototaxis is defined as the response of an organism in a direction away from the source of light.
In simple words, Phototaxis is called negative if the movement is away from the direction of the increasing light intensity.
It has been noticed that Negative phototaxis can be only observed in larval Drosophila melanogaster within the first three developmental instar stages. On the other hand, adult ones can display Positive phototaxis very well.
Being Negative Phototaxis is especially helpful to a lot of species. Like for example, in Earthworm.
Earthworms actually have receptor cells in their skin that are sensitive to light and touch. They will move away from the light that is they are negative phototaxis because the heat from the sun or a light source will dry out their skin and can kill them.
Same organism can also show both positive and negative phototaxis depending on their environmental scenarios.
For example, it is seen that Euglena grows and moves towards (positive phototaxis) or away (negative phototaxis) from light depending on their needs.
Explained: Phototaxis in Animals
In animals as well, phototaxis can be widely seen. A majority of the animals seem to be attracted to the intensity of light, let it be natural light or artificial light.
The attraction of insects like moth, flies, grasshoppers, etc. to the direction of a flame, lamp, etc. can be well seen because they confuse the animals’ navigational systems.
Being positive phototaxis is beneficial to organisms as well. As such, the behaviour of many invertebrate larvae contributes to upward migration in the water column and can facilitate larval dispersal. It’s because the eyes (most probably the eyespots) of the larvae have one to a few photoreceptor cells associated with shadowing pigment.
For instance, positive phototaxis in zooplankton has been well studied in the trochophore and metatrochophore larvae of some marine annelid Platynereis dumerilii.
The trochophore and metatrochophore larvae have simple eyespots that consist of a pigment cell and a photoreceptor cell. The eyes don’t have a higher resolution for vision but, the photoreceptor cells are positively phototactic that synapses directly onto ciliated cells, thus helping them swim in the direction where the light is coming.
This helps the larvae to migrate upward in the water column and facilities larval dispersal that is necessary for its development.
On the other hand, in many of the cases, both positive and negative phototropism can be seen in the same organism.
For example, positive and negative phototaxis can be found in several species of Jellyfish such as those from the genus Polyorchis.
Jellyfish use their ocelli (light-sensitive organs) to detect the presence and absence of light. The ocelli contain photoreceptors that help them in their feeding behaviour in the case of the presence of light, and can also help them to avoid predation, and also to re-adjust toward a new light source.
Explained: Phototaxis in Plants
Plants or plant-like (not true plants) species can exhibit both positive and negative phototaxis. In fact, there’s no physical factor that regulates and stimulates the development of plants as extensively as the sunlight does.
Most probably, plants that have the chlorophyll-containing pigment in its cell shows positive phototaxis, and those that have no chlorophyll-containing pigment in a wide majority can show negative phototaxis.
Phototaxis is the orientation of free moving organisms to light. In plants, the free movement of the whole body is restricted to lower organisms, because only among them do we find freely moving organisms.
In botany, such free moving phototaxis has been reported in diatoms, blue-green algae, bacteria, desmids, and many flagellates.
And, its also seen in the various unicellular stages (that is in the zoospores and gametes) of many green algae and lower fungi.
In both higher and lower plants, only the orientation movement of chloroplasts inside the cell can be seen and not the whole body movement.
When the sunlight is present, plants tend to align their chloroplast pigment that is present in the cell towards the direction of sunlight. And as soon as the sun sets, that is when the illumination is discontinued, the chloroplasts arrange themselves by distributing itself around the whole cell.
Evolution of Phototaxis in Prokaryotes & Eukaryotes
1. Evolution of Phototaxis in Prokaryotes
Not all prokaryotes, but most of the prokaryotes are able to detect light and its intensity using their photoreceptor pigments.
As such, some of the prokaryotes can even sense light direction and make directed turns towards or away from the light, but their phototactic movement is very slow, maybe because the phototactic stimulation in this organism is low.
Some species among both Eubacteria and Archaebacteria (Archaea) are phototactic. For example, Halophilic archaebacteria, such as Halobacterium salinarum, use sensory rhodopsins (SRs) for phototaxis.
Some can show movement towards the direction of light, but only when they are present on the surface of the water and not in suspension. For example, Anabaena and Synechocystis show phototactic behaviour by orienting themselves in filaments or colonies towards the direction of light.
Most probably as the eukaryotes have evolved from the prokaryotes, so the above examples were just of some of the many prokaryotes that are phototactic and have evolved to the present day phototactic eukaryotes over the course of millions of years of evolution.
2. Evolution of Phototaxis in Eukaryotes
Unlike the prokaryotes, the eukaryotes are much more advanced and follow a more kind of complex mechanism of being phototactic in nature.
In fact, Eukaryotes are those that evolved for the first time in the history of life to have the ability to follow the light direction in three dimensions in the space around the open water.
They have more complex photoreceptor organs, organelles with highly advanced pigments like rhodopsin, chlorophyll, and more to name so far.
As compared to that of the prokaryotes, the eukaryotes do have better sensory integration, sensory processing, working mechanics, and better locomotory movement speed towards or away from the intensity of light.
Different species of eukaryotes ranging from one-celled ones to the multi-celled ones have a wide variety of photoreceptors.
But, the phototactic signaling can happen via. direct light-triggered ion currents, adenylyl cyclase cell signaling, or by trimeric G-proteins signaling.
Is Phototaxis innate or learned?
Innate behavior is the behavior that is inherited genetically in the organism from their parents as the required genes goes on passing from generation after generation.
Whereas, learned behaviour is also known as the acquired behavior as it is the one that is developed as a result of experience when the animals learn various things about its environment.
Phototaxis is an innate behaviour as the animals who are phototactic know all by themselves naturally that whether they have to change their locomotory movement away from or towards the response to a light stimulus, as soon as they are born.
If phototaxis would have been a learned behaviour than the young ones that are phototactic might not have developed into mature ones, or they might have even died if they didn’t properly learn the phototactic behaviour within days or even minutes after birth.
For example, Earthworms are negatively phototactic, that is they will move away from the light because the heat from the sun or a light source will dry out their skin and can kill them.
So, just think for a while. If the earthworms would have been negatively phototactic as a learned behavioral pattern, then they would have probably died after their birth if they would have taken a very long time to learn their negative phototaxis behaviour.
