By understanding the microorganisms that live beyond what the human eye can see, we can better understand our history as living beings. Amongst this microscopic world of organisms are Chlamydomonas.
Chlamydomonas is a genus of 325 species of unicellular green algae. The flagellates can be found living in droplets of water in freshwater, seawater, stagnant water, and even within moist soil. Chlamydomonas are studied as model creatures thanks to their unique flagellar movements and physiology.
The study of creatures like Chlamydomonas has been crucial in the medical field. Microbiology has played a large role in the development of medical technologies and treatments such as fluorescent fusion.
Structure of Chlamydomonas
Chlamydomonas make such a great model organism thanks to their anatomy and structure. Its haploidic vegetative growth allows for mutant phenotypes to express right away. They are also capable of growing so quickly that they can double once every eight hours. The Chlamydomonas’ motile cilia share many of the same structural elements and proteins as mammals, meaning that they are hugely useful in understanding cilium issues in humans.
Let’s take a closer look at the structure and anatomy of the Chlamydomonas:
- Cilium: A pair of flagella are on the anterior of the Chlamydomonas. They are typically 10-12 micrometers long. They are a greater length than the thallus and originated from a blepharoplast which comes through a small opening in the cell wall.
- Cell Wall: The exterior of the wall is smooth, thin, and contains cellulose. At the anterior end of the cell, the cell wall extends itself to make an apical papilla. The cell wall has many layers and is complex in structure.
- Plasma Membrane: Just inside the cell wall is where you’ll find the plasma membrane. In Chlamydomonas, the plasma membrane is divided by an opaque area. Its role is to move nutrients throughout the cell and get rid of toxic substances from within the cell. The membrane also provides rigidity and protection.
- Cytoplasm: Located in the middle of the cell wall and chloroplast, it’s a structure that houses the nucleus, mitochondria, endoplasmic reticulum, dictyosomes, ribosomes, and more. The dictyosomes and Golgi apparatus are located around the nuclear and have small vesicles.
- Golgi Body: Also known as the Golgi apparatus, is an organelle that assists with packaging proteins and lipids. It has a stacked appearance of its membranes.
- Nucleus: The singular nucleus is big and dark. It stores all of the genetic information of the cell.
- Contractile Vacuoles: Chlamydomonas have two contractile vacuoles, one at each flagellum. They assist with excretion and osmoregulation.
- Chloroplast: The Chlamydomonas’ chloroplast is big in size and cup-shaped. They can be other sizes and shapes depending on the species. In some species, the chloroplast is shaped like the letter ‘H’. The chloroplast is the organelle that helps convert sunlight into usable energy and sugars through photosynthesis.
- Pyrenoids: These help with the synthesis of starch in the Chlamydomonas. There are approximately two to six thylakoids that join within the chloroplast. The pyrenoid can be thought of as a small compartment inside of the chloroplasts that is unique to algae.
- Eyespot: Located at the anterior end of the Chlamydomonas, the eyespot (also known as the stigma) is uniquely colored orange. It is photoreceptive and helps direct the movement of the flagella. Two to three pigmented plates curve and help orientate the Chlamydomonas, which also contain carotenoids.
How Are Chlamydomonas Classified?
All living creatures can be biologically classified by a ranked system that groups structurally or phylogenetically related species together. The ranking system starts broadly at the top with six different kingdoms, the classification system works its way down to subkingdoms, infrakingdoms, divisions, subdivisions, classes, orders, families, genus, and species.
The Chlamydomonas are one of the largest genera of green algae with over 325 known species. Let’s take a look at how the Chlamydomonas are classified:
- Kingdom: Plantae – This group contains all plants of the world. They can be either eukaryotic, multicellular, autotrophic, or unicellular like the Chlamydomonas. Defining characteristics include non-motility, the ability to make their own food (mainly from photosynthesis like the Chlamydomonas), asexual vegetative propagation of sexual reproductive methods, and more.
- Subkingdom: Viridiplantae – This is a clade of over 500,000 species that is made up of mostly green algae like the Chlamydomonas. It reflects organisms with chloroplasts that contain green chlorophyll.
- Division: Chlorophyta – This taxon consists of green algae that are highly paraphyletic which include over 7,000 species. They are mostly aquatic and about 90% of all known species live in freshwater.
- Subdivision: Chlorophytina – A proposed basal Tetraphytina clade. It’s known as the sister clade of the Pedinomonadaceae.
- Class: Chlorophyceae – A class of green algae that is identified mainly by their ultrastructural morphology. Each clade within is defined by how their flagella are organized. Members of the CW clade have flagella organized in a clockwise manner while the DO clade have flagella organized in a counterclockwise manner. Defining characteristics include a discoid, cup-shaped, spiral, or ribbon-shaped chloroplast.
- Order: Chlamydomonadales – Also known as Volvocales, this is an order of flagellated green algae which varies from Gonium to Volvox. This order forms spherical colonies that are organized based on size. The Gonium represents four to 32 cells while the Volvox represents up to 500 cells. Defining characteristics include two distinct flagella that assist with coordinated movement of the colony.
- Family: Chlamydomonadaceae – A family of green algae that includes the Carteria, Chloromonas, Lobochlamys, Lobomonas, and more.
- Genus: Chlamydomonas – Spherical and unicellular organisms with cup-shaped green chloroplasts and two anterior flagella that are the same length.
Genetics of Chlamydomonas
Genetic research of the Chlamydomonas became a hot topic in 1930 thanks to Franz Moewus. Moewus was a German biologist who made highly controversial claims about the Chlamydomonas that were later determined to have been false. The controversial claims blasted a shockwave of misinformation about the Chlamydomonas throughout the scientific community that is still being unraveled to this day.
The claims which were later overturned revolve around sexual hormones as they relate to carotenoids. Moewus claimed that the carotenoids selectively activated male and female gametes. Most of Moewus’ career focused on analyzing 200,000 zygotes for 10 phenotypes over a decade. The findings from his analysis have never been reproduced, even by himself, and are at the forefront of the accusations that Moewus was not honest with his work. Despite the allegations of fraud, Moewus’ studies brought to light verifiable and important information.
For example, his work demonstrated that mutant strains of Chlamydomonas could be isolated. This supported his claims that genes could be lost in enzyme activity. The experiment was later replicated and confirmed by Gilbert Morgan Smith, a botanist from Wisconsin that focused on the study of algae.
Gilbert Morgan Smith’s Impact on Chlamydomonas
Gilbert Morgan Smith was nearly a high school dropout when he discovered his love for biology. His father was a professor in Beloit, Wisconsin, and focused on the study of Chemistry. Smith attended his father’s college and focused his studies on botany and chemistry.
He went on to complete his PhD and worked in the Botany Department where he focused primarily on algae. He was eventually invited to Stanford University as a Professor of Botany. He became a well-respected Emeritus Professor in 1950 and continued with his scientific studies until his death in 1959.
During his time as a professor, Smith focused on recreating Moewus’ experiments on Chlamydomonas. In fact, he was so committed to repeating Moewus’ experiments and studying the Chlamydomonas that he retired as the President of the Botanical Society of America to focus on his work. The botanist spent the next year cultivating and separating cultures into isolates. Many of the 700 cultures that he isolated were Chlamydomonas.
Smith was incredibly generous with his isolated cultures and shared them with other scientists throughout the world. He traded cultures with other scientists and became interested in sexual inheritance to mutate the Chlamydomonas. Some of the most studied Chlamydomonas in modern science was thanks to mutations that arose from Smith’s isolated cultures. The mutations were so effective that many credit Smith with laying the foundation for cilia biology and photosynthetic research. Smith worked with mutants with a paralyzed flagella and used this to demonstrate that cilia are expendable for inconsistency in genetic analysis in a lab setting.
Common Types of Chlamydomonas
Of the 325+ known species of Chlamydomonas, there are only a few that stand out that have been widely researched and used as model organisms. Let’s take a look at some of the few species of Chlamydomonas that there is literature about:
- Chlamydomonas acidophila: An acid-tolerant microalgae associated with freshwater habitats. This species is widely distributed throughout the world. They are studied for their metabolic adaptive properties.
- Chlamydomonas caudata: A freshwater species with an unusual arrangement of daughter cells with older and thicker cell walls.
- Chlamydomonas ehrenbergii: A species that is found in freshwater but can also tolerate marine environments. Observed in the United Kingdom.
- Chlamydomonas elegans: A freshwater species that was first discovered in 1915.
- Chlamydomonas moewusii: A unique species as it is mainly terrestrial living within moist soils. Used as a model organism for the study of hydrogen evolution in both dark and light environments.
- Chlamydomonas nivalis: A red-colored species found mainly in the snowfield of the Appalachian Mountains as well as polar regions. They are responsible for the phenomenon referred to as ‘watermelon snow’ where small areas of snow appear red or pink. Red snow has been observed for centuries and was even documented by Aristotle. These visible algal blooms have been widely studied and may contribute to the lowering of ice and snow albedo. It causes areas of snow to melt more rapidly because of this which is a concern to climate scientists as it becomes more and more prevalent. It’s used as a model species in studying how cells respond to stressful conditions in harsh habitats. It’s becoming one of the leading organisms for the research of cold-weather adaptation thanks to unique antioxidant capabilities, impressive repair mechanisms, and more.
- Chlamydomonas reinhardtii: About 10 micrometers in diameter, this species is the most widely known amongst the genera. It is used as a model organism to study how cells move, how they respond to light, how they recognize one another, how they generate flagellar waves in patterns, how they control their flagellar length, and how they respond to nutrition such as nitrogen and sulfur. They are additionally useful in studying photosynthesis and protein synthesis. A complete nuclear genome was sequenced in 2007. This species is additionally used as a clean source of hydrogen production and also used in the production of biopharmaceuticals across the globe.
How do Chlamydomonas Produce Energy
Chlamydomonas are a green alga with specialized green Chlorophyll that helps them convert sunlight into usable energy and sugars. This process is called photosynthesis. They are capable of making their own food from the sun but also need water, the right temperature, and nutrients to grow. Chlorophyll is used as the factory in which produces the sugars that are used for the energy of the cell.
Chlamydomonas also need carbon dioxide in order to make sugars. This makes Chlamydomonas effective in converting pollutants from car exhaust pipes for example into breathable oxygen. Green algae are often referred to as the ‘lungs of the earth’ as they are responsible for a large amount of the oxygen that we depend on as humans.
Photosynthesis starts with sunlight as it is absorbed within the cell walls of the chlorophyll. Carbon dioxide and water are used in conjunction to create glucose and oxygen. Oxygen is let out of the cell and into the atmosphere while the sugars are used for cellular activities such as growth and movement.
The oxygen that is produced during photosynthesis in green algae like Chlamydomonas actually helps the algae float to the surface of the water so that they can be closer to the sunlight. The Chlamydomonas contain an air-like bladder that fills up and helps them float.
Other important nutrients include carbon, nitrogen, and phosphorous. Chlamydomonas absorb these nutrients from their surrounding environment. This is why Chlamydomonas are most commonly found in stagnant water – it tends to be nutrient-rich. Carbon is stripped from the carbon dioxide that is breathed in and helps convert the sunlight into usable energy during photosynthesis.
Researchers have attempted to deprive the Chlamydomonas of carbon dioxide to see how they would react. To the researcher’s surprise, the Chlamydomonas began cannibalizing each other and other green algae to get ahold of their carbon. It is the first study of its kind on Chlamydomonas that demonstrates that an organism that is capable of undergoing photosynthesis using sunlight and carbon from other organisms instead of carbon dioxide. The ability to break down cellulose and take up their sugars is a hot topic in the biofuels industry. In cellulosic ethanol, pricey enzymes are needed to break down cellulose and turn them into sugars that are converted into ethanol. The Chlamydomonas are equally capable of converting these fats into fuel.
Where do Chlamydomonas Live?
Chlamydomonas are found evenly distributed throughout the entire globe. They live in freshwater, marine, terrestrial, and even extreme environments. They were first discovered in Europe and are heavily studied in the United Kingdom. Recent isolates come from North America and Japan.
The Chlamydomonas habitats range from humid, to dry, to saline. They have been found in moist soils and are fairly tolerant to pollutants. So much so that they have been found in sewage and other wastewaters. The algae are so tolerant that they can be windblown from one environment to the next at a high altitude and still survive.
Their phototaxis ability to move towards the light allows them to live in soil or other environments where bacteria are rampant. When suspended in the water, the bacteria sink to the bottom while the Chlamydomonas survive by swimming towards the light at the surface of the water.
How do Chlamydomonas Reproduce?
Chlamydomonas can reproduce either sexually or asexually depending on the environment:
Asexual Reproduction
Zoospores form in favorable conditions and is preferred. A protoplast constricts and leaves the cell wall. The Chlamydomonas either drops off or absorbs its flagella. Afterward, the contractile vacuoles disintegrate. The protoplasm then separates longitudinally through mitotic division and gives life to two new protoplasts.
Another longitudinal separation occurs at a right angle to the first separation. This results in four new chloroplasts. At this time, the pyrenoids also separate. Each new cell continues to develop a cell wall, its own two flagella, and ends up as a zoospore. The cell wall ruptures, and the new zoospore is freed. It is identical to the parent in structure but not as large. They grow larger as they mature. This process can occur once per 25 hours.
The Chlamydomonas can also reproduce by aplanospores in unfavorable conditions. In extremely unfavorable conditions, the Chlamydomonas can asexually reproduce by hypnospores. If the environment is short of water, salts, or other nutrients, then the Chlamydomonas may asexually reproduce in its palmella stage.
Sexual Reproduction
Chlamydomonas can be either isogamous, anisogamous, or oogamous depending on the environment. A majority of Chlamydomonas are isogamous, meaning that sexual reproduction occurs with gametes of a similar shape and size. This is common amongst unicellular organisms. Since both gametes are similar, they cannot be classified as male or female.
Anisogamy results in two gametes that are separate in size: one is larger, and one is smaller. The smaller of the two is typically a male sperm cell while the larger is typically an egg cell. This is most common amongst multicellular organisms. Oogamy is much less common and only occurs in a handful of Chlamydomonas species. In this case, the female egg cell is much larger than the male gamete and is non-motile.
The History of Chlamydomonas
It wasn’t until the mid-20th century that the green algae known as Chlamydomonas were first used as a model organism to help explain foundational cellular occurrences like photosynthesis and the many functions of the cilia. Although as a model organism is where the Chlamydomonas rose to fame, it was first described in 1883 by a German.
German naturalist Christian Gottfried Ehrenberg was the man who first described the genus Chlamydomonas. He was a zoologist, anatomist, geologist, and microscopist. He was one of the most famous of impactful scientists of his time. Ehrenberg was the son of a judge which afforded him the opportunity to study theology as an evangelist.
In 1820, Ehrenberg went on a scientific exploration of his own to the middle east. It was on this exploration that he collected hundreds and hundreds of samples of plants and animals. His trip extended from Egypt to the Libyan Desert, all the way to the Red Sea. When Ehrenberg returned from his trip, he began publishing detailed reports about the creatures that he discovered along the way.
During his time on the northern coast of the Red Sea, Ehrenberg became fascinated with insects and corals. For the next 30 years of his career, Ehrenberg focused on microscopic organisms. He wrote two volumes of Symbolae physicae that first described the genus Chlamydomonas.
Throughout his career, Ehrenberg is credited with over 40,000 microscopic preparations, 5,000 microorganism samples, 3,000 microorganism drawings, and almost 1,000 scientific letters and publications. Amongst this list of accomplishments was the discovery of the Chlamydomonas.
Ehrenberg’s earliest drawings of the Chlamydomonas were not complex. His original slides and samples of the Chlamydomonas are displayed at the Museum fur Naturkunde in Berlin to this day. Ehrenberg detailed two key properties of the genus: sexual reproductive abilities and phototaxis. At the time, unicellular algae were widely studied and intriguing thanks to their sexual reproductive abilities and life cycle.
Using a microscope, Ehrenberg was able to discover that gametes of the Chlamydomonas aggregate and fuse. This is the earliest evidence that proves the Chlamydomonas have two separate mating types. Phototaxis is how an organism responds to and either moves towards or away from light. Phototaxis behaviors were reported during the reproductive process.
References
- https://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=180784#null
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC396372/
- https://elifesciences.org/articles/39233
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6713297/
- https://www.jstor.org/stable/23328724?seq=1
- https://www.biologydiscussion.com/algae/life-cycle-algae/chlamydomonas-position-occurrence-and-structure-with-diagrams/21052