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Single Celled Organisms

We are looking at single celled organisms. While they are often described as simple or basic organisms, this activity shows they can actually be quite complex. The purpose of this procedure is to expose you to a new vocabulary regarding cells while challenging any previous ideas you had about the complexity, size or function of cells.

To complete this activity, read each description provided for the different cells and fill in the provided chart. The name should have at least the scientific name and may include a common name. The size is listed for each cell. Important information is anything you consider to be important or defining about the cell. Where is it found? What is it’s source of genetic information? Are there any organelles listed? How does produce/acquire energy?

On the back of the page I want you to identify any shared characteristics, the different characteristics (life cycle, genetic information, ability to move), and write a brief paragraph explaining what you learned about single celled organisms. Remember, this is an introduction activity so keep it simple, keep it brief, but put the time in to learn something new.

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Single Celled Organisms

Station

#Cell Name Cell Size Important Info

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#1 Stentor

Photo credit: Protist Image DatabaseGrowing up to 2 milimeters long, the trumpet-shaped freshwater protozoa of the genus Stentor are easily visible to the naked eye and well-known among microbe enthusiasts for their size. 2 millimeters might not sound impressive, but remember that this makes Stentor larger than many multicellular invertebrates. Among unicellular organisms, it is an absolute colossus.One of the factors that enables Stentor to get so big is its internal anatomy. Unlike regular cells, Stentors (like most of the entries on this list) have more than one nucleus, the part of a cell that houses its DNA and acts as its control center. Having multiple nuclei seems to make it easier for bigger cells to properly manage their relatively large cellular bodies. Specifically in Stentor’s case, it has numerous small micronuclei that control reproduction and a single, giant, string-like macronucleus that manages its regular functions.Stentors are what biologists call a ciliate; they’re covered in fine, hair-like structures called cilia. Stentors and other ciliates use these to swim, beating them in unison to propel themselves, but that isn’t all cilia can do. While Stentors gain some nutrients from symbiotic algae that often lives inside them, they are primarily filter feeders. To catch food, Stentors anchor themselves to floating debris or sediment, unfold their trumpet-like “mouth,” and use a ring of modified feeding cilia to create a current that sucks in bacteria, smaller protists, and the occasional unlucky water flea.In other words, not only is the unicellular Stentor bigger than several multicellular animals, but it sometimes eats them.

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#2 Spirostomum

Photo credit: PicturepestWith the largest species growing up to 4 millimeters long, members of the worm-like Spirostomum genus dwarf their Stentor relatives. Found both in fresh and saltwater, it is often mistaken for a small worm. When viewed under a microscope, though, it becomes clear that it is in fact a single, really long cell.Despite its length, Spirostomum is also notable in the microbial world for its incredible shrinking ability. When it is disturbed, it can shrink down to a quarter of its original size in less than a hundredth of a second. This is the fastest-known contraction of any cell.Like Stentor, Spirostomum is a ciliate. The cilia are arranged in a spiral formation and both propel it forward and sweep bacteria into its small “mouth” along the side of its body. Also like Stentor, Spirostomum has one large macronucleus and multiple smaller micronuclei. This setup is largely unique to ciliates.They do differ from Stentor in terms of prey, though. While Stentors are big game hunters that can take down small multicellular life, Spirostomummostly sticks to bacteria.

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#3 Chaos Carolinensis

Photo credit: Dr. Tsukii YuujiPicture an amoeba. Now enlarge it to the size of a sesame seed. You have Chaos carolinensis. While their exact dimensions change with their shape, the largest individuals can stretch to 5 millimeters in length. It is so big that putting a cover slip on it under a microscope can harm it.Despite its large size, C. carolinensis behaves much the same way as a smaller amoeba would. It moves around using temporary gelatinous protrusions called pseudopods (Latin for “false foot”). It also uses these to feed. When it encounters prey, C. carolinensis literally engulfs it with its pseudopods and absorbs the prey in an internal, temporary cavity called a vacuole. There, the prey is digested alive, and the remains will eventually be expelled from the cell as waste. C. carolinensis feeds on other microbes as well as small invertebrates like water fleas or rotifers. It will continue feeding until it’s ready to reproduce.Like Stentor and Spirostomum, C. carolinensis has multiple nuclei, although they aren’t organized or specialized like in the other two. A single nucleus simply would not be able to control a cell this big. In fact, depending on its size, C. carolinensis can have up to 1,000 nuclei.Chaos carolinensis was subject to a decades-long naming controversy after its discovery, as scientists argued over how to classify it. For this reason, older sources referred to it by a variety of names, including Pelomyxa carolinensis and Chaos chaos. To avoid confusion, some writers simply introduced the protist as “the giant amoeba.”

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#4 Gromia Sphaerica

When researchers from the University of Texas dove to the seafloor off the Bahamas, they were baffled to find dozens of odd, grape-sized balls that, despite seeming motionless, had clearly left trails in the sand. Initial guesses ranged from a strange new type of snail to oddly-shaped fecal matter. However, upon closer examination, the truth turned out to be even weirder. The balls were actually giant, 3-centimeter (1.2 in) wide spherical protists that were rolling across the seabed at a near glacial pace.Gromia sphaerica, or the Bahamian Gromia, is what biologists call a testate amoeba. In other words, it is an amoeba-like creature that encases itself in a soft, porous shell called a test. By continuously sending out its thin pseudopods through holes in the test and grabbing onto the sea floor, the cell is able to slowly roll itself along the bottom, feeding on organic matter in the sediment as it goes.The discovery of this gentle giant of a protist had dramatic implications for scientists’ understanding of the evolutionary timeline. The earliest conclusive evidence for multicellular life dates back to 580 million years ago, but the discovery of fossilized tracks dating as far back as 1.8 billion years ago has led some scientists to push the starting date back to much earlier. Surely, they argued, no microbe could have produced them. Yet it turns out that those fossilized tracks bear a strong resemblance to those of G. sphaerica, meaning that its ancestors may have produced them. Thus, the earlier starting date for multicellular life seems much less likely.

Unfortunately, not much else is known about these rolling blobs of cytoplasm due to the difficulty of taking live samples. Despite having a kind of shell, they are squishy and fragile by our standards. Researchers have described them as softer than a grape.

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#5 Sailor’s Eyeball

Photo credit: Alexander VaseninSo far, all of the entries on this list have been “animal-like” protozoa, but in fact, there could be an entire list devoted to giant unicellular algae. Also known as bubble algae, Sailor’s Eyeball (Valonia ventricosa) easily grows to 4 centimeters (1.6 in) in diameter or more. Found in shallow tropical waters across the world, this marble-like protist is usually solitary but is sometimes found living in small clumps. Younger individuals have a beautiful translucent green color, but older ones are often encrusted with smaller types of algae and animals. In other words, Sailor’s Eyeball is so large that some multicellular life-forms actually live on it.

Although some admire it for its peculiar biology and exotic, gemstone-like appearance, Sailor’s Eyeball is best known as a despised pest for aquarium enthusiasts. Often accidentally introduced into tanks when owners bring in “live rocks” taken from the ocean, the algae goes on to overrun the tank, and killing or removing it is surprisingly difficult. Popping them is no use, either, since that’s actually how they reproduce.

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#6 Spiculosiphon Oceana

Photo credit: Silvia Garcia

With a maximum length of 5 centimeters (2 in), this strange aquatic protozoan has amazed scientists from the moment they first documented it. When divers first found it in 2013 in an underwater cave off the coast of Spain, they initially mistook it for a carnivorous sponge. (Yes, such sponges do exist.) However, this wasn’t the case.Spiculosiphon oceana belongs to a type of test-building amoeba called Foraminifera, but being a “testate amoeba” is about the only thing that it has in common with its not-so close relative Gromia sphaerica. Unlike the rolling, detritus-eating sea-grape, this one is fixed in place and is a filter feeder. To catch food, S. oceana simply extends its long, tentacle-like pseudopods through the pores in its test and lets them float in the water, trapping and digesting any plankton that gets ensnared. In this manner, S. oceana’s feeding strategy is remarkably similar to many marine invertebrates, including carnivorous sponges.For the very accomplishment of being a 5-centimeter-long unicellular organism, scientists named S. oceana one of the top 10 new species discovered in 2013.

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#7 Acetabularia

Photo credit: TigerenteAlso known as Mermaid’s Wineglass, Acetabularia is a unique genus of mushroom-shaped algae that grows up to 10 centimeters (4 in) in height. Found primarily living in clusters in shallow, rocky waters, it lives in subtropical waters around the world and they sometimes carpet large spots of seabed with their light green caps.Acetabularia significantly differs from the other entries on this list in terms of its internal composition. As discussed earlier, large unicellular organisms usually have more than one nucleus, and the number generally increases with size. Yet despite dwarfing all the previous entries, Acetabularia spends most of its life with only a single, giant nucleus located at the base of its “stem.” The only exception is when it is about to reproduce. At this point, the nucleus undergoes multiple rounds of division, and the daughter nuclei travel up to the cell’s top frond. There, they bud off into numerous spore-like reproductive cysts, ready to spread and give rise to new Acetabularia.The cell’s large size combined with its reliance of a single nucleus gave it a key role in the advancement of cellular biology. In a set of experiments during the 1930s and 1940s, German scientist Joachim Hammerling (whose work was funded by the Nazis) proved that the nucleus was the control center of a cell by grafting together the caps and nuclei of two species of Acetabularia. He found that the cell would take on the characteristics of whichever species its nucleus came from.

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#8 Syringammina Fragilissima

Photo credit: NOAA

The largest member of the Xenophyophore class (example pictured above), which already is known for producing unicellular giants, this giant amoeboid creature dwells at the bottom of the ocean and can grow up to 20 centimeters (8 in) in diameter. Like most of its relatives, the cell does not produce its own test but instead constructs it from the remains of smaller microorganisms and sponges. It glues these together with a slimy excretion to form a complex network of delicate tubes, which serve as the Amoeba’s home.Unfortunately, we still know very little about Syringammina fragilissima. Scientists suspect that it feeds on bacteria, but they don’t know how it does so. Guesses range from filter feeding to farming them inside its shell. Scientists aren’t even sure how S. fragilissima reproduces. Part of the issue is the creature’s deep-sea habitat, but it also has to do with its extremely delicate nature. Its scientific name means “very fragile sand pipe.”

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#9 Plasmodial Slime Molds

Photo credit: John Carl Jacobs

Originally classified as a type of fungus, plasmodial slime molds, also known as Myxomycetes, are an unusual category of unicellular life that blur the boundary between an individual organism and a group of them. Like all slime molds, they start life out as tiny, amoeba-like microbes that live in the dirt much like a regular single-celled organism, munching on bacteria. Under certain conditions though, something changes. The individual cells congregate together and begin to combine until they have merged into one colossal blob. Although most slime molds remain small by our standards even in this form, a few can grow to more than 1 meter (3 ft) in diameter, if not more.

Now living as a single organism, the slime mold will start to crawl across the ground at a glacial pace, consuming whatever food or unfortunate bacteria falls into its path. In essence, it acts like a giant amoeba and is capable of navigating around obstacles and sensing the best food sources from afar. This phase continues until it has eaten enough. At that point, the slimd mold will stop moving, produce fruiting bodies, and release spores to start the cycle anew.

But wait. If it originated from individual cells gathering together, then isn’t the slime mold not technically unicellular? Nope. Plasmodial slime molds truly are unicellular. Unlike the so-called “cellular slime molds,” in which the cells retain their distinct membranes, plasmodial slime mold cells fuse completely, dissolving the membranes separating one another and becoming a single, gargantuan cell with millions of nuclei.

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#10 Caulerpa Taxifolia (Aquarium Strain)

Photo credit: NOAAConsisting of a long string of fern-like fronds, this type of unicellular algae is a

giant even among its family of fellow macroscopic unicellular algae. In the

Mediterranean, where it thrives best, it can reach a total length of nearly 3

meters (10 ft). Caulerpa taxifolia is so large, so structurally complex, and so

multicellular-looking that some sources simply forget to mention that it is

actually all one, unfathomably long cell with countless nuclei and other parts

floating within.

C. taxifolia is not native to the Mediterranean, however, nor does it normally

even come close to this size in its natural tropical habitat. Instead, the colossal

Mediterranean variant is the result of human interference, somewhat like the

Africanized killer bee. Attractive and easy to care for, C. taxifolia lends itself

toward use in aquarium display tanks, and in the 1970s, a German aquarium

acquired some of the algae in order to breed it for this exact purpose.

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Exposing their C. taxifolia to harsh chemicals and mutation-inducing UV

light, the staff selectively cultivated it to be even hardier, faster growing, and

most importantly, better able to grow in colder water. Finally, in 1980, they

were satisfied, and in an act of generosity, they distributed the finished

product to other aquariums across Europe.

Four years later, the inevitable happened. Some of the cold-water strain

“escaped” from an aquarium in Monaco. Within years, it had overrun the

Mediterranean. Compared to its natural ancestor, the mutant strain is bigger,

grows faster and more aggressively, can survive pollution, and is capable of

regenerating from fragments as small as 1 centimeter (2.1). It’s also toxic.

Eradication efforts have failed, and the only question is how to keep it from

spreading even farther.

Because of the ecological devastation it has brought, C. taxifolia earned the

nickname “killer algae,” along with a place on the Global Invasive Species

Specialist Group’s list of the 100 worst invasive species.

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Daniel Chitwood, Ph.D., assistant member, and his research group at the Donald Danforth Plant Science Center's in St. Louis, in collaboration with the laboratory of Neelima Sinha, Ph.D., at the University of California, Davis, are using the world's largest single-celled organism, an aquatic alga called Caulerpa taxifolia, to study the nature of structure and form in plants. They have recently reported the results of their work in the online journal, PLOS Genetics.

"Caulerpa is a unique organism," said Chitwood. "It's a member of the green algae, which are plants. Remarkably, it's a single cell that can grow to a length of six to twelve inches. It independently evolved a form that resembles the organs of land plants. A stolon runs along the surface that the cell is growing on and from the stolon arise leaf-like fronds, and root-like holdfasts, which anchor the cell and absorb phosphorus from the substrate. All of these structures are just one cell."

"For many years, I've been interested in structure and form in plants, especially in tomato, which is the land plant that I've studied most," Chitwood continued. "As you might imagine, finding out what determines structure and form in a complex tomato plant is a challenging goal. It's critical to know how plants grow and develop to provide more tools to improve them and ultimately to make food production more reliable. Multicellularity is an important prerequisite that enables complex architectures in crops. Yet Caulerpa is a plant, too, and independently evolved a land plant-like body plan, but without multicellularity and as a single cell. How does that happen?"

Chitwood and his group reasoned that the structure of Caulerpa might be reflected in the RNA's present in various parts of the cell. (RNA's are the molecular products found when genes are expressed or "turned on.") For example, the frond part of the cell might show different RNA's from the holdfast part of the cell. When performed on Caulerpa, this type of analysis would also provide insights into the distributions of RNA's within single cells,

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a feat normally difficult to achieve because cells in multicellular organisms are so small.

"The result turned out to be even more interesting than we'd hoped," said Chitwood. "Not only do different parts of the Caulerpa cell show distinctly different RNA's, but there is also some correlation between RNA's that are expressed together within different parts of the Caulerpa cell with those expressed together in the multicellular organs of tomato. Even though the lineage that Caulerpa belongs to probably separated from that giving rise to land plants more than 500 million years ago, in many ways Caulerpa displays patterns of RNA accumulation shared with land plants today."

"Our work on Caulerpa has given me and my team a whole new way of thinking about plant structure and development," Chitwood continued enthusiastically. "It's clear that the basic form we associate with land plants can arise with and without multicellularity. In fact, higher plant cells are connected to each other by means of channels called plasmodesmata, and it has been argued that multicellular land plants exhibit properties similar to single-celled organisms like Caulerpa. What if we could really think of higher plants, like tomato, as one cell instead of multitudes? This idea of thinking of multicellular land plants, like tomato, and giant single-celled algae, like Caulerpa, similarly is supported by our results that demonstrate a shared pattern of RNA accumulation. Frankly, our results have caused us to think about plant structure from an entirely different perspective, which is the most important outcome from this research."

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#11 Mycoplasma

More than 150,000 of these tiny bacterial cells could fit onto the tip of a human hair. (Berkeley Lab)

For 20 years, scientists have debated the existence of super-small bacteria. The theoretically tiny organisms presented a big problem to researchers—they were delicate, hard to capture and simply too small to see. But that all changed this week, when researchers announced that they were able to capture the first-ever detailed microscopy images of the miniature life forms.

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An international team of scientists led by researchers from the U.S. Department of Energy’s Lawrence Berkeley National Laboratory and the University of California, Berkeley, theorized that ultra-small bacteria are actually quite common. But in order to capture images of the organisms, they had to capture the microbes first—and that proved no easy feat.

Using a series of tiny filters, they filtered groundwater down to a size of 0.2 microns. Then they flash-froze their delicate samples to -272°C and used a “cryo plunger” to transport the samples from the field to the lab. The team used state-of-the-art electron microscopes to capture an image of the super-small bacteria within before sequencing the organism’s genome. (They found about one million base pairs of DNA.)

So what did they see? Cells so tiny, 150,000 of them could fit on the tip of a single human hair. The team described the cells in a release:

They’re also quite odd, which isn’t a surprise given the cells are close to and in some cases smaller than several estimates for the lower size limit of life. This is the smallest a cell can be and still accommodate enough material to sustain life. The bacterial cells have densely packed spirals that are probably DNA, a very small number of ribosomes, hair-like appendages, and a stripped-down metabolism that likely requires them to rely on other bacteria for many of life’s necessities.

Though researchers believe the microbes are relatively common, several questions remain unanswered. How do the microbes, which lack many basic functions, work with other organisms? What purpose do the genes serve? And the existence of these infinitesimal organisms begs an even bigger question—are there even smaller life forms 0ut there? Those questions may remain open for now, but one thing is clear: we’ve come a long way since the first electron micrograph of an intact cell, which was photographed in 1945.

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