Amoebas are crafty, shape-shifting engineers | Science News for Students

Amoebas are crafty, shape-shifting engineers

They can change form, build shells, punch holes in prey — even farm their own food
Jan 17, 2019 — 6:45 am EST
A microscopic image of a spiky round orange amoeba snacking on an algal cell.

An amoeba (in orange) named Vampyrella lateritia finishes eating the contents of an algal cell (green). Though often overlooked, amoebas have an amazing range of unusual behaviors.

Sebastian Hess

In 2009, biologist Dan Lahr received an intriguing email from another researcher. It included a photo of a strange organism. The researcher had discovered the microbe in a floodplain in central Brazil. Its yellowish-brown shell had a distinctive, triangle-like shape.

The shape reminded Lahr of the wizard’s hat in The Lord of the Rings movies. “That’s Gandalf’s hat,” he remembers thinking.

Lahr is a biologist at the University of São Paulo in Brazil. He realized the one-celled life form was a new species of amoeba (Uh-MEE-buh). Some amoebas have a shell, as this one did. They may build those shells out of molecules they make themselves, such as proteins. Others may use bits of material from their environment, such as minerals and plants. Still other amoebas are “naked,” lacking any shell. To learn more about the newfound amoeba, Lahr would need more specimens.

a microscopic image of a brown amoeba that is shaped a lot like a wizard hat
Researchers discovered a new amoeba species in Brazil. Its shape resembles the hat worn by the wizard Gandalf in The Lord of the Rings movies.
D. J. G. Lahr, J. Féres

Two years later, another Brazilian scientist sent him pictures of the same species from a river. But the bonanza came in 2015. That’s when a third scientist emailed him. This researcher, Jordana Féres, had collected a few hundred of the triangular amoebas. It was enough for her and Lahr to begin a detailed study of the species.

They examined the microbes under a microscope. The amoeba, they found, built its hat-shaped shell from proteins and sugars that it made. The big question is why the microbe needs that shell. Perhaps it offers protection from the sun’s harmful ultraviolet rays. Lahr named the species Arcella gandalfi (Ahr-SELL-uh Gan-DAHL-fee).

Lahr suspects many more amoeba species await discovery. “People are just not looking [for them],” he says.

Scientists still know little about amoebas. Most biologists study organisms that are either simpler or more complex. Microbiologists, for instance, often focus on bacteria and viruses. Those microbes have simpler structures and can cause disease. Zoologists prefer to study larger, more familiar animals, such as mammals and reptiles.

Amoebas have largely “been ignored,” notes Richard Payne. He is an environmental scientist at the University of York in England. “They’ve been sort of caught in the middle for a long time.”

But when scientists do peer at these odd little organisms, they find big surprises. Amoebas’ foods range from algae to brains. Some amoebas carry bacteria that protect them from harm. Others “farm” the bacteria they like to eat. And still others may play a role in Earth’s changing climate.

What’s on the menu? Fungi, worms, brains

Though you can’t see them, amoebas are everywhere. They live in soil, ponds, lakes, forests and rivers. If you scoop up a handful of dirt in the woods, it will probably contain hundreds of thousands of amoebas.

But those amoebas may not all be closely related to one another. The word “amoeba” describes a wide variety of single-celled organisms that look and behave a certain way. Some organisms are amoebas for only part of their lives. They can switch back and forth between an amoeba form and some other form.

Like bacteria, amoebas have just one cell. But there the similarity ends. For one thing, amoebas are eukaryotic (Yoo-kair-ee-AH-tik). That means their DNA is packed inside a structure called a nucleus (NEW-klee-uhs). Bacteria have no nucleus. In some ways, amoebas are more similar to human cells than to bacteria.

Also unlike bacteria, which hold their shape, shell-free amoebas look like blobs. Their structure changes a lot, Lahr says. He calls them “shape-shifters.”

Their blobbiness can come in handy. Amoebas move using bulging parts called pseudopodia (Soo-doh-POH-dee-uh). The term means “false feet.” These are extensions of the cell’s membrane. An amoeba can reach out and grab some surface with a pseudopod, using it to crawl forward.

a microscopic image of a Chaos amoeba, a large orange unicellular organism with several "arms" reaching out in all directions
Amoebas come in many shapes. This one belongs to the genus Chaos.
Ferry J. Siemensma

Pseudopodia also help amoebas eat. A stretched-out pseudopod can engulf an amoeba’s prey. That allows this microbe to swallow bacteria, fungal cells, algae — even small worms.

Some amoebas eat human cells, causing sickness. In general, amoebas don’t cause as many human diseases as bacteria and viruses do. Still, some species can be lethal. For example, a species known as Entamoeba histolytica (Ehn-tuh-MEE-buh Hiss-toh-LIH-tih-kuh) can infect human intestines. Once there, “they literally eat you,” Lahr says. The disease they cause kills tens of thousands of people each year, mostly in areas that lack clean water or sewer systems.

The most bizarre illness caused by an amoeba involves the species Naegleria fowleri (Nay-GLEER-ee-uh FOW-luh-ree). Its nickname is the “brain-eating amoeba.” Very rarely, it infects people who swim in lakes or rivers. Once inside the nose, it can travel to the brain where it feasts on brain cells. This infection is usually deadly. The good news: Scientists know of only 34 U.S. residents who became infected between 2008 and 2017.

A tiny can opener

A scientist named Sebastian Hess recently discovered the tricks some amoebas use to eat. He studies eukaryotic microbes in Canada at Dalhousie University. That’s in Halifax, Nova Scotia. Hess has loved watching tiny critters through a microscope since he was a kid.

Ten years ago, Hess punched through the ice of a frozen pond in Germany. He collected a sample of water and took it back to his lab. Through the microscope, he saw something odd. Green spheres were wiggling inside strands of green algae, like tiny bubbles. He had “no idea” what the spheres were.

So Hess mixed the algae containing the green balls with other algae. The wiggling spheres popped out of the algae and started swimming. Then they invaded other algal strands.

an animated gif showing a lot of small green spheres inside a long rounded cylindrical algal cell. The green spheres are wiggling.
The wiggling green spheres are organisms called Viridiraptor invadens. They spend part of their lives as amoebas. Here they have taken over an algal cell.
S. Hess

Hess realized that the green spheres were microbes called amoeboflagellates (Uh-MEE-buh-FLAH-juh-laytz). That means they can switch between two forms. In one form, they swim or glide using tail-like structures called flagella (Fluh-JEH-luh). When the swimmers find food, they transform into amoebas. Their shape becomes less rigid. Then they begin crawling along a surface instead of swimming.

Through the microscope, Hess watched one of these amoebas cut a hole in an algal cell. The amoeba squeezed inside. Then it ate the alga’s innards. Afterward, the amoeba divided and made copies of itself. Those were the wiggling green spheres that Hess had seen earlier. The new amoebas punched more holes in the algal cell. Some invaded the neighboring cell in the algal strand. Others escaped. Hess named the species Viridiraptor invadens (Vih-RIH-dih-rap-ter in-VAY-denz).

He found a similar species in a bog. Also an amoeboflagellate, it didn’t crawl inside algae. Instead, it cut a C-shaped gash in an algal cell. Hess likens this amoeba to “a can opener.” The amoeba then lifted the “lid” and used its pseudopod to reach into the hole. It gobbled up the material it pulled out of the cell. Hess named this species Orciraptor agilis (OR-sih-rap-ter Uh-JIH-liss).

an animated gif showing an amoeba breaking into an algal cell and slurping up the green innards
An Orciraptor agilis amoeba slurps up the innards of an algal cell.
S. Hess

More recently, he discovered clues to how these two amoeboflagellates hack into algae. Both seem to get help from a protein called actin (AK-tihn). Human cells use the same actin protein to move.

In amoeboflagellates, actin forms a mesh. It helps the cell make a pseudopod. The mesh might also help the pseudopod latch onto algae. Actin can connect to other proteins in the microbe’s cell membrane that might attach to the walls of algal cells. Actin may even help guide other proteins — enzymes — that can cut into algal cell walls.

Results from studies by Hess and his colleagues suggest that these seemingly simple amoebas may be far more advanced than they first seemed. One might even consider them one-celled engineers. “In terms of their behavior,” Hess says, “they are just super-complex organisms.”

Bacterial buddies

The relationship between amoebas and bacteria is even more complicated.

Debra Brock is a biologist at Washington University in St. Louis, Mo. She studies an amoeba called Dictyostelium discoideum (Dihk-tee-oh-STEE-lee-um Diss-COY-dee-um). Many simply refer to them as Dicty. These soil-dwelling organisms dine on bacteria.

Dicty usually live solo. But when food is scarce, tens of thousands may merge, clumping into a dome. Usually, the dome morphs into a slug-like shape. This slug — really thousands of individual amoebas moving together — crawls toward the soil surface.

a photo of several long white shiny blobs show Dicty "slugs" moving through dirt
Tens of thousands of Dicty amoebas can join up to form a “slug” that can crawl through the soil.
Tyler J. Larsen/Wikimedia Commons (CC BY-SA 4.0)

Once it gets there, the slug forms a mushroom shape. Amoebas at the top of the “mushroom” surround themselves with a hard coat. This coated form is known as a spore. Insects, worms or larger animals that brush against these spores may unknowingly transport them to new places. Later, the spores will crack open, allowing the amoebas inside the coat to strike out in search of food at this new site.

Some Dicty bring bacteria along for food. They carry the bacteria inside themselves without digesting them. It’s “like a lunch box,” Brock explains. To do this, the amoebas get help from a different group of bacteria that they can’t eat. These helper microbes also live in the amoebas. The helpers prevent the food bacteria from being digested so that the amoebas can save them for later.

a photo of biologist Debra Brock on the forest floor putting soil into a small plastic vial
Biologist Debra Brock collects soil samples in Virginia. She is hoping to find the amoeba Dictyostelium discoideum, also known as Dicty. Some Dicty “farm” bacteria that they eat.
Joan Strassmann

Scientists call the bacteria-carrying amoebas “farmers.” Researchers suspect that when the amoebas reach a new home, they spit the food bacteria out into the soil. Those bacteria then divide to make more bacteria. It’s like the amoebas are carrying seeds and planting them to grow more food.

Recently, researchers discovered that the amoeba slug protects itself with special cells while it’s traveling. These cells are also Dicty amoebas. Known as sentinel cells, they mop up bacteria and toxic substances that could harm the other amoebas. When that’s done, the slug leaves its sentinels behind.

Brock wondered what that finding meant for Dicty farmers. The farmers wouldn’t want sentinel cells to kill their bacterial food. So did farmers have fewer sentinel cells than non-farmers?

To find out, Brock’s team let amoeba slugs form in the lab. Some slugs were all farmers. Others were all non-farmers. Researchers dyed the sentinel cells, then let the slugs move across a lab dish. Afterward, the researchers counted how many sentinel cells had been left behind. As expected, farmer slugs had fewer sentinel cells.

The scientists wondered if this put farmers at greater risk from toxic chemicals. To test that, Brock exposed farmers and non-farmers to a toxic chemical. The farmers could still reproduce. In fact, they fared better than non-farmers.

Brock now thinks that some of the bacteria carried by the farmers helped to fight off the toxic chemicals. These bacteria might break down the chemicals. So farmers have two weapons against toxic threats: sentinel cells and bacterial buddies.

A link to climate change?

a photo of Arcella dentata, a testate amoeba, a round orange shape with many pointy projections
Testate amoebas have shells. This species, Arcella dentata, builds a crown-shaped shell.
Ferry J. Siemensma

Hess and Brock study naked amoebas. Payne is intrigued by those with shells. Called testate (TESS-tayt) amoebas, these crafty microbes can fashion many types of shells. Those coverings can resemble discs, bowls — even vases. Some are “fantastically beautiful,” Payne says.

Many testate amoebas live in habitats called peat bogs. These sites are usually soggy and acidic. But during summers, the peat can dry out. Payne thinks shells might protect a bog’s amoebas during these droughts.

Not just curiosities, these peat-dwelling amoebas may play an important role in the environment, Payne says. Partly decayed plants build up in peat bogs. Bacteria eat those plants, releasing carbon-dioxide gas. In the atmosphere, that greenhouse gas can foster global warming. Bog amoebas eat these bacteria. So in that way, a bog’s amoebas may influence how big a role peatlands play in global warming.

Payne and his colleagues studied one peat bog in China where a wildfire had burned. Wildfires may become more frequent as the climate warms. So the scientists wanted to know how fire affected the bog’s testate amoebas.

Payne’s Chinese colleagues took samples from burned and unburned parts of the bog. Then the team analyzed differences between two types of testate amoebas. One makes its shell out of debris, such as sand grains and bits of plants. The other type builds a glassy shell using a mineral called silica.

In unburned patches, the scientists found similar numbers of both types of amoebas. But burned patches contained many more amoebas with shells made of sand and debris. The findings suggest that the fire had destroyed more of the amoebas with silica shells.

Payne doesn’t yet know what that means for climate change. It’s not clear whether the shift in amoebas will cause peat bogs to release more or less carbon. The process is “hugely complicated,” he says.

Many other details about amoebas remain unknown. How many species exist? Why do some have shells? How do amoebas affect the numbers of other microbes in some parts of the environment? How do they influence the ecosystem around them, such as plants?

Scientists have enough questions about amoebas to occupy themselves for a long time. That’s partly why researchers such as Payne find these organisms so intriguing. Plus, he says, “They’re just really cool.”

A photo of a peat bog. Up close grass grows in wet mud and grass stretches out to the horizon where it meets some trees.
Peat bogs contain many testate amoebas. Climate change may alter the number and types of amoebas that live there. And changes in the bogs’ amoeba populations may have a feedback on climate; they may change how much carbon dioxide is emitted by decaying peat.
R. Payne

Power Words

(more about Power Words)

acidic     An adjective for materials that contain acid. These materials often are capable of eating away at some minerals such as carbonate, or preventing their formation in the first place.

actin     A protein that can form long chains in cells. In muscle cells, these chains help the muscle contract. Actin chains also help cells move.

algae     Single-celled organisms, once considered plants (they aren’t). As aquatic organisms, they grow in water. Like green plants, they depend on sunlight to make their food.

amoeba     A single-celled microbe that catches food and moves about by extending fingerlike projections of a colorless material called protoplasm. Amoebas are either free-living in damp environments or they are parasites.

atmosphere     The envelope of gases surrounding Earth or another planet.

bacteria     (singular: bacterium) Single-celled organisms. These dwell nearly everywhere on Earth, from the bottom of the sea to inside other living organisms (such as plants and animals).

bacterial     Having to do with bacteria, single-celled organisms. These dwell nearly everywhere on Earth, from the bottom of the sea to inside animals.

behavior     The way something, often a person or other organism, acts towards others, or conducts itself.

biology     The study of living things. The scientists who study them are known as biologists.

bog     A type of wetland that forms peat from the accumulation of dead plant material — often mosses.

carbon     The chemical element having the atomic number 6. It is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules.

cell     The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall. Depending on their size, animals are made of anywhere from thousands to trillions of cells. Most organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell.

cell membrane     A structure that separates the inside of a cell from the outside of it. Some particles are permitted to pass through the membrane.

chemical     A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.

climate     The weather conditions that typically exist in one area, in general, or over a long period.

climate change     Long-term, significant change in the climate of Earth. It can happen naturally or in response to human activities, including the burning of fossil fuels and clearing of forests.

colleague     Someone who works with another; a co-worker or team member.

debris     Scattered fragments, typically of trash or of something that has been destroyed. Space debris, for instance, includes the wreckage of defunct satellites and spacecraft.

DNA     (short for deoxyribonucleic acid) A long, double-stranded and spiral-shaped molecule inside most living cells that carries genetic instructions. It is built on a backbone of phosphorus, oxygen, and carbon atoms. In all living things, from plants and animals to microbes, these instructions tell cells which molecules to make.

drought     An extended period of abnormally low rainfall; a shortage of water resulting from this.

ecosystem     A group of interacting living organisms — including microorganisms, plants and animals — and their physical environment within a particular climate. Examples include tropical reefs, rainforests, alpine meadows and polar tundra. The term can also be applied to elements that make up some an artificial environment, such as a company, classroom or the internet.

engineer     A person who uses science to solve problems. As a verb, to engineer means to design a device, material or process that will solve some problem or unmet need.

environment     The sum of all of the things that exist around some organism or the process and the condition those things create. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature and humidity (or even the placement of things in the vicinity of an item of interest).

enzymes     Molecules made by living things to speed up chemical reactions.

flagella     (sing. flagellum)   A thread-like structure that comes out of certain types of cells. The term derives from the Latin word for whip. And that's because the structures serve like oars to help these cells travel. 

floodplain     The nearly flat land that runs along the side of a river, for some distance out from the water. When the river floods, it spills over into this plain, which is built up, over time, with the silt left as the waters recede. That silt tends to be soil that eroded off of the upstream lands during rains.

global warming     The gradual increase in the overall temperature of Earth’s atmosphere due to the greenhouse effect. This effect is caused by increased levels of carbon dioxide, chlorofluorocarbons and other gases in the air, many of them released by human activity.

greenhouse gas     A gas that contributes to the greenhouse effect by absorbing heat. Carbon dioxide is one example of a greenhouse gas.

habitat     The area or natural environment in which an animal or plant normally lives, such as a desert, coral reef or freshwater lake. A habitat can be home to thousands of different species.

infection     A disease that can spread from one organism to another. It’s usually caused by some type of germ.

innards     Slang term for internal organs, such as the stomach and intestines.

insect     A type of arthropod that as an adult will have six segmented legs and three body parts: a head, thorax and abdomen. There are hundreds of thousands of insects, which include bees, beetles, flies and moths.

link     A connection between two people or things.

literally     A term that the phrase that it modifies is precisely true. For instance, to say: "It's so cold that I'm literally dying," means that this person actually expects to soon be dead, the result of getting too cold.

mammal     A warm-blooded animal distinguished by the possession of hair or fur, the secretion of milk by females for feeding their young, and (typically) the bearing of live young.

membrane     A barrier which blocks the passage (or flow through) of some materials depending on their size or other features. Membranes are an integral part of filtration systems. Many serve that same function as the outer covering of cells or organs of a body.

microbe     Short for microorganism. A living thing that is too small to see with the unaided eye, including bacteria, some fungi and many other organisms such as amoebas. Most consist of a single cell.

microbiology     The study of microorganisms, principally bacteria, fungi and viruses. Scientists who study microbes and the infections they can cause or ways that they can interact with their environment are known as microbiologists.

microscope     An instrument used to view objects, like bacteria, or the single cells of plants or animals, that are too small to be visible to the unaided eye.

mineral     Crystal-forming substances that make up rock, such as quartz, apatite or various carbonates. Most rocks contain several different minerals mish-mashed together. A mineral usually is solid and stable at room temperatures and has a specific formula, or recipe (with atoms occurring in certain proportions) and a specific crystalline structure (meaning that its atoms are organized in regular three-dimensional patterns). (in physiology) The same chemicals that are needed by the body to make and feed tissues to maintain health.

morph     Short for metamorphosis, it means to change from one form to another (such as from a caterpillar to a butterfly) or from one shape to another. Or it can mean to evolve or mutate, where one or more parts of the genome undergo some sort of change in their chemistry — and potentially in their function.

Naegleria fowleri     A single-celled freshwater parasite, sometimes called the “brain-eating amoeba.” It lives in hot springs and other surface waters that get very warm.

nucleus     Plural is nuclei. (in biology) A dense structure present in many cells. Typically a single rounded structure encased within a membrane, the nucleus contains the genetic information.

organism     Any living thing, from elephants and plants to bacteria and other types of single-celled life.

peat     Largely decomposed plant material that develops in the absence of oxygen within a water-saturated site, such as a bog. When dried out, peat can be burned as a low-grade fuel.

peer     (noun) Someone who is an equal, based on age, education, status, training or some other features. (verb) To look into something, searching for details.

prey     (n.) Animal species eaten by others. (v.) To attack and eat another species.

protein     A compound made from one or more long chains of amino acids. Proteins are an essential part of all living organisms. They form the basis of living cells, muscle and tissues; they also do the work inside of cells. Among the better-known, stand-alone proteins are the hemoglobin (in blood) and the antibodies (also in blood) that attempt to fight infections. Medicines frequently work by latching onto proteins.

pseudopod     An arm-like extension of the cell membrane in eukaryotic cells, including amoebas. The cell can use a pseudopod to move or engulf food.

range     The full extent or distribution of something. For instance, a plant or animal’s range is the area over which it naturally exists.

sentinel     (in ecology) Species that scientists monitor to get information about the environment in which those organisms live. These species might be more sensitive to some environmental hazards, and so can indicate to researchers when those hazards are reaching dangerous levels.

sewer     A system of water pipes, usually running underground, to move sewage (primarily urine and feces) and stormwater for collection — and often treatment — elsewhere.

silica     A mineral, also known as silicon dioxide, containing silicon and oxygen atoms. It is a basic building block of much of the rocky material on Earth and of some construction materials, including glass.

species     A group of similar organisms capable of producing offspring that can survive and reproduce.

spore     A tiny, typically single-celled body that is formed by certain bacteria in response to bad conditions. Or it can be the single-celled reproductive stage of a fungus (functioning much like a seed) that is released and spread by wind or water. Most are protected against drying out or heat and can remain viable for long periods, until conditions are right for their growth.

toxic     Poisonous or able to harm or kill cells, tissues or whole organisms. The measure of risk posed by such a poison is its toxicity.

ultraviolet     A portion of the light spectrum that is close to violet but invisible to the human eye.

Citation

Journal: J.C. Féres et al. Morphological and morphometric description of a novel shelled amoeba Arcella gandalfi sp. nov. (Amoebozoa: Arcellinida) from Brazilian continental waters. Acta Protozoologica. Vol. 55, 2016, p.221. doi: 10.4467/16890027AP.16.021.6008.

Journal: S. Hess and M. Melkonian. The mystery of clade X: Orciraptor gen. nov. and Viridiraptor gen. nov. are highly specialised, algivorous amoeboflagellates (Glissomonadida, Cercozoa). Protist. Vol. 164, September 2013, p. 706. doi: 10.1016/j.protis.2013.07.003.

Journal: A. Busch and S. Hess. The cytoskeleton architecture of algivorous protoplast feeders (Viridiraptoridae, Rhizaria) indicates actin-guided perforation of prey cell walls. Protist. Vol. 168, February 2017, p. 12. doi: 10.1016/j.protis.2016.10.004.

Journal: D.A. Brock et al. Sentinel cells, symbiotic bacteria and toxin resistance in the social amoeba Dictyostelium discoideum. Proceedings of the Royal Society B. Vol. 283, April 20, 2016. doi: 10.1098/rspb.2015.2727.

Journal: Y. Qin et al. Short-term response of testate amoebae to wildfire. Applied Soil Ecology. Vol. 116, August 2017, p. 64. doi: 10.1016/j.apsoil.2017.03.018.