Archaea

Archaea are a special group of tiny living things that scientists call a domain. They are different from bacteria and from the cells that make up most plants and animals, called eukaryotes. Archaea were first found in very tough places, like hot springs and salty lakes, where very few other life forms can survive. But later, scientists discovered that archaea are actually quite common and live almost everywhere — in soil, oceans, and even inside our own bodies.

These tiny organisms have some unique features. For example, their cell membranes are made differently from other life forms, using special kinds of fats. Some archaea can make their own food using chemicals instead of sunlight, while others can eat different substances like hydrogen gas. Many archaea live in harmony with other organisms. In our stomachs, certain archaea help break down food, showing that they can be very useful.

Archaea are important for the world’s ecosystems. They help control carbon and nitrogen in nature and support many communities of tiny life forms. Scientists also use archaea in technology, like making biogas from waste, because some archaea can survive very hot temperatures and harsh conditions. Even though we are just beginning to learn about them, archaea play a big role in life on Earth.

Discovery and classification

For much of the 20th century, tiny living things called prokaryotes were grouped together and sorted by looking at their chemistry, shapes, and what they eat. In 1965, scientists suggested using the instructions inside their genes to see how they are related. This idea is used a lot today.

Archaea were first set apart from bacteria in 1977 by scientists who studied their ribosomal RNA genes. They noticed these tiny creatures were different because they lacked certain materials in their outer layers and had unique traits. This led to a big change in how scientists group living things, splitting them into three main groups: Eukarya, Bacteria, and Archaea. The name "archaea" comes from ancient words meaning "ancient things," because early examples were thought to be very old and lived in extreme places like very hot or very salty spots. Later, scientists found archaea in many normal places too, showing just how common and varied they are.

Prokaryotic phyla

The following groups are types of archaea that scientists have officially recognized:

• Methanobacteriota
• Microcaldota
• Nanobdellota
• Promethearchaeota
• Thermoproteota

Some other groups have been suggested, but they still need more research to be officially recognized.

Origin and evolution

Further information: Timeline of evolution

The Earth is about 4.54 billion years old. Scientists believe life began on Earth at least 3.5 billion years ago. The earliest signs of life are tiny bits of carbon in very old rocks from Western Greenland and fossils of very simple living things in Western Australia.

Archaea are a group of very simple living things. They were once thought to be only one kind of very basic life, but we now know that they are very ancient and might be closely related to the ancestors of more complex life forms, including plants and animals. Scientists study archaea to learn about how life might have begun and evolved on Earth.

Archaea were recognized as a separate group because of differences in their cell structures, especially in a special kind of molecule called ribosomal RNA. This helped scientists see that archaea are different from both bacteria and more complex cells. Some archaea can live in very harsh places, like very hot or very salty water, because of special fats in their cell membranes. They also have unique ways of getting energy, such as making methane gas, which is important for the Earth's carbon cycle.

Morphology

Archaea are tiny living things that can be as small as 0.1 micrometers and as large as 15 micrometers across. They come in many shapes, such as spheres, rods, spirals, or flat plates. Some archaea have very unusual shapes, like square flat pieces or long, thin threads. These shapes are helped by their cell walls and special structures inside their cells.

Some archaea can stick together to form groups or long chains of cells. These groups can look like a string of pearls or form bush-like clusters. Scientists are still learning more about how these connected cells work together.

Structure, composition development, and operation

Archaea and bacteria look similar in their cell structure, but they differ in how their cells are built and organized. Like bacteria, archaea do not have internal membranes or organelles. They also usually have a cell wall and can move using structures called flagella. Most archaea have a single layer surrounding their cells, but one type, Ignicoccus, has an extra layer that contains small, membrane-bound bags.

Archaea often have a cell wall made from special proteins arranged in a pattern, which protects them. Unlike bacteria, archaea do not have a certain type of molecule in their walls. Their flagella work similarly to those of bacteria but are made from different materials and develop in a unique way. Archaea have special membranes made from molecules different from those in other living things. These membranes help protect archaea, especially in extreme conditions. Some archaea even have a single layer instead of a double layer in their membranes, which makes them very tough.

Metabolism

Archaea have many ways to get energy and nutrients. Some use simple chemicals like sulfur or ammonia, while others can use sunlight. Even though they use sunlight, they do not create oxygen like plants do.

Archaea share some basic ways of turning food into energy with all other living things. For example, they have a version of the process that turns sugar into energy. Some archaea can even make methane gas, especially in places without oxygen, like swamps. Others can take carbon from the air and turn it into food. All these different ways help archaea survive in many places on Earth.

Genetics

Further information: Plasmid and Genome

Archaea usually have one circular piece of DNA, called a chromosome, but some can have many copies. The largest known archaeal DNA was found in a microbe called Methanosarcina acetivorans, with over 5 million base pairs. On the smaller end, Nanoarchaeum equitans has a very tiny DNA piece, about 491,000 base pairs, and it is the smallest known archaeal DNA. Archaea also have smaller pieces of DNA called plasmids, which can move between cells through direct contact, similar to how some bacteria share DNA.

Archaea have unique patterns of DNA and proteins that set them apart from both bacteria and other complex cells. Up to 15% of the proteins in archaea are unlike those in other life forms, though many of these proteins’ roles are still unknown. The shared proteins among archaea, bacteria, and complex cells handle basic cell functions like building proteins and using energy. Archaea also group their DNA in special ways and have distinct features in the tiny structures that help read DNA. Their process of making proteins and handling DNA is more similar to complex cells than to bacteria, but it mixes features from both groups.

Reproduction

Further information: Asexual reproduction

Archaea make new organisms without help from others. They split into two parts, grow from pieces, or form small bumps that become new cells. They do not use the special splitting ways that plants and animals do. When an archaea cell wants to make a copy, it makes a copy of its instructions and then splits into two parts.

Some archaea have special ways to split that mix ideas from simple cells and more complex cells. Even though archaea are different from bacteria and plants and animals, they share some steps when they split. Some archaea can change their shape to survive in different water places, but these changes are not how they make new cells.

Behavior

Archaea, a type of tiny living thing, can talk to each other in special ways. Scientists once thought they couldn’t, but now they know some archaea can send and receive signals, much like how some bacteria do. This helps them work together when there are many of them close by.

Archaea can also live in groups called biofilms. These groups stick to surfaces and are protected by a slimy coating. This coating helps them stay safe, share helpful pieces of information, and work together. Making a biofilm takes several steps: sticking to a surface, forming small groups, building the coating, and sometimes leaving the group when they’re ready.

Ecology

Archaea live in many different places, from very hot to very cold, and from salty to fresh water. They are a big part of the tiny living things in the oceans, making up about 20% of all tiny cells there. Some archaea love very hot places, like geysers and deep-sea vents, where they can survive temperatures above 100 °C (212 °F). Others live in cold, salty, or very acidic water. Some archaea grow just fine in normal, mild conditions like swamps, soil, and even inside animals.

Archaea help nature by recycling important elements like carbon, nitrogen, and sulfur. They play a big role in the nitrogen cycle, helping to change nitrogen so plants can use it. In the sulfur cycle, they help release sulfur from rocks. And in the carbon cycle, some archaea help break down organic matter in places without oxygen, like marshlands and sewage treatment works.

Archaea also have interesting relationships with other living things. Some help other organisms by eating harmful waste products, which helps both stay healthy. Others simply live on or near other creatures without helping or hurting them. While archaea aren't usually known to make other organisms sick, some tiny archaea might live inside other cells and could be considered parasites.

Significance in technology and industry

Further information: Biotechnology

Some tiny living things called archaea can live in very hot or very acidic places. The special proteins they make can work in these tough conditions, and people use them in laboratories to study DNA more easily. These proteins are also used in food making, like creating milk without lactose.

Archaea can also help clean sewage and might be used to get valuable metals from rocks. Scientists are also studying proteins from archaea that could work as new medicines, different from the ones we usually use.