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. But scientists later found that archaea are actually quite common. They live almost everywhere — in soil, oceans, and even inside our own bodies.

These tiny organisms have some unique features. 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. Others can eat different substances like hydrogen gas. Many archaea live together with other organisms. In our stomachs, certain archaea help break down food.

Archaea are important for the world’s ecosystems. They help control carbon and nitrogen in nature and support many 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 by looking at their 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 live 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

These are the types of archaea that scientists have officially recognized:

• Methanobacteriota
• Microcaldota
• Nanobdellota
• Promethearchaeota
• Thermoproteota

Some other groups may exist, but they need more research before they can 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 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 changed 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. They can be very small, about 0.1 micrometers, or a bit larger, up to 15 micrometers. They come in many shapes, like spheres, rods, spirals, or flat plates. Some archaea have unusual shapes, such as square pieces or long, thin threads. Their cell walls and special parts inside their cells help give them these shapes.

Some archaea can stick together to form groups or chains of cells. These groups might look like a string of pearls or 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 are built differently. Like bacteria, archaea do not have internal parts called organelles. They usually have a cell wall and can move using tiny hairs called flagella. Most archaea have one layer around their cells, but one type, Ignicoccus, has an extra layer with tiny bags inside it.

Archaea have cell walls made from special proteins that protect them. Unlike bacteria, archaea do not have a certain molecule in their walls. Their flagella work like bacteria’s but are made from different stuff. Archaea have unique membranes made from different molecules than other living things. These membranes help archaea survive in tough places. Some archaea have just one layer in their membranes instead of two, which makes them very strong.

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. Some archaea can have many copies of their DNA. The largest known archaeal DNA was found in a microbe called Methanosarcina acetivorans. On the smaller end, Nanoarchaeum equitans has a very tiny DNA piece.

Archaea also have smaller pieces of DNA called plasmids. These plasmids can move between cells through direct contact.

Archaea have unique patterns of DNA and proteins that make them different from bacteria and other complex cells. Many of their proteins have roles that are still being studied. Archaea, bacteria, and complex cells share some proteins that help with basic cell functions. Archaea also have special ways of grouping their DNA and unique features in the tiny structures that help read DNA. Their process of making proteins is more similar to complex cells than to bacteria.

Reproduction

Further information: Asexual reproduction

Archaea make new organisms without needing help from others. They split into two parts, grow from pieces, or form small bumps that turn into new cells. They do not use the same 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 are tiny living things that can talk to each other in special ways. Scientists used to think they couldn’t, but now they know some archaea can send and receive signals, like some bacteria do. This helps them work together when they are 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 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 an important part of the tiny living things in the oceans. 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 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.

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. People use these proteins 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 studying proteins from archaea that could work as new medicines.