Archaea

Archaea are a fascinating group of tiny living things that are different from both plants and animals. They belong to one of the three main groups, or domains, of life on Earth, along with bacteria and eukaryotes. Even though they look similar to bacteria, archaea have special features that set them apart. For example, their cell membranes are made from unique materials, and they can survive in some of the most extreme places on the planet, like very hot springs and very salty lakes.

Scientists first thought archaea were just a type of bacteria, but we now know they are their own group. They can use many different ways to get energy, eating things like sugars or even using gases such as hydrogen. Some archaea can even turn sunlight into energy, similar to how plants do. These tiny organisms are very important for the environment. They help clean up the planet by recycling nutrients and even live inside our bodies, helping us digest food.

Archaea are found almost everywhere — in soil, oceans, and even in the human gut. Because they can survive in tough conditions, scientists study them to learn about life in extreme places, and they use enzymes from archaea in many technologies, like making clean energy from waste. Even though we have only recently learned about them, archaea play a big role in keeping Earth healthy and balanced.

Discovery and classification

For much of the 20th century, tiny living things without cells called prokaryotes were grouped together and sorted by looking at their chemistry, shapes, and the foods they could use. In 1977, scientists Carl Woese and George E. Fox studied these tiny beings and found that some were very different from others. They looked especially at the genes inside these organisms, particularly the ribosomal RNA (rRNA) genes, and discovered a whole new group they called Archaea. This helped scientists divide all living things into three big groups: Eukarya, Bacteria, and Archaea.

Archaea were first found in very harsh places like hot springs and salty lakes, leading people to think they only lived in extreme conditions. But by the end of the 20th century, scientists realized archaea are found everywhere — in normal places like soil and water too. This discovery was made possible by a technique called polymerase chain reaction (PCR), which lets scientists find and study these tiny beings even when they can't grow them in a lab. Today we know archaea are a large and diverse group all around us.

Prokaryotic phyla

Archaea are a group of very small, simple organisms. Some groups of these organisms have been officially named and recognized by scientists. These include Methanobacteriota, Microcaldota, Nanobdellota, Promethearchaeota, and Thermoproteota.

Other groups have been suggested but not yet officially named. These include "Aenigmatarchaeota", "Altarchaeota", "Augarchaeota", "Geoarchaeota", "Hadarchaeota", "Hadesarchaeota", "Huberarchaeota", "Hydrothermarchaeota", "Iainarchaeota", "Micrarchaeota", "Nanohalarchaeota", "Nezhaarchaeota", "Parvarchaeota", "Poseidoniota", and "Undinarchaeota".

Origin and evolution

Further information: Timeline of evolution

The Earth is about 4.54 billion years old, and scientists believe life began here at least 3.5 billion years ago. Early signs of life include tiny fossils and special rocks found in places like Western Greenland and Western Australia.

Archaea are a special group of very old organisms. They were once thought to be simple, but we now know they are closely related to the complex cells that make up plants and animals. Researchers discovered this by studying tiny parts inside these organisms, called ribosomal RNA, which helps make proteins. This showed that archaea are different from bacteria and share more in common with complex cells. Some archaea can even make methane gas, which is important for nature’s balance.

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, like spheres, rods, spirals, or even flat squares. Some archaea, like those in Haloquadratum walsbyi, live in very salty water and look like flat, square pieces.

These archaea can also form groups or chains of cells that stick together. For example, some archaea connect using long, thin tubes, while others form colonies that look like strings of pearls. These shapes help scientists study how these ancient organisms live and grow.

Structure, composition development, and operation

Archaea and bacteria look similar in their basic cell shape, but they differ in important ways. Like bacteria, archaea do not have internal membranes or organelles. They also usually have a cell wall and can move using structures called flagella. However, archaea stand out because of their unique cell walls and membranes.

Archaea often have a cell wall made of special proteins arranged in a pattern called an S-layer, which protects the cell. Their flagella, known as archaella, work similarly to bacterial flagella but are made from different materials and put together in a different way. Archaea have special membranes made from unusual fatty molecules. These molecules are different from those in bacteria and other organisms, which helps archaea survive in extreme environments. Some archaea even have a single layer of membrane instead of the usual double layer, making their cells very tough.

Metabolism

Further information: Microbial metabolism

Archaea have many different ways to get energy and nutrients, called metabolic reactions. Some archaea get energy from simple chemicals like sulfur or ammonia, which are called chemotrophs. In these reactions, one chemical gives electrons to another, releasing energy that the cell uses to make adenosine triphosphate (ATP), the cell's energy source.

Other archaea use sunlight for energy, called phototrophs, but they do not produce oxygen like plants do. Many archaea share basic steps for breaking down food, such as a modified form of glycolysis. Some archaea, called methanogens, live in places without oxygen, like swamps, and produce methane as they break down materials. Others can take carbon from the air and turn it into food through a process called carbon fixation.

Genetics

Further information: Plasmid and Genome

Archaea usually have a single circular piece of DNA called a chromosome, but some can have more than one copy. The biggest archaeal chromosome found so far has over 5 million base pairs, while the smallest has around 491,000 base pairs and contains very few genes. Small pieces of DNA called plasmids can also be found in archaea and can move between cells.

Archaea have unique genes that make them different from bacteria and eukaryotes. They share many important genes involved in basic cell functions with both bacteria and eukaryotes. Archaea also have special ways of organizing their genes and unique types of proteins. Their process of making proteins and copying DNA is a mix of bacterial and eukaryotic methods.

Reproduction

Further information: Asexual reproduction

Archaea reproduce without needing another organism. They can split into two parts, break into pieces, or grow buds to make new cells. They do not use mitosis or meiosis, which means all the new cells have the same genetic material as the parent. When a cell is ready to divide, it copies its chromosome, and then the cell splits into two.

Some archaea, like those in the group Sulfolobus, have a mix of features from both bacteria and eukaryotes in how they copy their DNA and divide. Certain archaea can change their shape and grow in different ways, which helps them survive in different environments, but these changes are not for making new cells.

Behavior

Archaea, like many tiny living things, can talk to each other in a special way called quorum sensing. This helps them share important messages when there are many of them close together. They also work well with bacteria to build groups called biofilms.

Biofilms are like sticky homes that protect Archaea from harm. They build these homes in steps: first, they stick to a surface, then they make a sticky matrix to hold together, and finally, some leave when they are done. These biofilms help them survive and share useful information.

Ecology

Archaea live in many different places, from very hot to very cold, and from salty to fresh water. They are found in oceans, swamps, soil, and even inside animals. Some archaea love very hot places, like geysers, while others can live in very salty or very acidic water.

Archaea help control important elements like carbon, nitrogen, and sulfur. They play a big role in the nitrogen cycle, helping to change nitrogen into forms that plants can use. In the sulfur cycle, they help release sulfur from rocks. In the carbon cycle, some archaea help break down dead material in places without oxygen.

Significance in technology and industry

Further information: Biotechnology

Some tiny living things called archaea, especially those that love very hot or very acidic places, make special proteins called enzymes that work in these tough conditions. These enzymes are very useful. For example, they help scientists copy DNA quickly in a process called the polymerase chain reaction. They are also used in food production to make milk without lactose, and they can help create environmentally friendly chemicals.

Archaea can also help clean sewage and may be used to get valuable metals from rocks. Scientists are also excited about special antibiotics from archaea that could work in new ways against bad germs.