Thermoproteota

Thermoproteota are a major phylum within the domain Archaea. Archaea are single-celled organisms that are distinct from bacteria and eukaryotes. For a long time, scientists believed these organisms were only extremophiles. An extremophile is a life form that thrives in very harsh conditions. Most known members were thought to be sulfur-dependent. This means they rely on sulfur to survive in extreme environments. However, recent studies have changed this view significantly. Scientists found specific ribosomal RNA sequences in many different places. These sequences suggest that Thermoproteota may be the most abundant archaea in the ocean.

These organisms show a wide variety of physical forms. They are morphologically diverse, meaning they come in many different shapes. Some cells are shaped like tiny balls, known as cocci. Others look like long rods or thin filaments. Some even have very odd, irregular shapes. When scientists use a Gram stain, these cells appear Gram negative. This is a method used to classify bacteria and archaea based on their cell walls. While they were once thought to lack certain proteins called histones, some members actually possess them. This diversity makes them a complex group to study.

One of the most famous members is Sulfolobus solfataricus. This organism was first found in geothermally heated sulfuric springs in Italy. It is a thermophile, which means it loves heat. It can grow in temperatures as high as 80 °C. It also prefers very acidic environments with a pH between 2 and 4. Unlike many other heat-loving organisms, it grows aerobically. This means it uses oxygen to live. It is also a chemoorganotroph. This means it gains its energy from organic sources like sugars. Because it is easier to grow in a lab, it is a model organism. Scientists use it to study hyperthermophiles and the viruses that infect them.

Sulfolobus solfataricus has a unique way of protecting its genetic code. When these cells are exposed to ultraviolet light, they react. The light induces the formation of type IV pili. These are tiny, hair-like structures on the cell surface. These pili cause the cells to undergo cellular aggregation. This means the cells clump together in groups. When they clump, they can exchange chromosome markers at a very high frequency. In fact, recombination rates can be three orders of magnitude higher than normal. This process helps them repair damaged DNA. Scientists view this as a primitive form of sexual interaction.

Our understanding of where these organisms live has expanded greatly. Before 1992, we mostly knew them from hot places. Since 1992, data has shown they are present in marine environments. Scientists can identify them in the open ocean by looking at their lipids. Lipids are the fats that make up their cell membranes. These specific membrane lipids are used in a measurement called TEX-86. These measurements show that Thermoproteota are major contributors to carbon fixation. Carbon fixation is the process of turning inorganic carbon into organic matter. They are also found in soil and freshwater, making them ubiquitous. Ubiquitous means they are found almost everywhere.

In 2005, a major discovery changed our view of their temperature limits. Scientists published evidence of the first cultured "low temperature" members. One such organism is named Nitrosopumilus maritimus. It was isolated from a marine aquarium tank. Unlike its heat-loving relatives, it grows at 28 °C. It is an ammonia-oxidizing organism. This means it gets energy by processing ammonia. This discovery proved that the phylum is not limited to extreme heat. It can thrive in much milder, everyday environments as well.

Thermoproteota are also deeply connected to the history of all life. In 1984, James A. Lake proposed the eocyte hypothesis. He suggested a group called Kingdom Eocyta. This group included both crenarchaea and asgards. Although the name Eocyta was later discarded, the core idea remains important. Recent DNA analysis from 2008 and 2017 supports this connection. The data suggests that eukaryotes evolved from Thermoproteota-like organisms. Eukaryotes are the complex cells that make up plants, animals, and humans. This implies that complex life may have started from these simple prokaryotes. This remains a highly debated and active area of scientific research.