Which of These is Most Closely Related to S. solfataricus? A Guide to the Thermophilic Archaea

Sulfolobus solfataricus is a species of archaea that thrives in extreme environments, such as volcanic hot springs. It is a model organism for studying the molecular, genetic, and biochemical aspects of archaeal life. But what other organisms are closely related to S. solfataricus, and how do they compare? In this article, we will explore the diversity and evolution of the thermophilic archaea, and highlight some of their unique features and applications.

What are Thermophilic Archaea?

Thermophilic archaea are a group of microorganisms that belong to the domain Archaea, one of the three major branches of life, along with Bacteria and Eukarya. Archaea are often considered to be ancient and primitive, but they are actually very diverse and adaptable, and can inhabit a wide range of environments, from the deep sea to the human gut.

Thermophilic archaea are those that can grow at high temperatures, usually above 50 °C. They are found in geothermal areas, such as hot springs, geysers, and hydrothermal vents, where they utilize various sources of energy, such as sulfur, hydrogen, or organic compounds. Thermophilic archaea have evolved several adaptations to cope with the heat, such as having more stable proteins, membranes, and DNA.

How are Thermophilic Archaea Classified?

Thermophilic archaea are classified into two main phyla: Euryarchaeota and Thermoproteota. Euryarchaeota includes methanogens, halophiles, and thermoplasmatales, while Thermoproteota includes crenarchaeota and thaumarchaeota. Within these phyla, there are several orders, families, genera, and species of thermophilic archaea, each with their own characteristics and preferences.

One of the most well-studied thermophilic archaea is Sulfolobus solfataricus, which belongs to the order Sulfolobales, family Sulfolobaceae, and genus Sulfolobus. It was first isolated from the Solfatara volcano in Italy, and can grow at temperatures between 60 and 90 °C, and pH levels between 2 and 4. It has a spherical shape with frequent lobes, and a single circular chromosome of about 3 million base pairs. It can exchange genetic material through processes of transformation, transduction, and conjugation, and has a large proportion of archaeal-specific genes.

Which of These is Most Closely Related to S. solfataricus?

According to the Encyclopedia MDPI, the closest relative of S. solfataricus is Saccharolobus caldissimus, which was formerly classified as Sulfolobus caldissimus. It was transferred to a new genus, Saccharolobus, in 2018, along with S. solfataricus and other members of the Sulfolobus genus. Saccharolobus caldissimus was isolated from a hot spring in Japan, and can grow at temperatures up to 92 °C, and pH levels between 2 and 5. It has a similar genome size and structure to S. solfataricus, but differs in some metabolic and physiological traits.

Other closely related thermophilic archaea to S. solfataricus are those that belong to the same order, Sulfolobales, such as Acidianus, Metallosphaera, Stygiolobus, and Sulfurisphaera. These genera share some common features, such as having a circular chromosome, a high GC content, and a preference for acidic and sulfur-rich environments. However, they also have some differences, such as their optimal growth temperatures, their cell shapes, and their metabolic pathways.

Why are Thermophilic Archaea Important?

Thermophilic archaea are important for several reasons. First, they are essential for the biogeochemical cycles of elements, such as carbon, nitrogen, and sulfur, and contribute to the diversity and stability of the ecosystems they inhabit. Second, they are valuable sources of information for understanding the origin and evolution of life, as they may represent some of the earliest forms of life on Earth. Third, they are potential candidates for biotechnology applications, as they produce enzymes and proteins that are stable and functional at high temperatures, and can be used for various purposes, such as industrial, medical, and environmental.


Sulfolobus solfataricus is a thermophilic archaeon that lives in extreme environments, and is a model organism for studying the molecular, genetic, and biochemical aspects of archaeal life. It is closely related to other members of the Sulfolobales order, such as Saccharolobus caldissimus, which is its closest relative. Thermophilic archaea are important for the biogeochemical cycles of elements, the origin and evolution of life, and the biotechnology applications. They are fascinating and diverse microorganisms that deserve more attention and research.