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Nanobdellati

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Nanobdellati is a unique kingdom of archaea that was first proposed in 2013. These organisms are often called nanoarchaea or ultra-small archaea because of their nanometric size. They are significantly smaller than most other types of archaea. This tiny size is directly linked to their very small genomes. A genome is the complete set of genetic instructions a cell needs to function. Because their genomes are so small, these microbes have limited catabolic capacities. This means they have a restricted ability to break down substances for energy.

Many members of Nanobdellati survive by being episymbionts. An episymbiont is an organism that lives on the surface of another organism. These microbes often depend on a symbiotic or parasitic association to stay alive. They lack the central biosynthetic pathways needed to make nucleotides, amino acids, and lipids. Because they cannot make these essential building blocks, they must rely on other microbes. For example, the ARMAN archaea depend on neighbors to meet their biological requirements. However, some Nanobdellati can live freely. These free-living forms are typically fermentative and aerobic heterotrophs. Most of the group remains anaerobic, meaning they live without oxygen, and they are difficult to cultivate in laboratories.

Scientists often use the acronym "DPANN" to describe this group. This name comes from the initials of the first five groups discovered. These include Diapherotrites, Parvarchaeota, Aenigmarchaeota, Nanoarchaeota, and Nanohaloarchaeota. Over time, more groups have been added to this superphylum. Woesearchaeota and Pacearchaeota were discovered and proposed later. In 2017, the phylum Altiarchaeota was also placed within the DPANN superphylum. The classification of these groups is based on the List of Prokaryotic names with Standing in Nomenclature (LPSN) and the NCBI.

The different phyla within Nanobdellati live in many different environments. Their lifestyles and metabolisms vary widely. Nanoarchaeota were the first to be discovered in 2002. They were found in a hydrothermal source near the coast of Iceland. These organisms live as symbionts of other archaea. Parvarchaeota and Micrarchaeota were discovered in 2006. They live in acidic mine drainage from a mine in the United States. These are provisionally called ARMAN, which stands for Archaeal Richmond Mine acidophilic nanoorganisms. Other groups like Nanohalarchaeota thrive in environments with high salinity.

Finding these microbes is a difficult task for researchers. They are rarely found in the open ocean or on the ground. Some have been detected in nitrate-rich groundwater. Others have been found on the water surface, but not below it. Aenigmarchaeota have been located in wastewater from mines and hot spring sediments. Woesearchaeota and Pacearchaeota are found in the sediments and surface waters of lakes and aquifers. These groups often abound in saline conditions. This wide distribution shows how diverse the kingdom can be.

There is a major scientific debate regarding the phylogeny of Nanobdellati. Phylogeny is the study of the evolutionary history and relationships of organisms. Some analyses suggest Nanobdellati might be the first divergent clade of archaea. However, other studies suggest they belong to the Euryarchaeota. It is even possible that they are polyphyletic. A polyphyletic group is one where the members do not share a single common ancestor. Instead, they might occupy various different positions within the Euryarchaeota. This uncertainty exists because these phyla have a high mutation rate.

This high mutation rate can cause a scientific error called long branch attraction (LBA). LBA is an artifact where lineages are grouped together artificially at the base of a phylogenetic tree. This happens even if the lineages are not actually related. Because of this, the exact placement of Nanobdellati remains a subject of intense study. Scientists continue to use genomic comparisons to understand these evolutionary origins. The relationship between Nanobdellati and other groups, such as the Altiarchaeota, is still being debated. Understanding these tiny organisms helps us learn more about the diversity of life on Earth.

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