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Acidobacteria is a phylum of bacteria. Its members are physiologically diverse and ubiquitous, especially in soils, but are under-represented in culture.

Description
Members of this phylum are physiologically diverse, and can be found in a variety of environments including soil, hot springs, oceans, caves, and metal-contaminated soils. The members of this phylum are particularly abundant in soil habitats representing up to 52% of the total bacterial community. Environmental factors such as pH and nutrients have been seen to drive Acidobacteria dynamics. Many Acidobacteria are acidophilic, including the first described member of the phylum, Acidobacterium capsulatum.

Other notable species are Holophaga foetida, Geothrix fermentans, Acanthopleuribacter pedis and Bryobacter aggregatus. Since they have only recently been discovered and the large majority have not been cultured, the ecology and metabolism of these bacteria is not well understood. However, these bacteria may be an important contributor to ecosystems, since they are particularly abundant within soils. Members of subdivisions 1,4,and 6 are found to be particularly abundant in soils.

As well as their natural soil habitat, unclassified subdivision 2 Acidobacteria have also been identified as a contaminant of DNA extraction kit reagents, which may lead to their erroneous appearance in microbiota or metagenomic datasets.

Members of subdivision 1 have been found to dominate in low pH conditions. Additionally, Acidobacteria from acid mine drainage have been found to be more adapted to acidic pH conditions (pH 2-3) compared to Acidobacteria from soils, potentially due to cell specialization and enzyme stability.

The G+C content of Acidobacteria genomes are consistent within their subdivisions - above 60% for group V fragments and roughly 10% lower for  group  III  fragments.

The majority of Acidobacteria are considered aerobes. There are some Acidobacteria that are considered anaerobes within subdivision 8 and subdivision 23. It has been found that some strains of Acidobacteria originating from soils have the genomic potential to respire oxygen at atmospheric and sub-atmospheric concentrations.

Members of the Acidobacteria phylum have been considered oligotrophic bacteria due to high abundances in low organic carbon environments. However, the variation in this phylum may indicate that they may not have the same ecological strategy.

History
The first species, Acidobacterium capsulatum, of this phylum was discovered in 1991. However, Acidobacteria were not recognized as a novel division until 1997, and were not recognized as a phylum until 2012.

Carbon
Some members of subdivision 1 are able to use D-glucose, D-xylose, and lactose as carbon sources, but are unable to use fucose or sorbose. Members of subdivision 1 also contain enzymes such as galctosidases used in the breakdown of sugars. Members of subdivision 4 have been found to use chitin as a carbon source.

Nitrogen
There has been no clear evidence that Acidobacteria are involved in N-cycle processes such as nitrification, denitrification, or nitrogen fixation. However, Geothrix fermantans was shown to be able to reduce nitrate and contained the norB gene. The NorB gene was also identified in Koribacter verstailis and Solibacter usitatus. In addition, the presence of the nirA gene has been observed in members of subdivision 1. Additionally, to date, all genomes have been described to directly uptake ammonium via ammonium channel transporter family genes. Acidobacteria can use both inorganic and organic nitrogen as their N sources.

Phylogeny
The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature (LPSN) and the phylogeny is based on 16S rRNA-based LTP release 123 by The All-Species Living Tree Project.

Notes:

♠ Strains found at the National Center for Biotechnology Information (NCBI) but not listed in the List of Prokaryotic names with Standing in Nomenclature (LSPN)