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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.
Members of this phylum are physiologically diverse, and can be found in a variety of environments including soil, decomposing wood,[1] hot springs, oceans, caves, and metal-contaminated soils.[2] The members of this phylum are particularly abundant in soil habitats representing up to 52% of the total bacterial community.[3] Environmental factors such as pH and nutrients have been seen to drive Acidobacteria dynamics.[4][5][6] Many Acidobacteria are acidophilic, including the first described member of the phylum, Acidobacterium capsulatum.[7]
Other notable species are Holophaga foetida,[8] Geothrix fermentans,[9] Acanthopleuribacter pedis[10] and Bryobacter aggregatus.[11] 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.[12] However, these bacteria may be an important contributor to ecosystems, since they are particularly abundant within soils.[13] Members of subdivisions 1, 4, and 6 are found to be particularly abundant in soils.[14]
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.[15]
Members of subdivision 1 have been found to dominate in low pH conditions.[4][16] 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,[17] potentially due to cell specialization and enzyme stability.[4]
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.[12]
The majority of Acidobacteria are considered aerobes.[18][19] There are some Acidobacteria that are considered anaerobes within subdivision 8[9] and subdivision 23.[20] 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.[19]
Members of the Acidobacteria phylum have been considered oligotrophic bacteria due to high abundances in low organic carbon environments.[4] However, the variation in this phylum may indicate that they may not have the same ecological strategy.[4]
The first species, Acidobacterium capsulatum, of this phylum was discovered in 1991.[21] However, Acidobacteria were not recognized as a novel division until 1997,[7] and were not recognized as a phylum until 2012.[22] First genome was sequenced in 2007.[23]
Some members of subdivision 1 are able to use D-glucose, D-xylose, and lactose as carbon sources,[4] but are unable to use fucose or sorbose.[24] Members of subdivision 1 also contain enzymes such as galactosidases used in the breakdown of sugars.[4] Members of subdivision 4 have been found to use chitin as a carbon source.[4][25][26]
There has been no clear evidence that Acidobacteria are involved in nitrogen-cycle processes such as nitrification, denitrification, or nitrogen fixation.[4] However, Geothrix fermantans was shown to be able to reduce nitrate and contained the norB gene.[4] The NorB gene was also identified in Koribacter verstailis and Solibacter usitatus.[4][27] In addition, the presence of the nirA gene has been observed in members of subdivision 1.[4] Additionally, to date, all genomes have been described to directly uptake ammonium via ammonium channel transporter family genes.[4][19] Acidobacteria can use both inorganic and organic nitrogen as their nitrogen sources.
The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature (LSPN)[28] and the National Center for Biotechnology Information (NCBI).[29]
Phylogeny of Acidobacteria using 16S rRNA (LTP release 132[30])
Phylogeny of Acidobacteria (Annotree v1.2.0,[31][32] GTDB 05-RS95[33][34])