Melainabacteria

Melainabacteria is a phylum related to Cyanobacteria. Organisms belonging to this phylum have been found in the human gut and various aquatic habitats such as groundwater. By analyzing genomes of Melainabacteria, predictions are possible about the cell structure and metabolic abilities. The bacterial cell is similar to cyanobacteria in being surrounded by two membranes. It differs from cyanobacteria in its ability to move by flagella (like gram-negative flagella), though some members (e.g. Gastranaerophilales) lack flagella. Melainabacteria are not able to perform photosynthesis, but obtain energy by fermentation.

Classification

 * Class "Melainabacteria" Soo et al. 2014 ["Vampirovibrionia"; "Vampirovibrionophyceae" corrig. Strunecký & Mareš 2022 ] (ACD20)
 * Order "Caenarcanales" corrig. Soo et al. 2014 ["Caenarcaniphilales" Soo et al. 2014 ]
 * Family "Caenarcanaceae"
 * Genus "Ca. Caenarcanum" Soo et al. 2014
 * "Ca. C. bioreactoricola" Soo et al. 2014
 * Order "Obscuribacterales" Soo et al. 2014
 * Family "Obscuribacteraceae"
 * Genus "Ca. Obscuribacter" Soo et al. 2014
 * "Ca. O. phosphatis" Soo et al. 2014
 * Order "Vampirovibrionales" Soo et al. 2015
 * Family "Vampirovibrionaceae" Soo 2015
 * Genus Vampirovibrio Gromov & Mamkayeva 1972 ex Gromov & Mamkaeva 1980
 * V. chlorellavorus Gromov & Mamkayeva 1972 ex Gromov & Mamkaeva 1980
 * Order "Gastranaerophilales" Soo et al. 2014
 * Family CAJFVJ01
 * Genus "Ca. Adamsella" Glendinning et al. 2020
 * "Ca. A. avium" Glendinning et al. 2020
 * Family RUG14156
 * Genus "Ca. Galligastranaerophilus" Gilroy et al. 2021
 * "Ca. G. faecipullorum" Gilroy et al. 2021
 * "Ca. G. gallistercoris" Gilroy et al. 2021
 * "Ca. G. intestinavium" Gilroy et al. 2021
 * "Ca. G. intestinigallinarum" Gilroy et al. 2021
 * Family "Gastranaerophilaceae"
 * Genus "Ca. Avigastranaerophilus" Gilroy et al. 2021
 * "Ca. A. faecigallinarum" Gilroy et al. 2021
 * Genus "Ca. Gastranaerophilus" Soo et al. 2014
 * "Ca. G. phascolarctosicola" Soo et al. 2014
 * "Ca. G. termiticola" Utami et al. 2018
 * Genus "Ca. Limenecus" Gilroy et al. 2021
 * "Ca. L. avicola" Gilroy et al. 2021
 * Genus "Ca. Scatenecus" Gilroy et al. 2021
 * "Ca. S. faecavius" Gilroy et al. 2021
 * Genus "Ca. Spyradomonas" Gilroy et al. 2021
 * "Ca. S. excrementavium" Gilroy et al. 2021
 * Genus "Ca. Stercorousia" Gilroy et al. 2021
 * "Ca. S. faecigallinarum" Gilroy et al. 2021
 * Genus "Ca. Scatousia" Gilroy et al. 2021
 * "Ca. S. excrementigallinarum" Gilroy et al. 2021
 * "Ca. S. excrementipullorum" Gilroy et al. 2021

Ecological Niche
Melainabacteria can be found in a range of environments, including soil, water, and animal habitats. They can be often be found in the gut of humans and in the respiratory tract, oral environments, and skin surface, though rarely. Melainabacteria is often found in natural environments such as groundwater aquifers and lake sediment, as well as soil and bioreactors. Melainabacteria are also found in the aphotic zone of aquatic environments such as lake sediment and aquifers. Cyanobacteria bloom in freshwater systems as a result of excess nutrients and high temperatures, resulting in a scum on the water surface that resembles spilled paint. Because Melainabacteria and Cyanobacteria are related, it has raised concern because Melainabacteria thrive in groundwater systems. The genomes of Melainabacteria were found to be bigger when found in aquifer systems and algal cultivation ponds than when in the mammalian gut environment.

Origin
The Great Oxygenation Event (GOE) that occurred 2.4 billion years ago altered the course of life on Earth forever by increasing the abundance of oxygen in the atmosphere. Bacteria that existed before the GEO did not rely on the presence of oxygen as a source for metabolism, such as the billion-year-old Cyanobacteria. Melainabacteria is a close relative to Cyanobacteria, though Melainabacteria diverged and do not photosynthesize. Cyanobacteria produced atmospheric oxygen and supported the development of early plant cells.

Genome
The genomes of Melainabacteria organisms isolated from ground water indicate that the organism has the capacity to fix nitrogen. Melainabacteria lack linked electron transport chains but have multiple methods to generate a membrane potential which can then produce ATP via ATP synthase. They are able to use Fe hydrogenases for production that can be consumed by other  microorganisms. Melainabacteria from the human gut also synthesize several B and K vitamins, which suggests that these bacteria are beneficial to their host because they are consumed along with plant fibers.

Animal Habitats
Melainabacteria have been found to potentially play a role in digesting fiber in the human gut, and are more commonly in herbivorous mammals and those with plant-rich diets. Because plant diets require more fiber break-down, Melainabacteria may aid in this digestive function. However, scientists are unsure of why these microbes are in the gut and how they got there. Ongoing studies such as, "The human gut and groundwater harbor non-photosynthetic bacteria belonging to a new candidate phylum sibling to Cyanobacteria," funded by various organizations such as the National Institutes of Health, the David and Lucile Packard Foundation, The Hartwell Foundation, the Arnold and Mabel Beckman Foundation, the U.S. Department of Energy, the European Molecular Biology Organization and the Wellcome Trust.