The Thermomicrobia is a group of thermophilic green non-sulfur bacteria. Based on species Thermomicrobium roseum (type species) and Sphaerobacter thermophilus, this bacteria class has the following description:
The class Thermomicrobia subdivides into two orders with validly published names: Thermomicrobiales Garrity and Holt 2001 and Sphaerobacterales Stackebrandt, Rainey and Ward-Rainey 1997. Gram negative. Pleomorphic, non-motile, non-spore-forming rods. Non-sporulating. No diamino acid present. No peptidoglycan in significant amount. Atypical proteinaceous cell walls. Hyper-thermophilic, optimum growth temperature at 70-75 °C. Obligatory aerobic and chemoorganotrophic.
As thermophilic bacteria, members of this class are usually found in environments which are distant from human activity. However, they have features like improved growth in antibiotics and CO oxidizing activity, making them interesting topics of research (e.g. for biotechnology application).
History
In 1973, a strain of rose-pink thermophilic bacteria was isolated from Toadstool Spring in Yellowstone National Park, which was later named Thermomicrobium roseum and proposed as a novel species of the novel genus Thermomicrobium. At that time the genus was categorized under family Achromobacteraceae, but it became a distinct phylum by 2001.
In 2004, it was proposed, on the basis of an analysis of genetic affiliations, that the Thermomicrobia should more properly be reclassified as a class belonging to the phylum Chloroflexota (formerly Chloroflexi). The bacteria Sphaerobacter thermophilus originally described as an Actinobacteria is now considered a Thermomicrobia. In the same year, another strain of rose-pink thermophilic bacteria was isolated from Yellowstone National Park, which was named Thermobaculum terrenum. Later analysis based on genome put this species under Thermomicrobia class. However, the current standing of Thermobaculum terrenum is disputed.
In 2012, a thermo-tolerant nitrite-oxidizing bacterium was isolated from a bioreactor, which was named Nitrolancetus hollandica and proposed as a novel species later in 2014. While it has nitrite-oxidizing activity, which is unique in the Thermomicrobia class, it is placed under the Thermomicrobia class based on 16s rRNA phylogeny.
In 2014, two thermophilic, Gram-positive, rod-shaped, non-spore-forming bacteria (strains KI3T and KI4T) isolated from geothermally heated biofilms growing on a tumulus in the Kilauea Iki pit crater on the flank of Kilauea Volcano (Hawai'i) were proposed as representatives of new species based on 16s rRNA phylogeny. The KI3T strain, later named as Thermomicrobium carboxidum, is closely related to Thermomicrobium roseum. The KI4T strain, later named as Thermorudis peleae, was proposed as a type strain of new genus Thermorudis.
In 2015, a thermophilic bacteria strain WKT50.2 isolated from geothermal soil in Waitike (New Zealand) was proposed to be a novel species, later named Thermorudis pharmacophila. Phylogenic analysis based on 16s rRNA place it within Thermomicrobia class, as close relative to Thermorudis peleae.
Characteristics
Living environment
Members of the class Thermomicrobia are broadly distributed across a wide range of both aquatic and terrestrial habitats. Thermomicrobium roseum was found in geothermally heated hot springs, Thermorudis pharmacophila and Thermobaculum terrenum from heated soils, and Thermomicrobium carboxidum and Thermorudis peleae from heated sediments In addition, Sphaerobacter thermophilus was found in sewage sludge that went through thermophilic treatment. The common features of their habitats include temperature ranging from around 65~75 °C and a pH around 6.0~8.0 (except for Nitrolancea hollandica which grow around 40 °C).
Metabolism
Members of Thermomicrobia class have variation in their basic metabolism. Nitrolancetus hollandica has nitrifying activity that utilize NO2− as energy source, which is unique in the whole Chloroflexota phylum. Thermomicrobium spp. and Sphaerobacter thermophilus have constitutive CO oxidizing not found in other species in this class. However, species of this class do share some features, as listed below:
All members except Thermobaculum terrenum have inability to utilize some common monosaccharides (e.g. glucose, fructose, etc.) as sole carbon source. The mechanisms behind this inability are currently unknown.