Thermomicrobia

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:^[3][4]

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. ^{[note 1]}

As thermophilic bacteria, members of this class are usually found in environments which are distant from human activity.^[5] 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.^[6] At that time the genus was categorized under family Achromobacteraceae, but it became a distinct phylum by 2001.^[3]

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 Chloroflexi. The bacteria Sphaerobacter thermophilus originally described as an Actinobacteria is now considered a Thermomicrobia.^[4][7] In the same year, another strain of rose-pink thermophilic bacteria was isolated from Yellowstone National Park, which was named Thermobaculum terrenum.^[8] Later analysis based on genome put this species under Thermomicrobia class.^[9] However, the current standing of Thermobaculum terrenum is disputed.^[10]

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.^[11] 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.^[12]

In 2014, two thermophilic, Gram-positive, rod-shaped, non-spore-forming bacteria (strains KI3^T and KI4^T) 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 KI3^T strain, later named as Thermomicrobium carboxidum, is closely related to Thermomicrobium roseum. The KI4^T strain, later named as Thermorudis peleae, was proposed as a type strain of new genus Thermorudis.^[13]

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.^[5]

Characteristics

Taxonomy

Thermomicrobiota has the following taxonomy:^[21][22]

Phylum Thermomicrobiota

• Class Thermomicrobia
  • Order Thermomicrobiales
    • Family Thermomicrobiaceae
      • Genus Thermomicrobium^[6]
        • Thermomicrobium roseum^[6]
        • Thermomicrobium carboxidum^[13]
      • Genus Thermorudis^[13]
        • Thermorudis peleae^[13]
        • Thermorudis pharmacophila^[5]
  • Order Sphaerobacterales
    • Family Sphaerobacteraceae
      • Genus Sphaerobacter^[15]
        • Sphaerobacter thermophilus^[15]
      • Genus Nitrolancea^{[note 2][12]}
        • Nitrolancea hollandica^{[note 2][12]}

Notes

References