The Chloroflexia are a class of bacteria in the phylum Chloroflexota. Chloroflexia are typically filamentous, and can move about through bacterial gliding. It is named after the order Chloroflexales.
Etymology The name "Chloroflexi" is a Neolatin plural of "Chloroflexus", which is the name of the first genus described. The noun is a combination of the Greek chloros (χλωρός) meaning "greenish-yellow" and the Latin flexus (of flecto)Lewis, Charlton T. and Charles Short, A Latin Dictionary. Oxford: Clarendon Press, 1879. Online version at Perseus meaning "bent" to mean "a green bending". The name is not due to chlorine, an element confirmed as such in 1810 by Sir Humphry Davy and named after its pale green colour.
Taxonomy and molecular signatures
The Chloroflexia class is a group of deep branching photosynthetic bacteria (with the exception of Herpetosiphon and Kallotenue species) that currently consist of three orders: Chloroflexales, Herpetosiphonales, and Kallotenuales. The Herpetosiphonales and Kallotenuales each consist of a single genus within its own family, Herpetosiphonaceae (Herpetosiphon) and Kallotenuaceae (Kallotenue), respectively, whereas the Chloroflexales are more phylogenetically diverse.
Microscopic distinguishing characteristics Members of the phylum Chloroflexota are monoderms and stain mostly Gram negative, whereas most bacteria species are diderms and stain Gram negative, with the Gram positive exceptions of the Bacillota (low GC Gram positives), Actinomycetota (high GC, Gram positives), and the Deinococcota (Gram positive, diderms with thick peptidoglycan).
Genetic distinguishing characteristics Comparative genomic analysis has recently refined the taxonomy of the class Chloroflexia, dividing the Chloroflexales into the suborder Chloroflexineae consisting of the families Oscillachloridaceae and Chloroflexaceae, and the suborder Roseiflexineae containing family Roseiflexaceae. The revised taxonomy was based on the identification of a number of conserved signature indels (CSIs) which serve as highly reliable molecular markers of shared ancestry.
Physiological distinguishing characteristics Additional support for the division of the Chloroflexales into two suborders is the observed differences in physiological characteristics where each suborder is characterized by distinct carotenoids, quinones, and fatty acid profiles that are consistently absent in the other suborder.
In addition to demarcating taxonomic ranks, CSIs may play a role in the unique characteristics of members within the clade: In particular, a four-amino-acid insert in the protein pyruvate flavodoxin/ferredoxin oxidoreductase, a protein which plays important roles in photosynthetic organisms, has been found exclusively among all members in the genus Chloroflexus, and is thought to play an important functional role.
Additional work has been done using CSIs to demarcate the phylogenetic position of Chloroflexia relative to other photosynthetic groups such as the Cyanobacteria. Chloroflexia shares a number of CSIs with Chlorobiota in the chlorophyll-synthesizing proteins. As the two lineages are not otherwise closely related, the interpretation is that the CSIs are the result of a horizontal gene transfer event between the two. Chloroflexia in turn acquired these proteins by another HGT from a "Clade C" marine cyanobacteria.
Phylogeny 16S rRNA based LTP_10_2024 120 marker proteins based GTDB release 10-RS226
Taxonomy
The currently accepted taxonomy is as follows:
Class Chloroflexia Gupta et al. 2013 Genus ?"Dehalobium" Wu et al. 2002 Genus ?"Candidatus Lithoflexus" Saghai et al. 2020 Genus ?"Candidatus Sarcinithrix" Nierychlo et al. 2019 Order "Chloroheliales" Tsuji et al. 2024 Family "Chloroheliaceae" Tsuji et al. 2024 Genus "Candidatus Chlorohelix" Tsuji et al. 2024 Order "Thermobaculales" Chuvochina et al. 2023 Family "Thermobaculaceae" Chuvochina et al. 2023 Genus "Thermobaculum" Botero et al. 2004 Order Kallotenuales Cole et al. 2013 Family Kallotenuaceae Cole et al. 2013 Genus Kallotenue Cole et al. 2013 Order Herpetosiphonales Gupta et al. 2013 Family Herpetosiphonaceae Gupta et al. 2013 Genus ?"Candidatus Anthektikosiphon" Ward, Fischer & McGlynn 2020 Genus Herpetosiphon Holt & Lewin 1968 Order Chloroflexales Gupta et al. 2013 Suborder Roseiflexineae Gupta et al. 2013 Family Roseiflexaceae Gupta et al. 2013 ["Kouleotrichaceae" Mehrshad et al. 2018] Genus ?"Candidatus Amarofilum" Petriglieri et al. 2023 Genus ?Heliothrix Pierson et al. 1986 Genus "Kouleothrix" Kohno et al. 2002 Genus "Candidatus Ribeiella" Petriglieri et al. 2023 Genus Roseiflexus Hanada et al. 2002 Suborder Chloroflexineae Gupta et al. 2013 Family Chloroflexaceae Gupta et al. 2013 Genus ?"Candidatus Chloranaerofilum" Thiel et al. 2016 Genus Chloroflexus Pierson & Castenholz 1974 ["Chlorocrinis"Ward et al. 1998] Family Oscillochloridaceae Gupta et al. 2013 Genus ?Chloronema ♪ Dubinina & Gorlenko 1975 Genus "Candidatus Chloroploca" Gorlenko et al. 2014 Genus Oscillochloris Gorlenko & Pivovarova 1989 Genus "Candidatus Viridilinea" Grouzdev et al. 2018
See also List of bacteria genera List of bacterial orders Green sulfur bacteria
References
Further reading
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