# Haloferacales

Haloferacales is an order of halophilic, chemoorganotrophic or heterotrophic archaea within the class Haloarchaea. The type genus of this order is Haloferax.
The name Haloferacales is derived from the Latin term Haloferax, referring to the type genus of the order and the suffix "-ales," an ending used to denote an order. Together, Haloferacales refers to an order whose nomenclatural type is the genus Haloferax.

## Biochemical characteristics and molecular signatures

Members are halophiles and can be chemoorganotrophs or heterotrophs and are isolated from high-salt environments such as marine solar salterns and the Dead Sea. Some members are motile and contain gas vesicles. Morphology is variable, including rod, coccus or flat square shapes. Members of this order grow optimally in neutral pH. The DNA G+C content for this order ranges between 55-66 mol%.
This order can be reliably distinguished from other Halobacteria by the presence of five conserved signature proteins (CSPs) and four conserved signature indels (CSIs) present in the following proteins: DNA gyrase B, prolyl t-RNA synthetase, acyl-CoA synthetase and aspartyl/glutamyl-tRNA amido-transferase subunit B.

## Historical systematics and current taxonomy

As of 2021, Haloferacales contains two families, Haloferacaceae and Halorubraceae. Members of this order was demarcated from the class Halobacteria, previously a large phylogenetically unrelated group of species with distinct biochemical characteristics and different ecological niches. The diverse range of morphological and physiological characteristics made it difficult to clarify the evolutionary relationship within the class beyond a genus level.
In 2015, Gupta et al. proposed the division of the class Halobacteria into the orders Halobacteriales, Haloferacales and Natrialbales based on comparative genomic analyses and the branching pattern of various phylogenetic trees constructed from several different datasets of conserved proteins and 16S rRNA sequences. Molecular markers, specifically conserved signature indels, specific to this order were also identified as evidence supporting the division independent of phylogenetic trees. Each order can be reliably distinguished from each other and other species based on the presence of these CSIs.

## Phylogeny

The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature (LPSN) and National Center for Biotechnology Information (NCBI).
| 16S rRNA based LTP_06_2022 | 53 marker proteins based GTDB 08-RS214 |
| Haloferacales Halobaculum Salinigranum Halolamina Halohasta Halonotius Haloferax Halopelagius Halogeometricum Halobellus Halobium Halogranum Halegenticoccus Haloprofundus Haloplanus Halalkaliarchaeum Halopenitus Haloparvum Halorubrum Haloferacaceae | Haloferacales Halobaculum Oren et al. 1995 Halolamina Cui et al. 2011 Natronocalculus Sorokin et al. 2023 "Halalkalirubrum" Zuo et al. 2021 Halohasta Mou et al. 2013 Halonotius Burns et al. 2010 Halalkaliarchaeum Sorokin et al. 2019 Halopenitus Amoozegar et al. 2012 Haloparvum Chen et al. 2016 Halorubrum McGenity & Grant 1996 Haloplanus Elevi Bardavid et al. 2007 Salinirubrum corrig. Cui and Qiu 2014 Haloprofundus Zhang et al. 2017 Halegenticoccus Liu et al. 2020 Halogranum Cui et al. 2010 Salinigranum Cui and Zhang 2014 Haloferax Torreblanca et al. 1986 Halopelagius Cui et al. 2010 Halogeometricum Montalvo-Rodriguez et al. 1998 Haloquadratum Burns et al. 2007 Halobellus Cui et al. 2011 Haloferacaceae Halorubraceae Haloferacaceae |