# Genomic Characterization of a Halovirus Representing a Novel Siphoviral Cluster
## 1 Introduction  
Hypersaline environments are distributed worldwide in different forms, such as salt lakes, sun-dried salt pans, and salt mines; archaea dominate hypersaline areas, but bacteria and some eukaryotes are also present [1]. The number of viruses in hypersaline environments can be 10-100 times more than that of the host [2, 3], reaching 10\({}^{9}\) virus-like particles (VLPs)/mL in some hypersaline water [4]. Viruses occupying hypersaline environments are known as haloviruses [5]. Compared with aquatic systems such as the ocean, there is less research on viruses in extreme environments such as hypersaline environments. Virus-like particles with various morphotypes have been described in hypersaline environments, ranging from 40 to 100 nm in diameter [6, 7]. There are six main forms, comprising viruses with head-tailed (contractile) \(\Phi\)Ch1 [8], head-tailed (noncontractile) BJ1 [9], head-tailed (short) HSTV-1 [10], pleomorphic HRPV-1 [11], spherical SH1 [12], and spindle-shaped His1 [13]. Over 100 haloviruses have been isolated to date, most of which infect extremehalophilic archaea. Among these 100 haloviruses, only \(\sim\)30 phages infect halophilic bacteria [14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27]. Currently, the research on culturable bacterial haloviruses mainly focuses on their morphological, physiological, and biochemical characteristics; research on the viral genome is still relatively scarce. Due to the small number of isolates, the genomes of only 16 bacterial haloviruses have been analyzed in detail [21; 22; 24; 25; 26; 27], hindering our understanding of virus-host interactions. Four Marseille viruses and one _Minivirus_ isolated from a hypersaline environment in Tunisia using the halophilic amoeba as the host are the only known eukaryotic haloviruses [28]. The genus _Halomonas_ is classified within the family _Halomonas_ and the order _Oceanospirillales_ in the class _Gammaproteobacteria_. The cells are Gram-negative and non-endospore-forming rods and are halophilic or halotolerant. Some species in _Halomonas_ are haloalkaliphilic or psychrotolerant [29]. The genus _Halomonas_ is a common bacterial group found in hypersaline samples using culture-dependent methods. The genus _Halomonas_ currently contains 121 species with validly published and correct names isolated from saline environments worldwide, including solar salt facilities, intertidal estuaries, the Dead Sea, and hypersaline lakes ([[https://lpsn.dsmz.de/genus/halomonas](https://lpsn.dsmz.de/genus/halomonas)]([https://lpsn.dsmz.de/genus/halomonas](https://lpsn.dsmz.de/genus/halomonas)), accessed on 17 June 2023). In addition to being widely distributed in various hypersaline environments, _Halomonas_ has substantial development potential due to its versatile properties, such as good emulsifying activities against the petroleum hydrocarbons and ability to produce polyhydroxybutyrate (PHB) and polyhydroxyalkanoate (PHA) [30; 31]. The phage-host interactions of five myoviruses infecting _H. venusta_ or _H. salina_ from the Great Salt Plains in Oklahoma [32] and three temperate sihowviruses infecting _H. halophila_ isolated from hypersaline soil have been characterized [33]. Moreover, a myovirus-like temperate phage has been successfully induced from ocean-isolated _H. aquararina_ using mitomycin C, and its genome has been sequenced [20]. However, to the best of our knowledge, there are no detailed characterizations to date of any virulent _Halomonas titanica_ viruses. The hypersaline environment has a high similarity with the early Earth environment and the Martian environment [34]. Thus, viruses in hypersaline environments may comprise remnants of ancestral viruses [4], and excavating and studying the physiological and ecological functions of uncultured haloviruses in hypersaline environments may help elucidate the origin and evolution of life on Earth and provide a theoretical reference for the exploration and discovery of life on Mars. This study isolated the first phage, YPHTV-1, infecting _H.