# A Novel Singleton Giant Phage Yong-XC31 Lytic to the _Pyropia_ Pathogen _Vibrio mediterranean_
## 1 Introduction  
Vibrios are ubiquitous in marine ecosystems living as well-described pathogens of aquatic fauna, for example, _Vibrio anguillarum_, _Vibrio harveyi_, _Vibrio coralliolyticus_ and _Vibrio aestariumus_[1, 2, 3, 4]. _Vibrio mediterranean_ is a potential emerging pathogen of marine animals such as corals and scallops [5, 6]. _V. mediterranean_ 117-T6 (CGMCC1.16311) was isolated from the bleached shell-born conchoelis of _Pyropia_ _yezensis_ and was pathogenic to the conchoelis of several _Pyropia_ species, including _P. yezensis_[7, 8]. _Pyropia_ culture has a long history in China, but there are no effective prevention and control measures for bacterial diseases, which seriously affect the economic income of farmers and the healthy and sustainable development of _Pyropia_ industry [8, 9]. Global warming has caused an increase in sea surface temperature that has undoubtedly led to the unseasonal outbreaks of Vibrios as well as their increased abundance and virulence in marine environments and aquaculture [10, 11]. Therefore, there are a growing need for effective methods for managing bacterial infections.  
Viruses, in particular bacteriophages are the most abundant biological entities in the oceans, showing an extremely high, uncharted diversity [12]. The potential of viruses as therapeutic agents to treat infectious diseases has been known for a long time, and they are considered to be important agents and resources as a solution to antibiotic resistance [13]. The phages with double-stranded (ds) DNA genomes larger than 200 kbp are defined as giant or \"jumbo\" bacteriophages [14]. Giant bacteriophages commonly contain many genes that do not exist in the small genome bacteriophages. For example, some giant phages have several paralogous genes for DNA polymerase and RNA polymerase (RNAP). Importantly, the proteins encoded by these additional genes may replace the function of the host proteins, thereby reducing the dependence of giant phages on their bacterial hosts [15]. In addition, many genes in giant phages are interpreted as coding hypothetical proteins which are not found in small phages, and their biological features are understudied [14]. These genes may be a new resource of proteins for industrial, agricultural, or medical applications in the future [16; 17; 18; 19]. However, large phages are not commonly isolated. Limited mobility on semi-solid plates, which prevented formation of visible plaques, may be the possible reason for the rare isolation of giant phage [15; 20]. A centrifugal force that is too high, in the centrifugation to remove bacteria and protists, may also be a reason.  
In recent years, bacteriophages have been increasingly applied for disease prevention and control in aquaculture [21; 22; 23]. To mine potential marine application resources for disease control of _Pyropia_ vibriosis, the first _V. mediterranean_ phage, named _Vibrio_ phage Yong-XC31 (abbreviated as Yong-XC31), was isolated from the coastal water of Meishan island (29 deg46.989 N 121 deg57.516 E), Ningbo, China. Characteristics and the complete genome of the phage were analyzed. _Vibrio_ phage Yong-XC31 is a giant phage presenting a new evolutionary lineage of phage.