**3. Halophilic viruses and their infection cycles**

The process in which these viruses infect their host are similar to those of other viruses. Firstly, viruses adhere to the surface of the targeted cell and work their way into the cell's cytoplasm without detection. The next step in this process would be for the virus to replicate its genetic material using the hosts cell machinery. This process takes place in different fashions and timeframes that will be discussed later. The majority of halophilic viruses that have been discovered and studied contain DNA as their genetic material. There has yet to be a discovery of a halophilic virus that is made up of RNA [26]. After the virus replicates its genome and it is transcribed into mRNA, that mRNA is then translated into more viral particles. These particles then assemble and eventually leave the host cell to go on and infect another host. There can be variations of this process between different types of viruses and their own specific cycles. For example, viruses that behave in a lytic fashion proceed to take over the hosts cell machinery to produce its own progeny, resulting in destruction or lysis of the cell. Viruses that partake in this lytic lifestyle are also termed to be virulent. There are also viruses known as temperate viruses. This type of virus is known to infect halophilic prokaryotes in one of two ways. The first being that after invasion of the host cell they are able to integrate their own genetic information into the genome of the host and exist as a prophage. As the host cell undergoes its own replication cycle, the viral DNA is replicated along with it and is being passed along to daughter cells. Once the infected cell is subjected to instances of stress, this can cause the prophage to enter into the lytic cycle, take over the hosts cell machinery and eventually lead to cell lysis. The second type of temperate virus known to invade halophilic prokaryotes works by replicating its genetic material

within the cytoplasm of the host cell similarly to a plasmid. Viruses are also able to partake in a chronic infection type lifestyle, also known as a persistent infection. This is where once the virus is inside the host cell it is able to continuously replicate its genetic material within the host without causing cell lysis. This type of lifestyle is best used to describe the non-lytic halophilic viruses. Examples of this type of virus would be the lemon-shaped virus His1 and also the pleomorphic virus, HRPV-1. How these viruses are able to leave the cell without causing cell lysis is not completely understood but it is hypothesized that it may be through budding of the plasma membrane [26].

### **4. Different Halophage morphologies**

Not only can these halophages take part in multiple different types of infection cycles, but they are also known to have varying morphological structures. Of the halophilic viruses that have been isolated, there are 4 general categories of morphology in which they can be placed. The first being viruses with an icosahedral shape with either a medium length contractile tail, a short non-contractile tail or a long non-contractile tail. Second being viruses also with an icosahedral shape but this virus type also has an internal cell membrane. The final two categories of possible cellular morphologies include viruses that are pleomorphic and viruses that are lemon-shaped [26]. These varying morphologies can help us better understand the process in which these viruses are able to come into contact with the host cells and integrate their genetic information. In general, viruses are able to adhere to the hosts cell surface through some sort of receptor molecule. However, for haloviruses there has not been a specific receptor molecule identified. There has been a hypothesis that the receptor molecule for a known halovirus ɸCh1 could be a galactose residue found on the surface of a well-known haloalkaliphilic archaeon, *Natrialba magadii* [26]. The supporting evidence for this hypothesis is that the tail of ɸCh1 actually contains a protein fiber that has a galactose binding domain. Thus, when this virus comes within a close vicinity of the archaeon, its tail is able to bind with this galactose residue and allow the virus to invade the host cell. This theory is also supported by the fact that when there is a disruption in the genetic sequence of this virus that alters this protein fiber binding domain, the virus is no longer able to adhere to the hosts cell surface [26]. Furthermore, this would support the notion that if the virus has more than one protein fiber present within the tail, it could possibly have the ability to bind to more than one potential prokaryotic host [26].

Of the different viral morphology types, the tailed icosahedral type is one of the most common [27]. So naturally, there have been some tailed icosahedral halophilic viruses uncovered. These tailed viruses are thought to belong to either the *myoviridae* family or the *siphoviridae* family and preferentially infect archaea. These viruses not only have structural similarities between known bacteriophages, but they also have some genetic similarities. These findings suggest that they might share a common ancestor or that recombination might have taken place between the halophilic viruses and other cells found within the hypersaline environment. Examples of well-studied tailed icosahedral viruses include HSTV-2, HVTV-1 and HSTV-1. These three viruses are known to live a virulent life cycle and they do not contain an integrase sequence within their genetic material [26]. Integrase is the enzyme that lysogenic viruses generally use in order to integrate their own genetic information into the host cell's DNA to be replicated and transcribed. After these viruses infect the host cell and replicate their genetic information, they generally lyse the cell within 24 hours after the start of the infection. They also share an interesting feature that they are able to inactivate and reactivate their infectivity

### *Viruses of Extremely Halophilic Prokaryotes DOI: http://dx.doi.org/10.5772/intechopen.96720*

in response to lower salinities [26]. When the virus is subjected to a less than ideal or higher salt concentration, the active infection comes to a halt. When the virus is then placed back into a more optimal or lower salt concentration, the infection can resume. This type of adaptation is useful to viruses that are found in natural aquatic habitats because with rainfall or lack thereof, there can be either an increase or decrease in the water's salinity. This adaptation helps ensure that the virus can survive in instances of drought, where it may be a longer period of time before there is a decrease in the water's salt concentration. All halophilic viruses however, are not of the tailed icosahedral shape. There have also been halophilic viruses uncovered that have either the spherical or pleomorphic type morphology. These pleomorphic viruses can be isolated from not only aquatic habitats but salt crystals as well. They are also known to carry out a non-lytic lifestyle within their host. Examples of this type of virus would include His2 and HRPV-6 [26]. Finally, there are also the lemon-shaped viruses, which could possibly be the most common morphology for halophilic viruses.
