**Abstract**

As viruses are known to be the most distinct source of biodiversity, it is not surprising that they are the most abundant biological group in hypersaline environments such as aquatic systems which have saturated salt concentrations. However, of more than 6000 known prokaryote viruses less than 100 are considered to be extremely halophilic (salt loving) and have the ability to infect bacteria. Combination of information obtained from culture dependent and culture independent methods allow better understanding of these viruses. This review will update the advances in halophilic viruses and its impact on the bacteriophage studies.

**Keywords:** halophiles, viruses, halophilic viruses, hypersaline environment, bacteriophages

### **1. Introduction**

Halophiles are considered to be a part of a larger group of microorganisms called extremophiles. Like their name suggests, these microorganisms are able to thrive and survive within an extreme environment that would prove to be impossible for others. The extreme environment in which halophiles live are environments of high salinity or high salt concentration. Originally there were two categories of microorganisms considered to be halophiles, archaea and bacteria. However, within the last 50 years there was a discovery of yet another type of halophile, halophilic viruses. Viruses are in-fact one of the most abundant organism types within our biosphere and are able to infect organisms from all three domains of life [1–3]. Thousands of prokaryotic viruses have been identified but only a small portion of these are able to infect halophilic prokaryotes [4]. The origin of these halophilic viruses is not yet known but there are two differing hypotheses in regard to their arrival. One being that these viruses were the ones to originally give rise to other cell types; the other being that the different cell types gave rise to the viruses [5–11]. It has also been hypothesized that these halophilic viruses, also known as halophages, have served as a mode of genetic information between prokaryotes [9, 12]. This last hypothesis is supported by the fact that some of the largest viral sequences known, have some genetic similarities with the bacteria in which they prey upon. These halophages are able to be isolated from many different hypersaline environments all over the world and since their discovery have captured the interest of many different scientists. This is likely because not only do these viruses possess the ability to survive in these extreme environments, but they also have the ability to infect other halophilic organisms and live a wide variety of different life cycles. While there has been some

progress on better understanding these viruses, further research is needed; to not only understand the effects they have had and continue to have on the environment in which they live but to also have a better idea of their potential uses across a wide variety of industries [9].

### **2. Discovery and initial studies of halophages**

The discovery of the first halophilic virus happened accidentally while scientists were studying a known halophile, *Halobacterium salinarum,* which it had infected [9]. H. *salinarum* is an extremely halophilic archaea, despite its possibly misleading genus name of halobacterium. This archaeon is known for the discovery of bacteriorhodopsin, which is a light driven proton pump, that it utilizes as an energy source [13]. Since this discovery, there have been nine more viruses found that have the ability to infect halophilic bacteria as well as 56 other viruses that infect different species of halophilic archaea [9]. There has been further investigation into these types of viruses, including the work done by scientists Daniels and Wais. Working out of hypersaline ponds found in Jamaica they hypothesized that the samples collected both before and after rain fall would have a differing amount of both prokaryotes and halophilic viruses [14]. They thought that rain would act as a diluting agent in these ponds and would affect both the salinity of the water as well as its microbial community [9]. To evaluate this hypothesis, samples were collected both before and after rain fall. It was later learned that sample results not only varied between pre- and post-rain samples but also varied between large versus small sample sizes [9]. When Daniels and Wais were evaluating their "pre-rain", smaller sample volumes, they noted that there were fewer halophages as well as fewer plaques present [9]. The larger sample sizes under the same conditions on the other hand showed a larger number of viruses present as well as larger plaques [9, 15–17]. It has been suggested that plaque size is directly correlated with virulence, meaning, the larger a plaque appears to be the more virulent a virus is [14, 18, 19]. Not only the size of the plaques within the samples gives us information but also their opaqueness. Unlike the directly correlated relationship seen between virulence and plaque size, the opaqueness of the viral sample and its virulence are indirectly correlated. Meaning, the opaquer the plaques appear to be, the less virulent the viruses are, while samples that are clearer, suggests the viruses present are more lytic (or virulent) [20]. There have been arguments that these findings are not directly correlated with virulence and that the plaque appearance is due to chance. This is because of the argument that with the larger sample sizes there would be a larger virus to host ratio. This is supported by the consensus that more viruses tend to be present in comparison to prokaryotes within aquatic samples. Thus, an increase in the number of viruses' present could ultimately lead to more lysis taking place [9].

Another finding by Wais and Daniels was that after rainfall, there was a decrease in number of total halophilic prokaryotes present in the hypersaline ponds but an increase in free halophages [9]. It is important to note that extremely halophilic microorganisms generally thrive at a salinity of about 10%–30% NaCl. When concentrations drop below this range, this can disrupt the normal osmotic gradient present within the cell, resulting in cell lysis. Halophilic viruses on the other hand, are active at lower saline concentrations. From this information they suggested that the decrease in salinity lead to mortality in the prokaryotic population but prior to cell death, the viruses present within that host cell utilized the hosts cell machinery to replicate [9, 21–23]. While the viral population increased immediately after rainfall, this increase appeared to be short lived. Samples taken 24 days after the last rainfall showed that the viral population had decreased as the halophile population

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

returned to baseline levels as well as the salinity of the water [9]. These findings led Wais and Daniels to hypothesize that the less-virulent strains of halophages can exist within these hypersaline environments, but they are not active when salt concentrations are too high. This means that when these prokaryotes are in their ideal hypersaline environments, they are safe from viral predation, but, once those water salt concentrations fall below ideal levels, they are more susceptible to active halophilic viral infections [9]. While these halophages may not be active at high salt concentrations, they are still present and living within the host cells genetic material as a prophage. When the salinity of the aquatic environment would decrease, due to instances such as rainfall, these same viruses would become active and utilize the hosts cell machinery prior to host cell death [9]. They believed this was the strategy employed by these viruses to ensure that they are able to remain stable despite the less favorable hypersaline environment. This type of relationship is viewed by some as mutually beneficial [24]. The prokaryotes present in the extreme hypersaline environment can live without the concern of viral predation, while the halophages are also able to co-exist within the host and make use of its cell machinery prior to inevitable cell death [9]. Similar studies and findings were conducted by Torvisk and Dundas while they were investigating their halophilic viral isolate, Hs-1 [25]. They observed that when infection of *Halobacterium salinarum* took place in a lower salinity atmosphere, the virus appeared to be more virulent. Conversely, when the viral infection took place in an environment of higher salinity the virus behaved in a more lysogenic fashion [9]. They also noted that the rate at which the virus could adhere to the surface of these prokaryotes also decreased with increasing salinity, suggesting that as salinity increases, adsorption decreases. The findings between the four previously mentioned scientists help further support their theories from both an environmental observation and laboratory point of view.
