Abundance of viruses

It is difficult to locatea paper about marine virology which does not state the sheer abundance ofviruses in our oceans. In the late 1980’s, the discovery of the magnitude ofviruses in our oceans brought interest into the study of marine virology (BØrsheimet al., 1990). Before thenresearchers knew that there was a presence of viruses in the ocean, but it wasnot thought that they were abundant enough to have significant impacts on the ecologythere.

What was once a topic of just curiosity, was now a topic vital to bestudied in depth in order to understand the physical and biological effectsthat viruses have in our ocean (Suttle and Fuhrman, 2010). In recent years, theprocesses for estimating the abundance of viruses in the ocean has moved fromtransmission electron microscopy to epifluorescence microscopy and flowcytometry. Epifluorescence microscopy uses an epifluorescence microscope with ahigh intensity light that passes through the sample being examined andmagnifies it. Several thousand samples were taken from oceans all over theworld in order to quantify and estimate the abundance of viruses (Breitbart, 2011).

Injust one litre of surface seawater, there is approximately 1010 virus-likeparticles. This would mean that virus-like particles occur more often in thewater than bacteria and archea, which are the second largest biological entityin our oceans (Wommack and Colwell, 2000). This easily makes viruses the largestgroup of predators in our oceans, rivers and lakes. Even though viruses are onlya very small size of 20-200nm in length, they still make up the oceans secondbiggest biomass (Lodish et al., 2000).

They are second only to the total biomass of prokaryotes occuring in the water(Suttle, 2005).   Thequantification of viruses occurring in the marine environment has shown thatvirioplankton is the most abundant plankton class, which can vary drasticallybetween season and geographical position (Wommack and Colwell, 2000). Virusestend to follow the same general abundance occurrences as bacteria. They usuallyoccur in the greatest amount at the euphotic zone (layer in ocean closest tothe surface with enough light to allow photosynthesis to occur) and will continueto decrease in abundance at increasing depth. Viral abundance is usuallygreater in coastal regions rather than oligotrophic (nutrient poor) offshorewaters (Marchant et al., 2000).

Their general abundance patterns are alsosimilar to bacteria with the fact that sea ice can contain a higher amount ofviruses than the water lying beneath it (Fuhrman, 1999).XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXFor several decades, the effect that viruses had oncertain marine organisms was studied rather than the effect that they had onecological systems as a whole (Fuhrman and Suttle, 1993). Viruses were not seento be ecologically important in the marine food web, until the late 1980’swhere the true abundance of viruses in the marine environment was discovered. Virusesare not visible using a normal light microscope, so special transmissionelectron microscopes were used in order to visualise the viral particles.

Special processes are needed in order to get a concentrated level of viralparticles from water (Fuhrman, 1999).The use of a transmission electron microscope (TEM) wasadapted from the use of an electron microscope, which was the first method of virusquantification and counting published (Sharp, 1949). If the use of a TEM wasapplied rather than an electron microscope back then, research in this area andthe high abundance of viral particles in water would have been discovered up to40 years earlier. This could have changed the development of biologicaloceanography, which has only moved within the last two decades towards microbiologicalresearch (Fuhrman, 1999). Viruses are sensitive toecological changes that can occur in the water, for example algal blooms. Inone particular study, the abundance of viruses was noted during a spring diatombloom. The population size of viruses varied throughout, with 5 × 105 occurringbefore the bloom and 1.3 × 107 viruses ml?1 occurringone week after the peak of the bloom.

A high concentration of viral particleswas observed in a mucus layer encircling dead diatoms after the collapse of thebloom. Approximately 23% of the entire virus population were joined to thediatoms. This shows that viruses are active components of the microbial foodweb, and not just inactive species as once thought (Bratbak et al., 1990). Fluctuations in the concentrationof viruses can also occur due to lysis of host cells causing viral particles tobe released into the water. These concentration changes can occur in a matterof minutes (Bratbak et al.

, 1996).Virus abundance is most often linked to the concentration of prokaryotes in anarea, which indicates that most viruses infect bacteria and archaea. Viruseswith an approximate diameter of 60nm usually dominate the virus population (Cochlanet al., 1993).