Abundance of viruses

It is difficult to locate
a paper about marine virology which does not state the sheer abundance of
viruses in our oceans. In the late 1980’s, the discovery of the magnitude of
viruses in our oceans brought interest into the study of marine virology (BØrsheim
et al., 1990). Before then
researchers knew that there was a presence of viruses in the ocean, but it was
not thought that they were abundant enough to have significant impacts on the ecology
there. What was once a topic of just curiosity, was now a topic vital to be
studied in depth in order to understand the physical and biological effects
that viruses have in our ocean (Suttle and Fuhrman, 2010). In recent years, the
processes for estimating the abundance of viruses in the ocean has moved from
transmission electron microscopy to epifluorescence microscopy and flow
cytometry. Epifluorescence microscopy uses an epifluorescence microscope with a
high intensity light that passes through the sample being examined and
magnifies it. Several thousand samples were taken from oceans all over the
world in order to quantify and estimate the abundance of viruses (Breitbart, 2011).

In
just one litre of surface seawater, there is approximately 1010 virus-like
particles. This would mean that virus-like particles occur more often in the
water than bacteria and archea, which are the second largest biological entity
in our oceans (Wommack and Colwell, 2000). This easily makes viruses the largest
group of predators in our oceans, rivers and lakes. Even though viruses are only
a very small size of 20-200nm in length, they still make up the oceans second
biggest biomass (Lodish et al., 2000).
They are second only to the total biomass of prokaryotes occuring in the water
(Suttle, 2005).  

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The
quantification of viruses occurring in the marine environment has shown that
virioplankton is the most abundant plankton class, which can vary drastically
between season and geographical position (Wommack and Colwell, 2000). Viruses
tend to follow the same general abundance occurrences as bacteria. They usually
occur in the greatest amount at the euphotic zone (layer in ocean closest to
the surface with enough light to allow photosynthesis to occur) and will continue
to decrease in abundance at increasing depth. Viral abundance is usually
greater in coastal regions rather than oligotrophic (nutrient poor) offshore
waters (Marchant et al., 2000). Their general abundance patterns are also
similar to bacteria with the fact that sea ice can contain a higher amount of
viruses than the water lying beneath it (Fuhrman, 1999).

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For several decades, the effect that viruses had on
certain marine organisms was studied rather than the effect that they had on
ecological systems as a whole (Fuhrman and Suttle, 1993). Viruses were not seen
to be ecologically important in the marine food web, until the late 1980’s
where the true abundance of viruses in the marine environment was discovered. Viruses
are not visible using a normal light microscope, so special transmission
electron microscopes were used in order to visualise the viral particles.
Special processes are needed in order to get a concentrated level of viral
particles from water (Fuhrman, 1999).

The use of a transmission electron microscope (TEM) was
adapted from the use of an electron microscope, which was the first method of virus
quantification and counting published (Sharp, 1949). If the use of a TEM was
applied rather than an electron microscope back then, research in this area and
the high abundance of viral particles in water would have been discovered up to
40 years earlier. This could have changed the development of biological
oceanography, which has only moved within the last two decades towards microbiological
research (Fuhrman, 1999).

 

Viruses are sensitive to
ecological changes that can occur in the water, for example algal blooms. In
one particular study, the abundance of viruses was noted during a spring diatom
bloom. The population size of viruses varied throughout, with 5 × 105 occurring
before the bloom and 1.3 × 107 viruses ml?1 occurring
one week after the peak of the bloom. A high concentration of viral particles
was observed in a mucus layer encircling dead diatoms after the collapse of the
bloom. Approximately 23% of the entire virus population were joined to the
diatoms. This shows that viruses are active components of the microbial food
web, and not just inactive species as once thought (Bratbak et al., 1990). Fluctuations in the concentration
of viruses can also occur due to lysis of host cells causing viral particles to
be released into the water. These concentration changes can occur in a matter
of minutes (Bratbak et al., 1996).
Virus abundance is most often linked to the concentration of prokaryotes in an
area, which indicates that most viruses infect bacteria and archaea. Viruses
with an approximate diameter of 60nm usually dominate the virus population (Cochlan
et al., 1993).