labeled bacterial DNA with protists present and with protists removed, attributing the
difference to viral lysis. Both protist feeding and viral infection balanced major
fractions of bacterial growth.
(^) The field of aquatic viral ecology is moving rapidly. Early reports of numbers and
activity show variable results that may in part be due to the specific methods used.
Viral abundances are typically determined by TEM (transmission electron
microscopy), EFM (epifluorescent microscopy) or FCM (flow cytometry). The most
widely applied method for measuring rates of virus production is “virus reduction and
grow-out”, which is similar to the dilution technique for measuring rates of grazing by
microzooplankton. New infections are decreased by reducing total virus abundance
(and virus–host contacts). This allows for enumeration of viruses released from
already infected cells. Water is ultrafiltered (0.2 μm) to reduce the viral population,
and added to water with naturally occurring bacteria and viruses to reduce the
populations of both to about 10% of their initial values. Frequent sampling (every 4–6
h) over a period of 20–36 h allows determination of short-term increases of viral
infections of natural communities of bacteria. Winter et al. (2004) found that viral
lysis generally occurs around noon, and that infections generally occur at night (Fig.
5.13). The two peaks observed in these incubation studies could result from two
different virus–host systems or two different infection events prior to collecting the
samples.
Fig. 5.13 Viral abundance over time in the virus dilution approach at three sites in the
North Sea. Abundances are normalized to a mean of zero to allow comparison of
relative changes among the three stations. Difference between average values of
duplicate incubations plotted against sampling time and the start of the incubations
corresponds to time needed to process the sample. Shaded areas represent night; error
bars are not shown for the sake of clarity.
(After Winter et al. 2004.)