FREE VIRUSES IN THE FRESHWATER ENVIRONMENT:
A SCOPING INVESTIGATION
PART 2: FINAL REPORT AND SURVEY OF FRESHWATER BODIES
THE OCCURRENCE AND SIGNIFICANCE OF VIRUSES
IN THE FRESHWATER ENVIRONMENT
1. EXECUTIVE SUMMARY
1.1 All viruses are obligate parasites that must infect and probably destroy a host cell in order to replicate. Free viruses (virus-like particles; vlps) inhabit the marine and freshwater aquatic environments in concentrations of many millions per millilitre of water. The virus content of fresh water, therefore, is of concern to suppliers, users and consumers.
1.2 In Part 1 of the project, fluorescence microscopy and transmission electron microscopy (TEM) methods were developed for the enumeration of free viruses in fresh water, and described in FWR Research Report: FR/INV 0005 – April 1998.
1.3 The fluorescence microscopy technique, using YO-PRO-1 nucleic acid specific stain, was primarily developed for routine laboratory use as a user-friendly, relatively inexpensive method of virus enumeration. The TEM technique was used to categorise the viruses and as a method of assessing the efficiency of the fluorescence microscopy technique. The two microscopy methods in a standardised form were applied in the current work programme (Part 2 of the project) to assess the free virus populations within the virus communities of a range of UK freshwater bodies.
1.4 In routine practice, both microscopy methods proved simple to use and appeared to work well. In 19 out of the 24 samples measured (79%), the difference between virus counts resulting from fluorescence and electron microscopy techniques for any one sample was calculated at better than " 10% variance-from-mean, and in 11 out of 24 samples (46%) this was close to " 5%.
1.5 The fluorescence microscopy enumeration method was shown to be reliable for those bodies of fresh water in which humic substances or detritus were absent, or present only in the minimal quantities that did not adversely affect fluorescence response or interfere with free virus counting.
1.6 The virus populations recorded ranged between >106 vlps ml-1 for an English Lake District oligotrophic lake in winter, and >108 vlps ml-1 for an Essex reservoir in spring.
1.7 The free virus data produced were probably the first for free virus numbers in a range of UK fresh waters sampled seasonally over a 12 month period, and may act as baseline data for future investigations.
1.8 The health and ecological significance of viruses in water is reported upon. Their involvement reaches new heights of importance in view of the vast numbers of viruses found to be present in the aquatic habitat.
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