Microbes and Plants - Freshwater Ecology: Concepts and Environmental Applications - page 128

(infectious proteins) and viroids (naked RNA) may be significant parasites,
although little is known about their importance in nature.
Viruses can be specific for one species or strain of organisms or more
widely infective. Those of the greatest interest to humans cause disease,
and many of these can be transmitted in water (Table 8.1). Viruses that in-
fect unicellular organisms are generally fatal if an infection proceeds be-
cause the reproductive virus lyses (bursts) the cell. Understanding the dy-
namics of microbial communities requires knowledge of how viruses are
transmitted.
A successful virus in an aquatic habitat must make contact with the
correct type of host cell. The virus must remain active long enough to ran-
domly encounter the appropriate host. The spread of viral infections is
greater when the density of host cells in the environment is high and the
length of time that a virus can remain viable outside the host is long. Many
things may inactivate viruses when outside of their hosts, including UV
light, absorption onto cells or remains of cells that are not proper hosts,
and predation by microflagellates that can ingest very small particles. In-
organic particles can enhance viral survival by limiting the previous factors
but can also lower infection rates because tight association with the inor-
ganic particles lowers the probability of contact with a host cell. Some of
these factors influence how long viruses can
survive in groundwater, which can be an im-
portant public health issue (Sidebar 8.1).

Sidebar 8.1.
Survival of Human Pathogenic Viruses
in Groundwater

Pathogenic viruses can enter groundwaters
through many different sources. Some of the most
common sources are land disposal of sewage,
overflow from septic systems, and livestock
waste. Leachate from solid waste landfills also
can contain viruses. For example, human infec-
tions were traced to viral contamination of
groundwater in Georgetown, Texas (coxsack-
ievirus and hepatitis A) and Meade County, Ken-
tucky (hepatitis A; Lipson and Stotzky, 1987), and
1500 people were infected with Norwalk virus
from a contaminated spring in Rome, Georgia
(Bitton, 1994). Contamination of drinking water
wells with hepatitis A, polio, or enteroviruses has
been documented throughout the world. Know-
ing how long these viruses can remain infective
is important to allow estimation of the probability
that groundwater flows will move them to drink-
ing water wells while they are still active.
Infective viruses have been demonstrated
to travel over 50 m (depth) from septic tanks
into drinking water wells. Controlled studies
have demonstrated movement up to 1.6 km hor-
izontally through soils (Gerba, 1987). Clearly,

ARCHAEA

The Archaea are prominent in extreme
environments including anaerobic waters,
hotsprings, and hypersaline environments
such as salt lakes. Some groups such as the
methanogens have global biogeochemical
importance, especially those populations
found in wetlands. The identification of
species or strains of archaebacteria gener-
ally is based on metabolic characteristics
(Table 8.2) and molecular analysis. These
organisms have similar morphology to the
Bacteria but are not closely related given ge-
netic and biochemical criteria. Analysis of
ribosomal RNA sequences has led to a clas-
sification of these organisms as separate
from the Bacteria (Fig. 7.1).

BACTERIA

The Bacteria are ubiquitous and may
have greater active biomass than any other
group of organisms on Earth (Whitman