in the transport of bacteria through aquifers has not been well studied. These eukaryotic predators of bacteria in the subsurface appear to be particularly abundant ( dry weight) in organically contaminated aquifer sediments.24,25 At least one organically contaminated sandy aquifer yielded evidence that protists might be more efficient at removing unattached bacteria being advected downgradient than are the organic and mineral coatings on the sediment grains.26 Also, growth of bacteria being transported downgradient in organically contaminated aquifers can be substantial and can offset the losses that occur as a result of attachment.21
The dearth of information on the importance of biologic controls on subsurface bacterial transport is due largely to the fact that many aspects of the subsurface ecosystem are so poorly understood that their effects on bacterial transport are not fully appreciated. A number of the biologic controls are interrelated and difficult to describe mathematically; this makes their study in field experiments difficult. However, the effects of several factors, such as microbial competition, predation, parasitism, and growth, on subsurface bacterial transport are ideal candidates for studies sponsored by the Biological and Environmental Research program that could build on the subsurface ecologic information already collected in previous research.
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