Environmental Engineering Reference
In-Depth Information
FIGURE 18.6 Virus-like particles attached to a bacterium from a freshwater lake (repro-
duced with permission from Pina et al., 1998).
Viral infection leads to lysis or breaking of cells and the cell contents
may be an important source of dissolved organic compounds that fuel the
microbial heterotrophic community. Researchers have long noticed that a
certain portion of organic carbon fixed by photosynthetic activity is lost to
solution, and viral infections may explain some of this loss. Interestingly,
release of dissolved organic matter by healthy planktonic cells may lower
rates of viral infection (Murray, 1995). The released substances bind the
viruses so they are no longer able to attach to cells. Thus, directly or indi-
rectly, viral activity may be a key feature in providing energy and nutrients
to the microbial loop (Bratbak et al., 1994).
Consumption of Small Cells
The primary difficulty in consuming very small particles from plank-
tonic systems is the energy required for extracting them from a dilute
aquatic environment. Very small particles are almost impossible to remove
via filtration because the Reynolds number is very low and the viscosity is
very high. A very fine-retention filter will not allow significant fluid flow
between the meshes. Thus, protozoa engulf the smallest particles individu-
ally, with particles as small as viruses effectively removed from solution
(Gonzalez and Suttle, 1993). Marine planktonic ciliates are major con-
sumers of very small bacteria and phytoplankton (Christaki et al., 1998).
Although less research has been done on them, ciliates can also be impor-
tant consumers of very small plankton in lakes (Porter et al., 1985). It has
been suggested that protozoa are the primary consumers of bacteria in all
aquatic environments and the only consumers of any importance in anoxic
environments (Finlay and Esteban, 1998).
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