Prey, predators and pollution

Bacteria can be used to clean soil effectively if they are protected from protozoans, their predator

Published: Friday 28 February 1997

contaminated soils can be cleaned if exposed to bacterial action. But the effectiveness of any such clean-up operation can seriously get hampered if the population of protozoa, which eat these bacteria, is not controlled. Bryan Travis and his team at Los Alamos National Laboratory in New Mexico, us, have discovered a way of suppressing the protozoan population while the bacteria do their work (New Scientist, Vol 153, No 2064).

The predator problem has to be taken into consideration because fast and cost-effective clean-ups depend on boosting the number of bacteria in the soil well beyond the level that occurs naturally. This is possible only if the prey (bacteria) are protected from the predators. To do this, nutrients are pumped into the soil. But when, as a consequence, the bacterial population explodes, so does the number of protozoa.

Travis and his team found that field results for bacterial cleaning operations often fell short of those achieved in laboratory conditions. They realised that this was because laboratory models took into account the mechanics of the flow of liquid or gases through the soil, but not the changes in the microbial community during the clean-up.

"The microbes aren't in a little microcosm, isolated from everything else," says Travis. "There are contaminants and various chemical fluids moving through their environment, so it is always changing." Travis' team is working on a model that includes microbial ecology with the soil's transport processes.

In field tests at the us department of energy's Savannah river site, Georgia, the researchers discovered that bioremediation (decontamination using bacteria) is most effective when nutrients for the bacteria are delivered in a series of small doses rather than all at once. This multicourse approach of feeding increases the number of bacteria while restricting the number of protozoa. The results were announced at the annual meeting of the American Geophysical Union in SanFransisco in December 1996.

Researchers are enthusiastic about Travis' idea. Manfred Lange, director of the Institute for Geophysics at the University of Munster in Germany, says that by using similar methods oil spills could be cleaned.

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