Dr. C. M. So & Prof. P. K. Wong
Department of Biology & Environmental Science Programme
Microbes have been used to solve environmental problems for many years. The triedand-true biological methods for waste treatment, like activated sludge, composting, and wetlands, are microbially-mediated and have long made major contribution to pollution abatement. Recently, remediation of polluted soil and groundwater by microorganisms, or bioremediation, have been studied extensively and applied with success. Microbes have also become a good source of alternatives to products which are hazardous to the environment.
Biopesticides based on bacteria and viruses can serve as substitutes to chemical pesticides which are highly toxic and persistent. Biodegradable plastics derived from microbial polymers may also replace the non-biodegradable synthetic polymers.
Microbes have been a good resource for solutions to environmental problems due to their characteristics. First, the vast variety of microorganisms possess very diverse metabolic capabilities which can be utilized in many different ways. In addition, these organisms naturally occur in and are well adapted to different environments. They can be readily isolated for use or utilized in situ. And since they are have been around in nature for millions of years, it is very unlikely that they will be harmful to the environment. But most of all, microbial solutions are usually more cost effective than other physical-chemical ones.
All sorts of microbes have found their roles in solving environmental problems. While bacteria are exploited most, fungi, algae and viruses have also been utilized in various applications. These organisms may be applied either as pure or mixed cultures. While the production of commodities like biodegradable plastics and biopesticides usually relies on pure cultures, waste treatment and bioremediation are predominantly achieved by the use of mixed cultures or indigenous microbes at the site.
In most cases, microbes help us to solve our environmental problems because they are benefiting from the processes too. All living organisms need energy to maintain cellular functions and propagate. Some metabolic processes relevant to the environmental applications to be discussed are: respiration, assimilation of essential elements, detoxification and co-metabolism.
Respiration is the major process for free energy production which involves oxidation of organics (and inorganics for lithotrophs) and the simultaneous reduction of electron acceptors. During aerobic respiration, an organic substrate like glucose is oxidized and oxygen (O
) is reduced. Such redox reaction generates energy as manifested by the formation of ATPs. Similarly, microbes can also oxidize various organic pollutants to yield the ATPs needed for cellular functions and growth, provided that the degradative enzymes are present in the organisms.
Microbes can also obtain energy by using electron acceptors other than O
. They can oxidize organic contaminants by coupling to the reduction of nitrate, iron(III) and sulfate.
Degradation of the contaminants can thus take place under anaerobic conditions. In rare cases, halogenated organics, such as 3-chlorobenzoate, can serve as electron acceptors themselves. The reduction of these compounds leads to dechlorination and detoxification.
Assimilatory processes are also utilized to solve environmental problems. Polymers like poly(
-hydroxyalkanoates) (PHA) can be biosynthesized from assimilated carbon by many bacteria. These PHA formed are extracted from the bacterial cells to produce biodegradable plastics. Nitrogen and phosphorus are also assimilated into biomass by various microbes and excess N and P nutrients can be removed from wastewaters during the process.
Furthermore, pollution by nitrogenous fertilizers can also be mitigated by microbial nitrogen fixation.
Microbes may degrade or transform pollutants for detoxification. For example, transformation of lindane to 2,3,4,5,6- pentachloro-1-cyclohexene detoxifies the pesticide.
Heavy metals like mercury, chromium and selenium can be enzymatically reduced to less toxic forms. These reactions sometime not only detoxify the toxicants but also yield energy by being incorporated into the respiratory process.
Cometabolism is the simultaneous transformation of an organic compound upon the utilization of another substrate for growth by the microbes. The transformed compound, however, cannot support growth. In many cases, cometabolism does not seem to benefit the microbes. However, it sometime act as be a detoxification mechanism. For example, the cometabolism of polychlorinated biphenyls (PCBs) during the degradation of biphenyl dechlorinates PCBs and detoxifies them.
New possibilities in environmental application of microbes are opened up in the advent of genetic engineering techniques. Not only some metabolic activities can be enhanced, biochemical pathways from two different microorganisms can also be combined to create novel pathways with new functions. Moreover, transgenic organisms are being developed to provide even more possibilities of applications.
Finally, certain examples of microbial solutions to environmental problems, both implemented locally and in foreign countries, are reviewed in further details.
1. What are the prerequisites for microbes to carry out degradation and transformation of chemical toxicants?