Problems of Assessing the Effect of Pollutants on Microbial Activity

Pollution is defined as the ''introduction of materials or effects at a harmful level" (Davis, 1971). The microflora plays an important role because of degradation and consequent detoxification of pollutants, but on the other hand some micro organisms, by causing epidemics through pollutants etc., may produce undesirable effects. These problems have been examined extensively for many years. The effect of pollutants on the ecology and activities of the indigenous microflora of a specific environment has, however, received less attention. The terms of reference of this panel are to discuss the problems encountered when assessing the effect of pollutants on microbial activity. The organisers have emphasized that the purpose of this symposium is a discussion on methods. However, I feel, that for this panel the most important problem is the decision which of the numerous microbial ac tivities should be investigated. How to measure these is a secondary consideration, since other sessions will discuss techniques at length. It would be desirable to designate one indicator activity by which alone effects of pollutants on the microflora could be assessed; analo gous to indicator organisms that demonstrate contamination by sewage, for instance. This has not been possible, since various microbial activi ties are affected to a different degree by various pollutants. The selec tion of microbial activities and eventually techniques depends on the aims of the research, whether it is a fundamental study or one orientated towards practical applications. In the latter case investigations will be confined to ascertaining whether pollutants have adverse effects on those microbial activities that are believed to be economically important. The term pollution implies an undesirable effect, but in the case of micro organisms, stimulation of growth and metabolic activity can also occur. Furthermore, the term pollutant in the strict sense of the definition is hardly applicabable to non-persistent chemicals such as many herbicides, which are applied at very low rates in the field and which exhibit no toxic effects. The impact of the action of pollutants on certain microbial activities may vary according to the polluted environment. For instance, in soil rapid decomposition of fresh organic matter is often desirable. In an aquatic situation this may lead to the de-oxyg?nation of water. Thus a pollutant which slows down decomposition of organic residues under aquat ic conditions may not be harmful under all circumstances. The total inhibition of mineralization of nitrogen or nitrification by a pollutant in, for instance, a sewage plant could have a serious adverse effect, whilst in an agricultural environment an inhibition of nitrification may be de sirable, in that it preserves mineral nitrogen in the form of ammonia. Apart from these qualifying considerations, the selection of criteria and techniques for studying the effects of pollutants on the microflora is similar whatever the nature of the agent (Table 1).