Functional impact of genetically modified micro-organisms on the soil ecosystem
Most attempts to monitor the effects of introductions of Genetically Modified Micro-organisms (GMMs) have centred on the enumeration of specific populations. However for a significant perturbation to be measured, changes of between 100% and 300% (0.3 and 0.5 on a log scale) are necessary for the impact to be significant. Standard population measurements, assessing the survival, dissemination and effect on total indigenous populations do not give an indication of the functioning of the ecosystem. There is very little literature regarding the functional impact of GMMs, for instance the effect upon nutrient cycling or functionally important groups of organisms. Through out this review a number of methods for the detection of perturbation in the rhizosphere/soil ecosystem are assessed. A range of non functional methods are summarised first, including molecular and non molecular genetic population diversity studies. Functional methodology, for example nutrient cycling, is then assessed for application as indicators of impact in the soil ecosystem and the significance of measuring functional impact is highlighted. These functional methods are classified into those relying upon the culturability of the target organisms and those that do not. Non culture methods discussed include biomass, respiration, nutrient cycling, mRNA studies and soil enzyme assays. Recently a range of soil enzyme assays have been used as alternatives to population measurements. The impact of a chromosomally marked Pseudomonas fluorescens (SBW25), on soil chitobiosidase, N-acetyl glucosaminidase, acid and alkaline phosphatases, phosphodiesterase, aryl sulphatase and urease were studied. Using these enzyme assays impacts of less than 20% could be detected. A series of interactions were observed which depended on whether a mixture of soil enzyme substrates were added to soil. Generally, microbial inoculation increased the enzyme activity of the biomass, but effects are likely to be 2 dependent upon the nature of the genetic modification. The environmental implications of these effects are discussed.